Maturation of lipid metabolism in the fetal and newborn sheep heart

At birth, the fetus experiences a dramatic change in environment that is accompanied by a shift in myocardial fuel preference from lactate and glucose in fetal life to fatty acid oxidation after birth. We hypothesized that fatty acid metabolic machinery would mature during fetal life in preparation for this extreme metabolic transformation at birth. We quantified the pre- (94-day and 135-day gestation, term ∼147 days) and postnatal (5 ± 4 days postnatal) gene expression and protein levels for fatty acid transporters and enzymes in hearts from a precocial species, the sheep. Gene expression of fatty acid translocase (CD36), acyl-CoA synthetase long-chain 1 (ACSL1), carnitine palmitoyltransferase 1 (CPT1), hydroxy-acyl dehydrogenase (HADH), acetyl-CoA acetyltransferase (ACAT1), isocitrate dehydrogenase (IDH), and glycerol phosphate acyltransferase (GPAT) progressively increased through the perinatal period, whereas several genes [fatty acid transport protein 6 (FATP6), acyl-CoA synthetase long chain 3 (ACSL3), long-chain acyl-CoA dehydrogenase (LCAD), very long-chain acyl-CoA dehydrogenase (VLCAD), pyruvate dehydrogenase kinase (PDK4), phosphatidic acid phosphatase (PAP), and diacylglycerol acyltransferase (DGAT)] were stable in fetal hearts and had high expression after birth. Protein expression of CD36 and ACSL1 progressively increased throughout the perinatal period, whereas protein expression of carnitine palmitoyltransferase 1a (fetal isoform) (CPT1a) decreased and carnitine palmitoyltransferase 1b (adult isoform) (CPT1b) remained constitutively expressed. Using fluorescent-tagged long-chain fatty acids (BODIPY-C12), we demonstrated that fetal (125 ± 1 days gestation) cardiomyocytes produce 59% larger lipid droplets (P < 0.05) compared with newborn (8 ± 1 day) cardiomyocytes. These results provide novel insights into the perinatal maturation of cardiac fatty acid metabolism in a precocial species.NEW & NOTEWORTHY This study characterized the previously unknown expression patterns of genes that regulate the metabolism of free fatty acids in the perinatal sheep myocardium. This study shows that the prenatal myocardium prepares for the dramatic switch from carbohydrate metabolism to near complete reliance on free fatty acids postnatally. Fetal and neonatal cardiomyocytes also demonstrate differing lipid storage mechanisms where fetal cardiomyocytes form larger lipid droplets compared with newborn cardiomyocytes.

Intrauterine growth restriction elevates circulating acylcarnitines and suppresses fatty acid metabolism genes in the fetal sheep heart

At birth, the mammalian myocardium switches from using carbohydrates as the primary energy substrate to free fatty acids as the primary fuel. Thus, a compromised switch could jeopardize normal heart function in the neonate. Placental embolization in sheep is a reliable model of intrauterine growth restriction (IUGR). It leads to suppression of both proliferation and terminal differentiation of cardiomyocytes. We hypothesized that the expression of genes regulating cardiac fatty acid metabolism would be similarly suppressed in IUGR, leading to compromised processing of lipids. Following 10 days of umbilicoplacental embolization in fetal sheep, IUGR fetuses had elevated circulating long-chain fatty acylcarnitines compared with controls (C14: CTRL 0.012 ± 0.005 nmol/ml vs. IUGR 0.018 ± 0.005 nmol/ml, P < 0.05; C18: CTRL 0.027 ± 0.009 nmol/mol vs. IUGR 0.043 ± 0.024 nmol/mol, P < 0.05, n = 12 control, n = 12 IUGR) indicative of impaired fatty acid metabolism. Uptake studies using fluorescently tagged BODIPY-C12-saturated free fatty acid in live, isolated cardiomyocytes showed lipid droplet area and number were not different between control and IUGR cells. mRNA levels of sarcolemmal fatty acid transporters (CD36, FATP6), acylation enzymes (ACSL1, ACSL3), mitochondrial transporter (CPT1), β-oxidation enzymes (LCAD, HADH, ACAT1), tricarboxylic acid cycle enzyme (IDH), esterification enzymes (PAP, DGAT) and regulator of the lipid droplet formation (BSCL2) gene were all suppressed in IUGR myocardium (P < 0.05). However, protein levels for these regulatory genes were not different between groups. This discordance between mRNA and protein levels in the stressed myocardium suggests an adaptive protection of key myocardial enzymes under conditions of placental insufficiency. KEY POINTS: The fetal heart relies on carbohydrates in utero and must be prepared to metabolize fatty acids after birth but the effects of compromised fetal growth on the maturation of this metabolic system are unknown. Plasma fatty acylcarnitines are elevated in intrauterine growth-restricted (IUGR) fetuses compared with control fetuses, indicative of impaired fatty acid metabolism in fetal organs. Fatty acid uptake and storage are not different in IUGR cardiomyocytes compared with controls. mRNA levels of genes regulating fatty acid transporter and metabolic enzymes are suppressed in the IUGR myocardium compared with controls, while protein levels remain unchanged. Mismatches in gene and protein expression, and increased circulating fatty acylcarnitines may have long-term implications for offspring heart metabolism and adult health in IUGR individuals. This requires further investigation.

Down-regulation of MEIS1 promotes the maturation of oxidative phosphorylation in perinatal cardiomyocytes

Fetal cardiomyocytes shift from glycolysis to oxidative phosphorylation around the time of birth. Myeloid ecotropic viral integration site 1 (MEIS1) is a transcription factor that promotes glycolysis in hematopoietic stem cells. We reasoned that MEIS1 could have a similar role in the developing heart. We hypothesized that suppression of MEIS1 expression in fetal sheep cardiomyocytes leads to a metabolic switch as found at birth. Expression of MEIS1 was assayed in left ventricular cardiac tissue and primary cultures of cardiomyocytes from fetal (100- and 135-d gestation, term = 145 d), neonatal, and adult sheep. Cultured cells were treated with short interfering RNA (siRNA) to suppress MEIS1. Oxygen consumption rate was assessed with the Seahorse metabolic flux analyzer, and mitochondrial activity was assessed by staining cells with MitoTracker Orange. Cardiomyocyte respiratory capacity increased with advancing age concurrently with decreased expression of MEIS1. MEIS1 suppression with siRNA increased maximal oxygen consumption in fetal cells but not in postnatal cells. Mitochondrial activity was increased and expression of glycolytic genes decreased when MEIS1 expression was suppressed. Thus, we conclude that MEIS1 is a key regulator of cardiomyocyte metabolism and that the normal down-regulation of MEIS1 with age underlies a gradual switch to oxidative metabolism.-Lindgren, I. M., Drake, R. R., Chattergoon, N. N., Thornburg, K. L. Down-regulation of MEIS1 promotes the maturation of oxidative phosphorylation in perinatal cardiomyocytes.

Diet Supplementation with Soy Protein Isolate, but Not the Isoflavone Genistein, Protects Against Alcohol-Induced Tumor Progression in DEN-Treated Male Mice

Diethylnitrosamine-treated male mice were assigned to 4 groups: a casein-based 35% high fat ethanol liquid diet (EtOH), an EtOH diet made with soy protein isolate protein (EtOH/SOY), an EtOH liquid diet supplemented with genistein (EtOH/GEN) and a chow group. EtOH feeding, final concentration 5% (v/v), continued for 16 wks. EtOH increased incidence and multiplicity of basophilic lesions and adenomas compared to the chow group, (p < 0.05). The EtOH/SOY group had reduced adenoma progression when compared to the EtOH and EtOH/GEN group, (p < 0.05). Genistein supplementation had no protective effect. Soy feeding significantly reduced serum ALT concentrations (p < 0.05), decreased hepatic TNFα and CD-14 expression and decreased nuclear accumulation of NFκB protein in EtOH/SOY-treated mice compared to the EtOH group (p < 0.05). With respect to ceramides, high resolution MALDI-FTICR Imaging mass spectrometry revealed changes in the accumulation of long acyl chain ceramide species, in particular C18, in the EtOH group when compared to the EtOH/SOY group. Additionally, expression of acid ceramidase and sphingosine kinase 1 which degrade ceramide into sphingosine and convert sphingosine to sphingosine-1-phosphate (S1P) respectively and expression of S1P receptors S1PR2 and S1PR3 were all upregulated by EtOH and suppressed in the EtOH/SOY group, p < 0.05. EtOH feeding also increased hepatocyte proliferation and mRNA expression of β-catenin targets, including cyclin D1, MMP7 and glutamine synthase, which were reduced in the EtOH/SOY group, p < 0.05. These findings suggest that soy prevents tumorigenesis by reducing inflammation and by reducing hepatocyte proliferation through inhibition of EtOH-mediated β-catenin signaling. These mechanisms may involve blockade of sphingolipid signaling.

MALDI Mass Spectrometry Imaging of N-Linked Glycans in Cancer Tissues

Glycosylated proteins account for a majority of the posttranslation modifications of cell surface, secreted, and circulating proteins. Within the tumor microenvironment, the presence of immune cells, extracellular matrix proteins, cell surface receptors, and interactions between stroma and tumor cells are all processes mediated by glycan binding and recognition reactions. Changes in glycosylation during tumorigenesis are well documented to occur and affect all of these associated adhesion and regulatory functions. A MALDI imaging mass spectrometry (MALDI-IMS) workflow for profiling N-linked glycan distributions in fresh/frozen tissues and formalin-fixed paraffin-embedded tissues has recently been developed. The key to the approach is the application of a molecular coating of peptide-N-glycosidase to tissues, an enzyme that cleaves asparagine-linked glycans from their protein carrier. The released N-linked glycans can then be analyzed by MALDI-IMS directly on tissue. Generally 40 or more individual glycan structures are routinely detected, and when combined with histopathology localizations, tumor-specific glycans are readily grouped relative to nontumor regions and other structural features. This technique is a recent development and new approach in glycobiology and mass spectrometry imaging research methodology; thus, potential uses such as tumor-specific glycan biomarker panels and other applications are discussed.

Molecular pathology of prostate cancer

This chapter includes discussion of the molecular pathology of tissue, blood, urine, and expressed prostatic secretions. Because we are unable to reliably image the disease in vivo, a 12 core method that oversamples the peripheral zone is widely used. This generates large numbers of cores that need to be carefully processed and sampled. In spite of the large number of tissue cores, the amount of tumor available for study is often quite limited. This is a particular challenge for research, as new biomarker assays will need to preserve tissue architecture intact for histopathology. Methods of processing and reporting pathology are discussed. With the exception of ductal variants, recognized subtypes of prostate cancer are largely confined to research applications, and most prostate cancers are acinar. Biomarker discovery in urine and expressed prostatic secretions would be useful since these are readily obtained and are proximate fluids. The well-known challenges of biomarker discovery in blood and urine are referenced and discussed. Mediators of carcinogenesis can serve as biomarkers as exemplified by mutations in PTEN and TMPRSS2:ERG fusion. The use of proteomics in biomarker discovery with an emphasis on imaging mass spectroscopy of tissues is discussed. Small RNAs are of great interest, however, their usefulness as biomarkers in clinical decision making remains the subject of ongoing research. The chapter concludes with an overview of blood biomarkers such as circulating nucleic acids and tumor cells and bound/free isoforms of prostate specific antigen (PSA).

Connexin-independent ganciclovir-mediated killing conferred on bystander effect-resistant cell lines by a herpes simplex virus-thymidine kinase-expressing colon cell line

A novel gap junction-independent mechanism for ganciclovir-mediated bystander effect killing by a herpes simplex virus thymidine kinase (HSV-TK)-expressing SW620 human colon tumor cell line has been characterized. The mechanism of the HSV-TK/GCV bystander effect for many tumor cell lines has been demonstrated to be due to connexin gap junction transfer of phosphorylated ganciclovir (GCV) metabolites; however, there may be as yet uncharacterized connexin-independent mechanisms for the effect. To address this, the bystander effect was further evaluated in a panel of cell lines mixed with homologous HSV-TK-expressing cell lines, a SW620.TK cell line, or a high connexin43-expressing PA-317.TK cell line. Of the 10 cell lines tested, 4 were found to be resistant to bystander effect killing by their homologous HSV-TK-expressing cell lines and the PA-317.TK cells, but all of the cell lines were sensitive to GCV killing when mixed with the SW620.TK cells. The SW620.TK cells were then further evaluated for any indication of extracellular GCV metabolite efflux. Culture medium from SW620.TK cells labeled with [(3)H]GCV was evaluated for the presence of GCV nucleotides by ion-exchange column separation and HPLC analysis. The presence of GCV mono-, di-, and triphosphate metabolites in the medium was detected. Inclusion in the medium of inhibitors of extracellular phosphatases and ecto-ATPases increased the proportion of GCV metabolites recovered. These results indicate that phosphorylated GCV metabolites can be effluxed from SW620.TK cells and that some type of cellular uptake mechanism independent of gap junctions exists for nucleotide entry into neighboring cells.

Conservative mutations of glutamine-125 in herpes simplex virus type 1 thymidine kinase result in a ganciclovir kinase with minimal deoxypyrimidine kinase activities

The herpes simplex virus type 1 thymidine kinase (HSV-1 TK) is the major anti-herpes virus pharmacological target, and it is being utilized in combination with the prodrug ganciclovir as a toxin gene therapeutic for cancer. One active-site amino acid, glutamine-125 (Gln-125), has been shown to form hydrogen bonds with bound thymidine, thymidylate, and ganciclovir in multiple X-ray crystal structures. To examine the role of Gln-125 in HSV-1 TK activity, three site-specific mutations of this residue to an aspartic acid, an asparagine, or a glutamic acid were introduced. These three mutants and wild-type HSV-1 TK were expressed in E. coli and partially purified and their enzymatic properties compared. In comparison to the Gln-125 HSV-1 TK, thymidylate kinase activity of all three mutants was decreased by over 90%. For thymidine kinase activity relative to Gln-125 enzyme, the K(m) of thymidine increased from 0.9 microM for the parent Gln-125 enzyme to 3 microM for the Glu-125 mutant, to 6000 microM for the Asp-125 mutant, and to 20 microM for the Asn-125 mutant. In contrast, the K(m) of ganciclovir decreased from 69 microM for the parent Gln-125 enzyme to 50 microM for the Asn-125 mutant and increased to 473 microM for the Glu-125 mutant. The Asp-125 enzyme was able to poorly phosphorylate ganciclovir, but with nonlinear kinetics. Molecular simulations of the wild-type and mutant HSV-1 TK active sites predict that the observed activities are due to loss of hydrogen bonding between thymidine and the mutant amino acids, while the potential for hydrogen bonding remains intact for ganciclovir binding. When expressed in two mammalian cell lines, the Glu-125 mutant led to GCV-mediated killing of one cell line, while the Asn-125 mutant was equally as effective as wild-type HSV-1 TK in metabolizing GCV and causing cell death in both cell lines.

Two-drug combinations that increase apoptosis and modulate bak and bcl-X(L) expression in human colon tumor cell lines transduced with herpes simplex virus thymidine kinase

Herpes simplex virus thymidine kinase (HSV-TK) and ganciclovir (GCV) gene therapy can induce apoptosis in tumor cells that are normally resistant to this type of cell death, although the cellular mechanisms by which this occurs remain to be elucidated. Human colon tumor cell lines expressing HSV-TK were treated with GCV or four other inducers of apoptosis: butyrate, camptothecin (CPT), Taxol (paclitaxel), or 7-hydroxystaurosporine (UCN-01). Over a 2-4 day treatment period with GCV or the other four drugs, protein levels of the apoptosis agonist Bak increased 1.5- to 3-fold, whereas a corresponding decrease in the levels of the apoptosis antagonist, Bcl-X(L), was observed in butyrate-, CPT-, and 7-hydroxystaurosporine (UCN-01)-treated cells. GCV and paclitaxel treatments resulted in increased levels of Bcl-X(L). In two-drug combinations with GCV plus one of the four other drugs, increased tumor cell killing was found with GCV plus UCN-01 or with some GCV/butyrate combinations; the other two tested combinations were largely antagonistic. The GCV/UCN-01 and GCV/butyrate combinations resulted in increased Bak and decreased Bcl-X(L) protein levels, while the GCV/CPT and GCV/paclitaxel combinations resulted in increased levels of both proteins. The results highlight the potential for new combination therapies of HSV-TK/GCV and chemotherapeutic drugs that result in increased tumor cell apoptosis for future treatments of colon cancer.

Ganciclovir-mediated cell killing and bystander effect is enhanced in cells with two copies of the herpes simplex virus thymidine kinase gene

Delivery and expression of the herpes simplex virus thymidine kinase (HSVtk) gene in combination with the prodrug ganciclovir is currently being evaluated for the treatment of many types of cancer. After initial phosphorylation by HSVtk, cellular kinases generate the toxic triphosphate form of ganciclovir (GCV). To further define the role of GCV metabolism in cells expressing HSVtk, two human tumor cell lines, UMSCC29 and T98G, were transduced with HSVtk and screened for insertion of one or two copies of the viral transgene by Southern blot analysis. Both the relative capacities for incorporating labeled GCV and the levels of GCV metabolites were determined for each of the parental cell lines and their derivatives containing either one or two copies of the HSVtk gene. The efficiency of GCV killing and the magnitude of the bystander effect were compared for the single- and double-copy HSVtk cell lines. Consistently, cells that expressed two copies of HSVtk metabolized GCV more efficiently, were more sensitive to GCV, and demonstrated improved bystander killing relative to single-copy HSVtk cells. The implications of these results for future and current therapies employing HSVtk and GCV are discussed.

Identification and modification of the uridine-binding site of the UDP-GalNAc (GlcNAc) pyrophosphorylase

UDP-GalNAc pyrophosphorylase (UDP-GalNAcPP; AGX1) catalyzes the synthesis of UDP-GalNAc from UTP and GalNAc-1-P. The 475-amino acid protein (57 kDa protein) also synthesizes UDP-GlcNAc at about 25% the rate of UDP-GalNAc. The cDNA for this enzyme, termed AGX1, was cloned in Escherichia coli, and expressed as an active enzyme that cross-reacted with antiserum against the original pig liver UDP-HexNAcPP. In the present study, we incubated recombinant AGX1 with N(3)-UDP-[(32)P]GlcNAc and N(3)-UDP-[(32)P]GalNAc probes to label the nucleotide-binding site. Proteolytic digestions of the labeled enzyme and analysis of the resulting peptides indicated that both photoprobes cross-linked to one 24-amino acid peptide located between residues Val(216) and Glu(240). Four amino acids in this peptide were found to be highly conserved among closely related enzymes, and each of these was individually modified to alanine. Mutation of Gly(222) to Ala in the peptide almost completely eliminated UDP-GlcNAc and UDP-GalNAc synthesis, while mutation of Gly(224) to Ala, almost completely eliminated UDP-GalNAc synthesis, but UDP-GlcNAc was only diminished by 50%. Both of these mutations also resulted in almost complete loss of the ability of the mutated proteins to cross-link N(3)-UDP-[(32)P]GlcNAc or N(3)-UDP-[(32)P]GalNAc. On the other hand, mutations of either Pro(220) or Tyr(227) to Ala did not greatly affect enzymatic activity, although there was some reduction in the ability of these proteins to cross-link the photoaffinity probes. We also mutated Gly(111) to Ala since this amino acid was reported to be necessary for catalysis (Mio, T., Yabe, T., Arisawa, M., and Yamada-Okabe, H. (1998) J. Biol. Chem. 273, 14392-14397). The Gly(111) to Ala mutant lost all enzymatic activity, but interestingly enough, this mutant protein still cross-linked the radioactive N(3)-UDP-GlcNAc although not nearly as well as the wild type. On the other hand, mutation of Arg(115) to Ala had no affect on enzymatic activity although it also reduced the amount of cross-linking of N(3)-UDP-[(32)P]GlcNAc. These studies help to define essential amino acids at or near the nucleotide-binding site and the catalytic site, as well as peptides involved in binding and catalysis.

Differential ganciclovir-mediated cell killing by glutamine 125 mutants of herpes simplex virus type 1 thymidine kinase

The therapeutic combination of the herpesvirus simplex virus type 1 (HSV-1) thymidine kinase (TK) gene and the prodrug, ganciclovir (GCV), has found great utility for the treatment of many types of cancer. After initial phosphorylation of GCV by HSV-1 TK, cellular kinases generate the toxic GCV-triphosphate metabolite that is incorporated into DNA and eventually leads to tumor cell death. The cellular and pharmacological mechanisms by which metabolites of GCV lead to cell death are still poorly defined. To begin to address these mechanisms, different mutated forms of HSV-1 TK at residue Gln-125 that have distinct substrate properties were expressed in mammalian cell lines. It was found that expression of the Asn-125 HSV-1 TK mutant in two cell lines, NIH3T3 and HCT-116, was equally effective as wild-type HSV-1 TK for metabolism and sensitivity to GCV, bystander effect killing and induction of apoptosis. The major difference between the two enzymes was the lack of deoxypyrimidine metabolism in the Asn-125 TK-expressing cells. In HCT-116 cells expressing the Glu-125 TK mutant, GCV metabolism was greatly attenuated, yet at higher GCV concentrations, cell sensitivity to the drug and bystander effect killing were diminished but still effective. Cell cycle analysis, 4', 6'-diamidine-2'-phenylindoledihydrochloride staining, and caspase 3 activation assays indicated different cell death responses in the Glu-125 TK-expressing cells as compared with the wild-type HSV-1 TK or Asn-125 TK-expressing cells. A mechanistic hypothesis to explain these results based on the differences in GCV-triphosphate metabolite levels is presented.

Variability of human hepatic UDP-glucuronosyltransferase activity

The availability of a unique series of liver samples from human subjects, both control patients (9) and those with liver disease (6; biliary atresia (2), retransplant, chronic tyrosinemia type I, tyrosinemia, Wilson's disease) allowed us to characterize human hepatic UDP-glucuronosyltransferases using photoaffinity labeling, immunoblotting and enzymatic assays. There was wide inter-individual variation in photoincorporation of the photoaffinity analogs, [32P]5-azido-UDP-glucuronic acid and [32P]5-azido-UDP-glucose and enzymatic glucuronidation of substrates specific to the two subfamilies of UDP-glucuronosyltransferases. However, the largest differences were between subjects with liver disease. Glucuronidation activities toward one substrate from each of the UDP-glucuronosyltransferases subfamilies, 1A and 2B, for control and liver disease, respectively, were 1.7-4.5 vs 0.4-4.7 nmol/mg x min for hyodeoxycholic acid (2B substrate) and 9.2-27.9 vs 8.1-75 nmol/mg x min for pchloro-m-xylenol (1A substrate). Microsomes from a patient with chronic tyrosinemia (HL32) photoincorporated [32P]5-azido-UDP-glucuronic acid at a level 1.5 times higher than the other samples, was intensely photolabeled by [32P]5-azido-UDP-glucose and had significantly higher enzymatic activity toward p-chloro-m-xylenol. Immunoblot analysis using anti-UDP-glucuronosyltransferase antibodies demonstrated wide inter-individual variations in UDP-glucuronosyltransferase protein with increased UDP-glucuronosyltransferase protein in HL32 microsomes, corresponding to one of the bands photolabeled by both probes. Detailed investigation of substrate specificity, using substrates representative of both the 1A (bilirubin, 4-nitrophenol) and 2B (androsterone, testosterone) families was carried out with HL32, HL38 (age and sex matched control) and HL18 (older control). Strikingly increased (5-8-fold) glucuronidation activity was seen in comparison to HL18 only with the phenolic substrates. The results indicate that one or more phenol-specific UDP-glucuronosyltransferase 1A isoforms are expressed at above normal levels in this tyrosinemic subject.

Variability of human hepatic UDP-glucuronosyltransferase activity

The availability of a unique series of liver samples from human subjects, both control patients (9) and those with liver disease (6; biliary atresia (2), retransplant, chronic tyrosinemia type I, tyrosinemia, Wilson's disease) allowed us to characterize human hepatic UDP-glucuronosyltransferases using photoaffinity labeling, immunoblotting and enzymatic assays. There was wide inter-individual variation in photoincorporation of the photoaffinity analogs, [32P]5-azido-UDP-glucuronic acid and [32P]5-azido-UDP-glucose and enzymatic glucuronidation of substrates specific to the two subfamilies of UDP-glucuronosyltransferases. However, the largest differences were between subjects with liver disease. Glucuronidation activities toward one substrate from each of the UDP-glucuronosyltransferases subfamilies, 1A and 2B, for control and liver disease, respectively, were 1.7-4.5 vs 0.4-4.7 nmol/mg x min for hyodeoxycholic acid (2B substrate) and 9.2-27.9 vs 8.1-75 nmol/mg x min for pchloro-m-xylenol (1A substrate). Microsomes from a patient with chronic tyrosinemia (HL32) photoincorporated [32P]5-azido-UDP-glucuronic acid at a level 1.5 times higher than the other samples, was intensely photolabeled by [32P]5-azido-UDP-glucose and had significantly higher enzymatic activity toward p-chloro-m-xylenol. Immunoblot analysis using anti-UDP-glucuronosyltransferase antibodies demonstrated wide inter-individual variations in UDP-glucuronosyltransferase protein with increased UDP-glucuronosyltransferase protein in HL32 microsomes, corresponding to one of the bands photolabeled by both probes. Detailed investigation of substrate specificity, using substrates representative of both the 1A (bilirubin, 4-nitrophenol) and 2B (androsterone, testosterone) families was carried out with HL32, HL38 (age and sex matched control) and HL18 (older control). Strikingly increased (5-8-fold) glucuronidation activity was seen in comparison to HL18 only with the phenolic substrates. The results indicate that one or more phenol-specific UDP-glucuronosyltransferase 1A isoforms are expressed at above normal levels in this tyrosinemic subject.

Lack of bystander killing in herpes simplex virus thymidine kinase-transduced colon cell lines due to deficient connexin43 gap junction formation

The efficacy of herpes simplex virus thymidine kinase (HSV-TK) gene therapy for colorectal carcinoma has been investigated in an in vitro system. The magnitude and the mechanism of the HSV-TK bystander effect was determined in three human colon tumor cell lines: HCT-116, HCT-8, and HT-29. Each HSV-TK(+) cell line was generated by stable transduction with a bicistronic retroviral vector containing the HSV-TK and neomycin resistance (neo) genes; each exhibited an IC50 for GCV of < or =4 microM. When GCV was added to HSV-TK(+) cells mixed with parental cells or known bystander-positive cell lines, no bystander killing was evident in the HT-29 or HCT-8 cells. Western blots detected the expression of the gap junction protein connexin43 (Cx43) in HCT-8 and HT-29 cells; however, immunolocalization studies indicated predominantly cytoplasmic staining of Cx43 and no cell surface staining in these cell lines. Stable transfection of HCT-8 and HT-29 cells with Cx43 resulted in increased levels of Cx43 expression with the same subcellular distribution as before, yet there was again no apparent bystander killing. In contrast, Cx43 expression was localized to the cell surface in the bystander-positive colon tumor cell line HCT-116. These results demonstrate that expression and proper surface localization of Cx43 gap junctions are necessary components of the bystander effect in human colon tumor cells. They also indicate that future combination gene therapy approaches using coexpression of HSV-TK and Cx43 genes may not be applicable to all tumor systems.

Synthesis of 5-azido-UDP-N-acetylhexosamine photoaffinity analogs and radiolabeled UDP-N-acetylhexosamines

Nuleotide sugar photoaffinity analogs have proven to be useful in the identification and characterization of glycosyltransferases. A radioenzymatic synthesis of [32P]5-azido-UDP-N-acetylglucosamine has been accomplished using 5-azido-UTP, [gamma-32P]ATP, porcine N-acetylgalactosamine kinase, and Escherichia coli UDP-N-acetylglucosamine pyrophosphorylase, GlmU. This general enzymatic scheme was useful for the synthesis of [32P]5-azido-UDP-N-acetylgalactosamine and high-specific-activity [3H] or [32P]UDP-N-acetylhexosamines. A new chemical synthesis method for generating 5-azido-uridine compounds was also developed. [32P]5-Azido-UDP-N-acetylglucosamine was functionally characterized using different soluble and membrane-associated glycosyltransferases which utilize UDP-GlcNAc as a substrate. Site-specific photoincorporation was observed for partially purified GlmU and porcine UDP-GlcNAc pyrophosphorylase. The photoprobe also effectively photoincorporated into the alpha- and beta-subunits of purified bovine UDP-N-acetylglucosamine:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase. Lastly, the photoprobe was also effective at photolabeling Streptococcus pyogenes hyaluronate synthase in membrane preparations.

Identification of a nucleotide binding site in HIV-1 integrase

HIV-1 integrase is essential for viral replication and can be inhibited by antiviral nucleotides. Photoaffinity labeling with the 3'-azido-3'-deoxythymidine (AZT) analog 3',5-diazido-2', 3'-dideoxyuridine 5'-monophosphate (5N3-AZTMP) and proteolytic mapping identified the amino acid 153-167 region of integrase as the site of photocrosslinking. Docking of 5N3-AZTMP revealed the possibility for strong hydrogen bonds between the inhibitor and lysines 156, 159, and 160 of the enzyme. Mutation of these residues reduced photocrosslinking selectively. This report elucidates the binding site of a nucleotide inhibitor of HIV-1 integrase, and possibly a component of the enzyme polynucleotide binding site.

Identification and molecular cloning of a unique hyaluronan synthase from Pasteurella multocida

Type A Pasteurella multocida, a prevalent animal pathogen, employs a hyaluronan [HA] polysaccharide capsule to avoid host defenses. We utilized transposon insertional mutagenesis to identify the P. multocida HA synthase, the enzyme that polymerizes HA. A DNA fragment from a wild-type genomic library could direct HA production in vivo in Escherichia coli, a bacterium that normally does not produce HA. Analysis of truncated plasmids derived from the original clone indicated that an open reading frame encoding a 972-residue protein was responsible for HA polymerization. This identification was confirmed by expression cloning in E. coli; we observed HA capsule formation in vivo and detected activity in membrane preparations in vitro. The polypeptide size was verified by photoaffinity labeling of the native P. multocida HA synthase with azido-UDP sugar analogs. Overall, the P. multocida sequence is not very similar to the other known HA synthases from streptococci, PBCV-1 virus, or vertebrates. Instead, a portion of the central region of the new enzyme is more homologous to the amino termini of other bacterial glycosyltransferases that produce different capsular polysaccharides or lipopolysaccharides. In summary, we have discovered a unique HA synthase that differs in sequence and predicted topology from the other known enzymes.

Metabolism and activities of 3'-azido-2',3'-dideoxythymidine and 2',3'-didehydro-2',3'-dideoxythymidine in herpesvirus thymidine kinase transduced T-lymphocytes

T-lymphocytes transduced with the conditionally toxic herpesvirus thymidine kinase gene (HSV-1 TK) are increasingly becoming important tools in genetic therapy approaches for treating viral infections and cancers. Therefore, the effects of different antiviral nucleoside drugs on the growth inhibition of parental and HSV-1 TK-transduced human T-lymphocyte cell lines (H9 and CEM TK-) were examined. As expected, both transduced cell lines were most sensitive to growth inhibition by ganciclovir (GCV). While the presence of HSV-1 TK did not potentiate 3'-azido-2',3'-dideoxythymidine (AZT) growth inhibition of H9 cells containing cellular TK; transduction of HSV-1 TK into the cellular TK-deficient CEM cells (CEM TK-) restored sensitivity to AZT. In both transduced cell lines, an HSV-1 TK-dependent growth inhibition with 2',3'-didehydro-2',3'-dideoxythymidine (d4T) was observed and a Km of 143 microM for d4T and HSV-1 TK was determined. Metabolic labeling analysis showed that drug metabolism correlated with the observed effects on cell growth. The effects of HIV-1 replication in the CEM TK- cell lines in the presence of AZT or d4T was evaluated. CEM TK- cells are largely resistant to AZT or d4T inhibition of HIV-1 replication, however, transduction of HSV-1 TK into the CEM TK- cells completely restored AZT and d4T inhibition of HIV-1 replication. These studies confirm the requirement for a thymidine kinase activity for the anti-HIV activities of d4T and suggest that AZT, but not d4T, could be potentially administered to patients receiving HSV-1 TK-transduced lymphocytes.

Biosynthesis of chondroitin sulfate. Purification of glucuronosyl transferase II and use of photoaffinity labeling for characterization of the enzyme as an 80-kDa protein

A photoaffinity analogue, [beta-32P]5-azido-UDP-GlcA, was used to photolabel the enzymes that utilize UDP-GlcA in cartilage microsomes and rat liver microsomes. SDS-polyacrylamide gel electrophoresis analysis of photolabeled cartilage microsomes, which are specialized in chondroitin sulfate synthesis, showed a major radiolabeled band at 80 kDa and other minor radiolabeled bands near 40 and 60 kDa. Rat liver microsomes, which are enriched for enzymes of detoxification by glucuronidation, had a different pattern with multiple major labeled bands near 50-60 and 35 kDa. To determine that the photolabeled 80-kDa protein is the GlcA transferase II, we have purified the enzyme from cartilage microsomes. This membrane-bound enzyme, involved in the transfer of GlcA residues to non-reducing terminal GalNAc residues of the chondroitin polymer, has now been solubilized, stabilized, and then purified greater than 1350-fold by sequential chromatography on Q-Sepharose, heparin-Sepharose, and WGA-agarose. The purified enzyme exhibited a conspicuous silver-stained protein band on SDS-polyacrylamide gel electrophoresis that coincided with the major radiolabeled band of 80 kDa. SDS-polyacrylamide gel analysis of photoaffinity-labeled active fractions from the Q-Sepharose, heparin-Sepharose, and WGA-agarose also indicated only the single radiolabeled band at 80 kDa. Intensity of photolabeling in each of the fractions examined coincided with enzyme activity. The photolabeling of this 80-kDa protein was saturable with the photoprobe and could be inhibited by the addition of UDP-GlcA prior to the addition of the photoprobe. Thus, the photolabeling with [beta-32P]5-azido-UDP-GlcA has identified the GlcA transferase II as an 80-kDa protein. The purified enzyme was capable of transferring good amounts of GlcA residues to chondroitin-derived pentasaccharide with negligible transfer to pentasaccharides derived from hyaluronan or heparan.

Photoaffinity labeling studies of the human recombinant UDP-glucuronosyltransferase, UGT1*6, with 5-azido-UDP-glucuronic acid

Recombinant human liver UDP-glucuronosyltransferase (UGT), UGT1*6, which catalyzes the glucuronidation of small phenols, previously expressed in a V79 cell line (1) was photolabeled with [beta-32P]5N3UDP-glucuronic acid ([beta-32P]5N3UDP-GlcUA). Two polypeptides with an approximate molecular weight of 54 kDa were extensively photolabeled in the recombinant cell line while the nontransfected cell line showed no photoincorporation in this area. The identity of the two polypeptides as UGTs, which correspond to two different glycosylation forms of the same enzyme, was confirmed by Western blot using a polyclonal monospecific antibody directed against the 120 amino acids of the N-terminal end of UGT1*6. Preincubation with UDP-glucuronic acid (UDP-GlcUA) inhibited the photoincorporation of the probe into the polypeptides indicating competition of both the photoprobe and the nucleotide-sugar for the same binding site. It was further shown that photoincorporation of [beta-32P]5N3UDP-GlcUA into the UDP-GlcUA-binding site was saturable. The lack of photoincorporation of a related photoprobe, [beta-32P]5N3UDP-glucose ([beta-32P]5N3UDP-Glc), into UGT1*6 demonstrated specificity of this enzyme for UDP-GlcUA. In enzymatic assays, unlabeled 5N3UDP-GlcUA was shown to be an effective cosubstrate of the glucuronidation of 4-nitrophenol catalyzed by UGT1*6. The studies were further extended by demonstrating that photolabeling of UGT1*6 was inhibited by several active site-directed inhibitors. Finally, photoaffinity labelling was used in the purification of the labeled UGT1*6 using preparative gel electrophoresis. In conclusion, we have demonstrated that photoaffinity labeling with [beta-32P]5N3UDP-GlcUA is an effective tool for the characterization of enzymes such as recombinant UGTs that use UDP-GlcUA.

Bovine UDP-N-acetylglucosamine:lysosomal-enzyme N-acetylglucosamine-1-phosphotransferase. II. Enzymatic characterization and identification of the catalytic subunit

The kinetic properties of UDP-N-acetylglucosamine:lysosomal-enzyme N-acetylglucosamine-1-phosphotransferase (GlcNAc-phosphotransferase) purified to homogeneity from lactating bovine mammary gland have been investigated. GlcNAc-phosphotransferase transferred GlcNAc 1-phosphate from UDP-GlcNAc to the synthetic acceptor alpha-methylmannoside, generating GlcNAc-1-phospho-6-mannose alpha-methyl, the structure of which was confirmed by mass spectroscopy. GlcNAc-phosphotransferase was active between pH 5.7 and 9.3, with optimal activity between pH 6.6 and 7.5. Activity was strictly dependent on Mg2+ or Mn2+. The Km for Mn2+ was 185 microM. The Km for UDP-GlcNAc was 30 microM, and that for alpha-methylmannoside was 63 mM. The enzyme was competitively inhibited by UDP-Glc, with a Ki of 733 microM. The 166-kDa subunit was identified as the catalytic subunit by photoaffinity labeling with azido-[beta-32P]UDP-Glc. Purified GlcNAc-phosphotransferase utilizes the lysosomal enzyme uteroferrin approximately 163-fold more effectively than the non-lysosomal glycoprotein ribonuclease B. Antibodies to GlcNAc-phosphotransferase blocked the transfer to cathepsin D, but not to alpha-methylmannoside, suggesting that protein-protein interactions are required for the efficient utilization of glycoprotein acceptors. These results indicate that the purified bovine GlcNAc-phosphotransferase retains the specificity for lysosomal enzymes as acceptors previously observed with crude preparations.

Two kinetically-distinct components of UDP-glucuronic acid transport in rat liver endoplasmic reticulum

Previous studies have documented the presence of protein-mediated transport of UDP-glucuronic acid (UDP-GlcUA) in rat liver endoplasmic reticulum (ER). Measurement of uptake at varying concentrations of high specific activity [beta-32P]UDP-GlcUA has revealed the presence of a two component UDP-GlcUA transporting system. Transport at low substrate concentrations occurred predominantly via a high affinity component (K(m) = 1.6 microM), whereas a low affinity component (K(m) = 38 microM) predominated at high substrate concentrations. The K(m) for the high affinity system is in agreement with that previously published, while the low affinity component is a new finding. The uptake of UDP-GlcUA was temperature-sensitive, time dependent, and saturable for both components. The high affinity transport was affected by trans-stimulation and cis-inhibition by UDP-N-acetylglucosamine (UDP-GlcNAc); however, the same concentrations of UDP-GlcNAc had less effect on the low affinity system. In order to further study the two transport components, various inhibitors of anion transport carriers were tested. The high affinity component was strongly inhibited by 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS) and furosemide, while the low affinity system was less sensitive to these reagents. Dose-dependent inhibition by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) was found for both transport systems. Probenecid was found to be a weak inhibitor of both components of the UDP-GlcUA uptake. Finally, the major metabolite of 3'-azido-3'-deoxythymidine, 3'-azido-3'-deoxythymidine monophosphate (AZTMP), was able to inhibit the uptake of UDP-GlcUA by both components. The results indicate the presence of two carrier-mediated UDP-glucuronic acid transporting components in rat liver ER.

Synthesis of aryl azide derivatives of UDP-GlcNAc and UDP-GalNAc and their use for the affinity labeling of glycosyltransferases and the UDP-HexNAc pyrophosphorylase

The chemical synthesis and utilization of two photoaffinity analogs, 125I-labeled 5-[3-(p-azidosalicylamido)-1-propenyl]-UDP-GlcNAc and -UDP-GalNAc, is described. Starting with either UDP-GlcNAc or UDP-GalNAc, the synthesis involved the preparation of the 5-mercuri-UDP-HexNAc and then attachment of an allylamine to the 5 position to give 5-(3-amino)allyl-UDP-HexNAc. This was followed by acylation with N-hydroxysuccinimide p-aminosalicylic acid to form the final product, i.e., 5-[3-(p-azidosalicylamido)-1-propenyl]-UDP-GlcNAc or UDP-GalNAc. These products could then be iodinated with chloramine T to give the 125I-derivatives. Both the UDP-GlcNAc and the UDP-GalNAc derivatives reacted in a concentration-dependent manner with a highly purified UDP-HexNAc pyrophosphorylase, and both specifically labeled the subunit(s) of this protein. The labeling of the protein by the UDP-GlcNAc derivative was inhibited in dose-dependent fashion by either unlabeled UDP-GlcNAc or unlabeled UDP-GalNAc. Likewise, labeling with the UDP-GalNAc probe was blocked by either UDP-GlcNAc or UDP-GalNAc. The UDP-GlcNAc probe also specifically labeled a partially purified preparation of GlcNAc transferase I.

Effect on substrate binding of an alteration at the conserved aspartic acid-162 in herpes simplex virus type 1 thymidine kinase

Despite the extensive use of antiviral drugs for the treatment of herpesvirus infections and as pro-drugs for ablative gene therapy of cancer, little structural information about the drug activating enzyme, herpes simplex virus type 1 thymidine kinase (TK), was available until recently. In the absence of the three-dimensional structure we sought to elucidate the function of the key aspartic acid residue (Dl62) present within a highly conserved tri-peptide motif that is thought to function in nucleoside binding. In this study we generated a mutant, D162Q, by site-directed mutagenesis, purified both the wild-type and mutant TKs to near homogeneity by single-step affinity chromatography and determined the kinetic parameters for thymidine, ATP, dTMP and dTTP interactions. A 12-fold increase in Km for thymidine by D162Q TK (Km = 6.67 microM) relative to wild-type enzyme (Km = 0.56 microM) was observed and the absence of any alteration in Km for ATP suggests that D162 participates in nucleoside binding. Furthermore, the Ki for dTMP is significantly higher for D162Q TK than for HSV-1 TK which is indicative of a shared or overlapping binding site with thymidine. This assessment is further supported by the different inhibition patterns of D162Q and wild-type TKs observed using [alpha-32P]5-N3dUMP photoaffinity labelling in the presence of thymidine, ganciclovir or dTMP. Interestingly, the Ki for dTTP was 30-fold lower for D162Q TK (Ki = 2.2 microM) than for the wild-type enzyme (Ki = 65.8 microM) which provides further evidence of the importance of D162 in TK function.

Trehalose-P synthase of mycobacteria: its substrate specificity is affected by polyanions

The trehalose-P synthase was purified to near homogeneity from the cytoplasmic fraction of Mycobacterium smegmatis. At the final stage of purification, the enzyme preparation showed one major band of 59 kDa on SDS gels. The 59 kDa band became labeled with N3-UDP[32P]-glucose, and this labeling was inhibited in a concentration-dependent manner by either unlabeled UDP-glucose or GDP-glucose. The native enzyme also had a molecular weight of about 60 kDa by gel filtration, indicating that the active enzyme is a monomer. The 59 kDa protein was subjected to endoproteinase Lys-C digestion, and three peptides isolated by HPLC were sequenced. The sequences of 56 amino acids in these three peptides showed 60% identity to the trehalose-P synthases of Saccharomyces cerevesiae and Schizosaccharomyces pombe. The purified mycobacterial enzyme catalyzed the synthesis of trehalose-P from glucose-6-P and a variety of nucleoside diphosphate glucose derivatives, depending on whether a polyanion was absent or present. Thus, UDP-glucose and GDP-glucose were the best glucosyl donors, but maximum activity with UDP-glucose required the presence of a polyanion such as heparin, whereas activity with GDP-glucose was relatively independent of polyanion. The presence of heparin in the incubation mixture increased the affinity of the enzyme for UDP-glucose by a factor of 100, or more. However, the affinity for GDP-glucose was only twofold better in the presence of heparin. The purified synthase also utilized ADP-glucose and CDP-glucose, but the K(m) for these glucosyl donors was quite high even in the presence of polyanion. The effect of heparin on UDP-glucose activity was dose-dependent and maximum at about 1-2 micrograms of heparin/incubation. However, the size of the heparin molecule (i.e., the number of monosaccharide residues) was critical for activation, and only those heparins with 18 or more monosaccharide units were effective in stimulating activity.

Proteolytic mapping of the thymidine/thymidylate binding site of herpes simplex virus type 1 thymidine kinase: a general photoaffinity labeling method for identifying active-site peptides

The herpes simplex virus type 1 thymidine kinase (HSV-1 TK) is an important pharmacological target of antiviral nucleoside drugs and it uniquely possesses both a thymidine kinase and a thymidylate kinase activity. The structural relationship between these two activities is addressed in this study using a combination of active-site directed photoaffinity analogs, proteases, and tricine-SDS-polyacrylamide gel electrophoresis. For analysis of the thymidylate binding site, the thymidylate analog [32P]5-azido-dUMP was specifically photocrosslinked to the active site of HSV-1 TK. Because the amino acid sequence of HSV-1 TK is known, endoprotease Lys-C, V8 protease, trypsin, or chymotrypsin was used to generate a proteolytic map of photoincorporated peptides by separation on high-resolution tricine-SDS-polyacrylamide gels. Analysis of the resulting peptides indicated that the photoprobe was localized to one region comprising amino acids Ile112-Tyr132. Photolabeling of this region indicates that the thymine base of thymidine and TMP bind at one shared site in HSV-1 TK. In addition, the results reported in this study demonstrate that photolabeling with azidonucleotides can be used to identify photolabeled peptides by proteolytic mapping. This technique bypasses the problems of peptide purification and sequencing and yields rapid results when the primary amino acid structure of the protein of interest is known.

Photoaffinity labeling of human recombinant sulfotransferases with 2-azidoadenosine 3',5'-[5'-32P]bisphosphate

Photoaffinity labeling with 2-azidoadenosine 3', 5'-[5'-32P]bisphosphate was used to identify and characterize adenosine 3',5'-bisphosphate-binding proteins in human liver cytosol and recombinant sulfotransferase proteins. The sulfotransferases investigated in these studies were the human phenol sulfotransferases, HAST1, -3, and -4, dehydroepiandrosterone sulfotransferase, and estrogen sulfotransferase. The cDNAs for these enzymes have been previously cloned and expressed in COS-7 cells or Escherichia coli. Photoaffinity labeling of all proteins was highly dependent on UV irradiation, was protected by co-incubation with unlabeled adenosine 3',5'-bisphosphate and phosphoadenosine phosphosulfate, and reached saturation at concentrations above 10 microM. To verify that the 31 35-kDa photolabeled proteins were indeed sulfotransferases, specific antibodies known to recognize human sulfotransferases were used for Western blot analyses of photolabeled proteins. It was shown unequivocally that the proteins in the 31-35-kDa region recognized by the antibodies also photoincorporated 2-azidoadenosine 3',5'-[5'-32P]bisphosphate. This is the first application of photoaffinity labeling with 2-azidoadenosine 3',5'-[5'-32P]bisphosphate for the characterization of recombinant human sulfotransferases. Photoaffinity labeling will be also useful in the purification and functional identification of other adenosine 3',5'-bisphosphate-binding proteins and to determine amino acid sequences at or near their active sites.

Purification, photoaffinity labeling, and characterization of a single enzyme for 6-sulfation of both chondroitin sulfate and keratan sulfate

A soluble sulfotransferase that could 6-sulfate both chondroitin sulfate and corneal keratan sulfate was purified 27,500-fold using a sequence of affinity chromatographic steps with heparin-Sepharose, wheat germ agglutinin-agarose, and 3',5'-ADP-agarose. The essentially pure enzyme had a specific activity 40 times greater than the most purified chondroitin 6-sulfotransferase previously reported and exhibited a single sharp Coomassie Blue-stained and a heavy silver-stained protein band of 75 kDa on SDS-polyacrylamide gel electrophoresis. Chromatography of the purified enzyme on Sephacryl demonstrated a size of 150 kDa, which indicated that the native enzyme exists as a dimer. In addition to 6-sulfation of nonsulfated GalNAc, the purified serum enzyme had the ability to sulfate GalNAc 4-sulfate residues to give GalNAc 4,6-disulfate residues. The purified enzyme exhibited a Km of 40 microM for adenosine 3'-phosphate 5'-phosphosulfate when either chondroitin sulfate or corneal keratan sulfate were used as the acceptors. Use of both chondroitin sulfate and keratan sulfate in the same experiment demonstrated mutual competition, establishing that the sulfation of these substrates is by the same enzyme. Photoaffinity labeling of the purified enzyme with 2-azidoadenosine 3',5'-di[5'-32P]phosphate occurred only with the 75-kDa protein, confirming that this is the chondroitin 6-sulfotransferase/keratan sulfotransferase.

Synthesis of a photoaffinity analog of 3'-azidothymidine, 5-azido-3'-azido-2',3'-dideoxyuridine. Interactions with herpesvirus thymidine kinase and cellular enzymes

Long term administration of 3'-azidothymidine (AZT) for the treatment of AIDS has led to detrimental clinical side effects in some patients, the biochemical causes of which are still being delineated. Base-substituted, azido-nucleotide photoaffinity analogs have routinely proven to be effective tools for identifying and characterizing nucleotide-utilizing enzymes. Therefore, we have synthesized 5-azido-3'-azido-2',3'-dideoxyuridine, which is a potential photoaffinity analog of two human immunodeficiency virus drugs, AZT and 3'azido-2',3'-dideoxyuridine. A partially purified herpes simplex virus type 1 thymidine kinase and [gamma-32P]ATP were used to make an AZT monophosphate analog, [32P]5-azido-3'-azido-2',3'-dideoxyuridine monophosphate. The photoaffinity properties of this analog were initially tested with herpes simplex virus type 1 thymidine kinase. Photoaffinity labeling of this enzyme was saturable (half-maximal, 30 microM) and could be specifically inhibited by AZT, AZT monophosphate, thymidine, and thymidine monophosphate. Photolabeling of rat liver microsomal membranes was also done, and several membrane proteins that interact with AZT monophosphate were identified. The antiviral and cytotoxic activities of 5-azido-3'-azido-2',3'-dideoxyuridine were determined using human immunodeficiency virus, type 1 strain IIIB and an AZT drug-resistant strain in human T lymphocyte H9 cells.

Characterization of human liver microsomal UDP-glycosyltransferases using photoaffinity analogs

The photoaffinity analogs [beta-32P]5-azido-UDP-glucuronic acid ([32P]5N3UDP-GlcUA) and [beta-32P]5-azido-UDP-glucose ([32P]5N3UDP-Glc) were used to characterize UDP-glycosyltransferases of microsomes prepared from human liver. Photoincorporation of both probes into proteins in the 50- to 56-kdalton range, known to contain UDP-glucuronosyl transferases (UGTs), was concentration dependent, and photolabeled proteins were susceptible to trypsin digestion only in the presence of detergent. The latter was demonstrated by the appearance on Western blots of the trypsin-treated, detergent-disrupted microsomes of a protein band of slightly lower molecular mass than, and presumably derived from, the UGTs. However, a labeled cleavage product was found only in samples photolabeled with [32P]5N3UDP-GlcUA and not in those labeled with [32P]5N3UDP-Glc. In detergent-treated microsomes, all of the nucleotide sugars that were tested protected better against photoinsertion of [32P]5N3UDP-GlcUA than of [32P]5N3UDP-Glc, with UDP-glucose being the most effective, followed by UDP-GlcUA and UDP-galactose. The pattern of inhibition of a series of uridinyl analogs toward photolabeling by the two probes was quite different: one inhibitor that was ineffective in blocking photoincorporation of [32P]5N3UDP-GlcUA (L-DPASiU) was one of the most potent inhibitors of photolabeling with [32P]5N3UDP-Glc. A similar dichotomy was seen with several inhibitors in enzymatic assays measuring hyodeoxycholic acid 6-O glucuronidation and glucosidation activities; the most potent inhibitors of HDCA glucosidation were not as effective against glucuronidation. The results indicate a lumenal orientation for human microsomal UGTs and provide substantial evidence that two distinct enzyme systems are involved in 6-O glucuronidation and 6-O glucosidation of HDCA.

Inhibition of glycosylation by amphomycin and sugar nucleotide analogs PP36 and PP55 indicates that Haloferax volcanii beta-glucosylates both glycoproteins and glycolipids through lipid-linked sugar intermediates: evidence for three novel glycoproteins and a novel sulfated dihexosyl-archaeol glycolipid

Arachaebacteria have been recently placed in evolution as a separate kingdom of organisms between procaryotes and eucaryotes. Although these organisms contain both glycolipids and glycoproteins, they possess no Golgi. No biosynthetic work has been published on the complex carbohydrates of these newly reassigned organisms. This report describes preliminary results from one member of this kingdom, Haloferax volcanii, which suggest that all glycosylation proceeds through lipid intermediates. Evidence for novel glycolipid structure was also found during this study. H. volcanii plasma membranes contain all of the enzyme activities for synthesis of N-linked glycoproteins and archaeol-based glycolipids. For glucose transfer, all reactions apparently proceed through glucose-phosphopolyisoprenol using UDP-glucose as primary donor. Incorporation of D-[3H]glucose from UDP-D-[3H]glucose into glycoproteins and glycolipids of H. volcanii was stimulated by addition of C55-polyisoprenol phosphate, but not by C85-105 dolichol phosphate, and was inhibited by amphomycin and two recently described sugar nucleotide analogs, PP36 (5'-[N-(2-decanoylamino-3-hydroxy-3-phenylpropyloxy carbonyl)glycyl]amino]-5'-deoxyuridine) and PP55 (5'-O-[[(2-decanoylamino-3-phenylpropyloxycarbonyl) amino]sulfonyl]uridine). All three inhibitors are reported to block transfer of sugar from UDP-sugars to phosphopolyisoprenols in eucaryotes. However, in H. volcanii these inhibitors apparently block transfer of glucose from polyprenyl intermediates to final glycoproteins and glycolipid products. The sulfodihexosyl archaeol glycolipid fraction was partially characterized by mass spectrometry and was found to contain a previously unreported structure with sulfate on the reducing-end sugar. Four major glycoproteins 190, 105, 56, and 52 kDa and an archaeol-based glycolipid fraction were labeled by amphomycin-sensitive pathways. Photoaffinity labeling of H. volcanii homogenate with 5-azido-[32P]UDP-Glc tagged only one 45-kDa polypeptide which is a probable glucosyl-phosphorylpolyisoprenol synthase. The fact that only one polypeptide band was photoaffinity-labeled indicated that no other transferase utilized UDP-glucose directly in H. volcanii. The salt requirement of the UDP-glucose-dependent pathways suggests that cytoplasmic enzymes function in a high salt environment in H. volcanii. The archaebacterial plasma membrane thus expresses many functions for glycosylation of both glycoproteins and glycolipids, normally found in the endoplasmic reticulum and Golgi of eucaryotes.

Purification and photoaffinity labeling of herpes simplex virus type-1 thymidine kinase

The molecular basis for the treatment of human herpesviruses with nucleoside drugs is the phosphorylation of these drugs by the viral-encoded thymidine kinases. In order to better understand the structural and enzymatic mechanisms by which herpesviral thymidine kinases recognize their substrates, photoaffinity labeling with [alpha-32P]5-azido-2'-deoxyuridine-5'-monophosphate and [ gamma-32P]8-azidoadenosine-5'-triphosphate was used to characterize the thymidine, thymidylate, and ATP active sites of the herpes simplex virus-1 (HSV-1) thymidine kinase. For this study, HSV-1 thymidine kinase and a site-specific mutant enzyme (C336Y, known to confer acyclovir resistance) were expressed in bacteria and purified by a rapid, two-step protocol. The specificity of photoaffinity labeling of these HSV-1 thymidine kinases was demonstrated by the ability of site-directed substrates such as thymidine, thymidylate, acyclovir, 5-bromovinyl-2'-deoxyuridine, and ATP to inhibit photoinsertion. Differences in inhibition patterns of photoaffinity labeling correlated with kinetic differences between the wild-type and C336Y HSV-1 thymidine kinases. Cumulative results suggest that the acyclovir-resistant cysteine 336 mutation primarily affects the ATP binding site; yet it also leads to alteration in the binding affinity of nucleoside drugs in the thymidine site. In this study, azidonucleotide photoaffinity analogs are shown to be effective tools for studying the active-site environment of HSV-1 thymidine kinase and related site-specific mutants.

Cloning, expression and purification of full length Rep78 of adeno-associated virus as a fusion protein with maltose binding protein in Escherichia coli

The adeno-associated virus (AAV) Rep78 protein is required for many aspects of AAV's life cycle including its DNA replication and the regulation of its gene expression. Because of increasing utilization of AAV as a gene therapy vector and its possible use as an anti-cancer/anti-viral agent, the complete characterization of its Rep78 regulatory protein is important. In order to study various functional aspects of Rep78, we have cloned and expressed the Rep78 gene in Escherichia coli using an inducible expression plasmid. The entire Rep78 open reading frame (nt 321 to 2185) was cloned into the LacZ inducible expression vector pMALc2. Upon induction of the Ptac promoter with isopropyl thio-beta-D-galactopyranoside (IPTG), Rep78 is produced as a fusion protein with maltose binding protein (MBP). This recombinant MBP-Rep78 protein displayed all the biochemical activities which are described for the wild type protein including binding to the AAV terminal repeats (TR), endonuclease activity, and helicase activity. Furthermore, for the first time, ATP binding by Rep78 is demonstrated.

Synthesis of a new photoaffinity probe, 5-azido-[32P]UDPxylose, by UDPglucuronate carboxylyase from wheat germ

The enzyme, UDPglucuronic acid carboxylase (EC 4.1.1.35), was extensively purified from wheat germ, and was used to convert 5-azido-[32P]UDPglucuronic acid to 5-azido-[32P]UDPxylose, for use as a new photoaffinity probe. The carboxylyase was purified approximately 1200-fold using conventional methods, and the enzyme preparation, at the final stage of purification, was stable to storage at -20 degrees C for at least 9 months with little or no loss of activity. The partially purified carboxylyase catalyzed the conversion of 5-azido-[32P]UDPglucuronic acid to 5-azido-[32P]UDPxylose in good yield, and the UDPxylose probe was purified by ion-exchange chromatography, and characterized. The newly synthesized photoaffinity analog, 5-azido-[32P]UDPxylose, should be a valuable tool in the purification of various xylosyltransferases.

Characterization of a new class of inhibitors of the recombinant human liver UDP-glucuronosyltransferase, UGT1*6

The inhibitory effect of a series of novel structurally related compounds on the human UDP-glucuronosyltransferase UGT1*6 stably expressed in a V79 cell line was investigated. The inhibitors contain a lipophilic N-acyl phenylaminoalcohol residue and a uridine moiety connected by a spacer varying for each compound. The effects of these compounds on the glucuronidation reaction measured with 4-methylumbelliferone as substrate were determined. The best inhibitor of the series, D-DPMSU, had an IC50 of 39 microM in the assay conditions. Low Ki values were found toward both UDP-glucuronic acid and 4-methylumbelliferone (17 and 21 microM, respectively). The inhibition was competitive toward both substrates. A similar strong and competitive inhibitory effect was observed with two other inhibitors, DHPASU and DHPASiU. Another compound, D-DPASiU, showed a pure competitive inhibition towards UDP-glucuronic acid, but a non-competitive inhibition towards the acceptor substrate. These data and the optimization of the structures of the inhibitors by molecular modeling suggest that D-DPMSU and DHPASiU compounds may be transition state analog inhibitors of the recombinant UGT1*6 enzyme.

(1,3) beta-Glucan synthase activity of Neurospora crassa: identification of a substrate-binding protein

(1,3)beta-Glucan synthase activity from the filamentous Ascomycete Neurospora crassa was purified 1300-fold to a specific activity of 14,000 nmol glucose incorporated/min per mg protein. Hyphae were disrupted and crude membrane fractions obtained by high-speed centrifugation. Membrane fractions were extracted with Tergitol NP-40 and a second high-speed particulate fraction was obtained. Enzyme activity was solubilized with (3-((3-cholamidopropyl)dimethylammonio)-1-propanesulfonate and octyl-beta-D-glucoside from Tergitol-extracted membrane preparations. Solubilized enzyme activity was purified by product entrapment and recovered by low-speed centrifugation through a layer of sucrose. SDS-PAGE analysis revealed 2 proteins of 165 and 100 kDa as likely candidates for subunits of the (1,3)beta-glucan synthase enzyme complex. 5-Azido-[32P]UDP-glucose was photo-crosslinked to UDP-glucose-binding proteins in each fraction of the purification procedure. Autoradiograms of SDS-PAGE gels revealed a single protein of 165 kDa enriching with enzyme activity and labeling with the substrate analog. Photoincorporation of 5-azido-[32P]UDP-glucose by the 165 kDa protein was competed by 0.25 mM UDP-glucose (80%) and TDP-glucose (65%) while ADP-glucose (27%), CDP-glucose (36%), and GDP-glucose (8%) where less effective. These results were similar to in vitro inhibition of enzyme activity by the same compounds. These data strongly suggest that the 165 kDa protein is a substrate-binding subunit of (1,3)beta-glucan synthase.

Characterization of UDP-glucuronic acid transport in rat liver microsomal vesicles with photoaffinity analogs

The endoplasmic reticulum (ER) of rat liver contains several well characterized UDP-glucuronosyltransferases (UGTs), membrane-bound proteins of 50-54 kDa, and also less well identified UDP-glucosyltransferases, with nucleotide binding sites located on the lumenal surface. There is evidence that the substrates for these enzymes, UDP-glucuronic acid (UDP-GlcUA) and UDP-glucose (UDP-Glc), biosynthesized in the cytosol, are transported into the lumen of the ER via unknown mechanisms, the characteristics of which are poorly defined. A new approach for the study of the transport process has been devised using two active-site directed photoaffinity analogs, [beta-32P]5-azido-UDP-GlcUA and [beta-32P]5-azido-UDP-Glc. Photoincorporation of these probes into the lumenally oriented UGTs of intact rat liver microsomal vesicles was used as an indicator of transport. In intact vesicles, [32P]5N3UDP-GlcUA was efficiently incorporated into UGTs in a time, temperature and concentration dependent manner. In contrast, [32P]5N3UDP-Glc apparently was not transported effectively; maximal photolabeling of the 50-54 kDa proteins by this probe was dependent on detergent disruption of the vesicles. Vesicular uptake of and subsequent photolabeling of the 50-54 kDa proteins by [32P]5N3UDP-GlcUA were inhibited by UDP-GlcUA and 5N3UDP-GlcUA while UDP-Glc, 5N3UDP-Glc, UDP-xylose and UDP-N-acetylglucosamine were less inhibitory, suggesting a high degree of specificity for the uptake/photolabeling process. The anionic transport inhibitors DIDS and SITS inhibited [32P]5N3UDP-GlcUA photoincorporation into UGTs in intact vesicles, but also inhibited photolabeling of these and other enzymes in detergent disrupted vesicles. These data suggest the presence in rat liver microsomal vesicles of a specific, carrier-mediated transport process for UDP-GlcUA which is distinct from the mechanism of UDP-Glc transport.

GDP-mannose pyrophosphorylase. Purification to homogeneity, properties, and utilization to prepare photoaffinity analogs

Pig liver GDP-mannose pyrophosphorylase was purified 5,000-fold to apparent homogeneity using standard techniques. The native enzyme showed a single band on gels of about 450 kDa and two subunits of 43 and 37 kDa on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The 37-kDa (beta-) subunit had only methionine at its amino terminus and a surprisingly hydrophobic sequence: Met-Lys-Ala-Leu-Ile-Leu-Val-Gly-Gly-Tyr-Gly-Thr-Arg-Leu- Arg-Pro-Leu-Thr-Leu-Ser-Ile-Pro-Lys. The 43-kDa (alpha-) subunit was blocked at the amino terminus, but a 29-kDa CNBr fragment had the following sequence: Leu-Asp-Ala-His-Arg-His-Arg-Pro-His-Pro- Phe-Leu-Leu-. Substrate specificity studies done in the direction of formation of nucleoside triphosphate and sugar-1-P indicated that the enzyme was most effective with GDP-glucose as substrate (100%) followed by IDP-mannose (72%) and then GDP-mannose (61%). That GDP-mannose and GDP-glucose activities were indeed catalyzed by the same enzyme was indicated by the following. (i) Various studies indicated that the enzyme was homogeneous. (ii) A staining procedure for production of GTP stained the same single band on native gels when either GDP-mannose or GDP-glucose was the substrate. (iii). GDP-mannose inhibited the utilization of GDP-glucose by the enzyme, and vice versa. When 8-azido-[32P]GTP was incubated with native enzyme and exposed to UV light, both the 43-kDa and the 37-kDa subunits became labeled, although the 37-kDa subunit reacted more strongly. On the other hand, 8-azido-GDP-[32P]mannose only photolabeled the 43-kDa band. Most importantly, the purified enzyme can be utilized to produce 8-azido-[32P]GDP mannose or 8-azido-[32P]GDP glucose.

A novel UDP-Glc-specific glucosyltransferase catalyzing the biosynthesis of 6-O-glucosides of bile acids in human liver microsomes

Two active site-directed photoaffinity analogs, 5-[beta-32P]azido-UDP-glucuronic acid and 5-[beta-32P]azido-UDP-glucose, were used for the characterization of UDP-sugar-utilizing enzymes in human liver microsomes. Both compounds were recognized by human microsomal proteins: major photolabeled bands of 50-56 kDa were detected. Both photoincorporations were competitively decreased by increasing concentrations of either UDP-Glc or UDP-GlcUA, indicating a high affinity for both nucleotides. The patterns of photoaffinity labeling in the 50-56-kDa range by the two probes were significantly different, indicating the presence of different UDP-GlcUA- and UDP-Glc-specific enzymes of similar molecular mass. The presence of a UDP-Glc-dependent transferase was confirmed by the identification of an enzymatic activity catalyzing the formation of glucosides of the 6 alpha-hydroxylated bile acid hyodeoxycholic acid (3 alpha, 6 alpha-diOH (HDCA)) in the presence of UDP-Glc. The specific activity of 1.5-3.2 nmol/min/mg of protein was similar to that of 6 alpha-glucuronidation of HDCA. The apparent Km for UDP-Glc estimated with HDCA was 280 microM, and the formation of HDCA glucosides was strongly inhibited by UDP-GlcUA (apparent Ki = 7 microM). Evidence is presented that HDCA-specific UDP-glucuronosyltransferase (clone UGT2B4) expressed in V79 cells is not involved in glucosidation of HDCA and is not photolabeled with 5-[beta-32P]azido-UDP-Glc. Rigorous structure identification of the biosynthetic product proved that HDCA was glucosidated at the 6-position. Thus, this UDP-Glc-dependent activity catalyzing the biosynthesis of 6-O-glucosides of 6 alpha-hydroxylated bile acids represents a new pathway in the metabolism of these bile acids.

Synthesis and characterization of a new class of inhibitors of membrane-associated UDP-glycosyltransferases

A new class of compounds designed to inhibit membrane-associated glycosyltransferases were synthesized and their biological activities were characterized in liver microsomes and human lymphoma cell lines. These inhibitors are composed of N-acyl phenylaminoalcohol derivatives linked to uridine via different spacers. One inhibitor, termed PP36 (5'-[[N-(2-decanoylamino-3-hydroxy-3-phenylpropyloxycarbonyl )-glycyl[amino]-5'-deoxyuridine) competitively inhibited the enzyme glucosyl phosphoryldolichol synthase (Glc-P-Dol synthase) in rat liver microsomes. In rat and human liver microsomes incubated with PP36 and photolabeled with [beta-32P]5-azido-UDP-Glc, Glc-P-Dol synthase was the only protein observed to have decreased photoincorporation. Two other inhibitors, PP37 (5'-O-[[(2-decanoylamino-3-hydroxy-3-phenyl- propyloxycarbonyl)amino]sulfonyl]uridine and PP55 (5'-O-[[(2-decanoylamino-3- phenylpropyloxycarbonyl)amino]sulfonyl]uridine), were also shown to be competitive inhibitors of Glc-P-Dol synthase activity and photolabeling. Activities of glycosyltransferases involved in glycosphingolipid biosynthesis were little affected by these compounds. Analysis of the effects of PP36, PP37, and PP55 on the incorporation of [3H] leucine and [14C]galactose into glycoprotein and glycolipid fractions from two human cell lines indicated the following: PP36 reduced incorporation into both fractions, PP37 was ineffective, and PP55 only decreased incorporation into glycolipids.

Analysis of the streptococcal hyaluronic acid synthase complex using the photoaffinity probe 5-azido-UDP-glucuronic acid

The mucopolysaccharide, hyaluronic acid, is an important component of both mammals and pathogenic streptococci. This high molecular weight polymer is synthesized by a membrane-associated, multisubunit hyaluronate synthase which utilizes UDP-glucuronic acid and UDP-N-acetylglucosamine as substrates. Using the photoaffinity probe, [beta-32P]5-azido-UDP-glucuronic acid, three streptococcal membrane proteins (42, 33, and 27 kDa) specifically photoincorporated this probe. Labeling of these proteins was enhanced in the presence of UDP-N-acetylglucosamine, whereas UDP-galactose or UDP-glucose had no effect on incorporation. UDP-glucuronic acid inhibited the labeling of the three proteins in a dose-dependent manner. Detergent-solubilized membrane proteins from transposon-inactivated hyaluronic acid capsule mutants no longer incorporated the probe. This was also the case when membranes from stationary phase organisms were tested. Finally, glucuronic acid no longer was incorporated into high molecular weight hyaluronic acid with either the mutant or stationary phase preparations. Further biochemical analysis will be required to demonstrate the exact role each of the proteins play in hyaluronic acid biosynthesis.

Photoaffinity labelling of glycosyltransferases

The photoaffinity analogues 5-azido-UDP-glucose and 5-azido-UDP-glucuronic acid have proven to be valuable biochemical tools in the studies of nucleoside diphosphate sugar-utilizing enzymes, especially membrane-associated glycosyltransferases. A summary of the past and current uses of these analogues is presented, as well as photoaffinity data for the enzyme UDP-glucose: dolichylphosphate glucosyltransferase (Glc-P-Dol synthase). This enzyme has served as a model membrane-associated glycosyltransferase for demonstrating the uses of 5-azido-UDP-glucose. The advantages of using photoaffinity analogues for the purification and characterization of glycosyltransferases are presented, as well as an outline of the general procedures which can be used in conjunction with these analogues.

Application of 5-azido-UDP-glucose and 5-azido-UDP-glucuronic acid photoaffinity probes for the determination of the active site orientation of microsomal UDP-glucosyltransferases and UDP-glucuronosyltransferases

A new approach to determining the active site orientation of microsomal glycosyltransferases is presented which utilizes the photoaffinity analogs [32P]5-Azido-UDP-glucose ([32P]5N3UDP-Glc) and [32P]5-Azido-UDP-glucuronic acid ([32P]5N3UDP-GlcA). It was previously shown that both photoprobes could be used to photolabel UDP-glucose:dolichol phosphate glucosyltransferase (Glc-P-Dol synthase), as well as the family of UDP-glucuronosyltransferases in rat liver microsomes. The effects of detergents, proteases, and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) on the photolabeling of these enzymes were examined in intact rat liver microsomes. Photolabeling of Glc-P-Dol synthase by either photoprobe was the same in intact or disrupted vesicles, was susceptible to trypsin digestion, and was inhibited by the nonpenetrating inhibitor DIDS. Photolabeling of the UDP-glucuronosyltransferases by [32P]5N3UDP-GlcA was stimulated 1.3-fold in disrupted vesicles as compared to intact vesicles, whereas photolabeling of these enzymes by [32P]5N3UDP-Glc showed a 14-fold increase when vesicles were disrupted. Photolabeled UDP-glucuronosyltransferases were only susceptible to trypsin digestion in disrupted vesicles, and this was further verified by Western blot analyses. The results indicate a cytoplasmic orientation for access of UDP-sugars to Glc-P-Dol synthase and a lumenal orientation of most UDP-glucuronosyltransferases.

Application of an N-(4-azido-2,3,5,6-tetrafluorobenzoyl)tyrosine-substituted peptide as a heterobifunctional cross-linking agent in a study of protein O-glycosylation in yeast

In order to investigate the O-mannosyltransferase involved in the initial O-mannosylation of glycoproteins in Saccharomyces cerevisiae, a photoactive hexapeptide, [125I]-N-(4-azido-2,3,5,6-tetrafluorobenzoyl)-3-iodo-Tyr-Asn-Pro-T hr-Ser-Val ([125I]azidoTyr-peptide), was synthesized by solid-phase techniques using a new photoactive cross-linking reagent, N-(4-azido-2,3,5,6-tetrafluorobenzoyl)tyrosine, and resin-bound Asn-Pro-Thr(tBu)-Ser(tBu)-Val. When this modified hexapeptide substrate was incubated with O-mannosyltransferase preparations, the hexapeptide was an acceptor of [14C]-mannose from dolichol phosphate-[14C]mannose. After partially purifying the O-mannosyltransferase and photolabeling these enzyme preparations with [125I]azidoTyr-peptide, a ca. 82-kDa protein was shown to be the only apparent photolabeled protein that was protected by unmodified hexapeptide. This ca. 82-kDa protein may be the catalytic subunit of the O-mannosyltransferase. The susceptibility of the N-(4-azido-2,3,5,6-tetrafluorobenzoyl) moiety to reducing agents in aqueous buffers was also examined.

Synthesis and characterization of 5-azido-UDP-glucuronic acid. A new photoaffinity probe for UDP-glucuronic acid-utilizing proteins

A new active site-directed photoaffinity analogue, [beta-32P]5-azido-UDP-glucuronic acid (UDP-GlcA), was enzymatically synthesized from [beta-32P]5-N3UDP-Glc using UDP-glucose dehydrogenase. The product was characterized by its mobility on ion exchange and two thin-layer chromatographic systems, by its UV absorbance at 288 nm, and the loss of this absorbance after UV irradiation of the compound. Photoincorporation of [beta-32P]5-N3UDP-GlcA into bovine liver UDP-Glc dehydrogenase (EC 1.1.1.22) was saturable with an apparent Kd of 12.5 microM, and was inhibited by the known active-site effectors UDP-GlcA, UDP-Glc, and UDP-xylose. When human liver microsomes with known UDP-glucuronosyltransferase (EC 2.4.1.17) activities were photolabeled with [beta-32P]5-N3UDP-GlcA, major photolabeled bands of 35-37 and 50-54 kDa were detected. When rat liver microsomes from phenobarbital-injected rats were photolabeled with [beta-32P]5-N3UDP-GlcA, there was a marked increase in photoincorporation of a 51-kDa protein as compared with control animals. Evidence is presented which suggests that the photolabeled 51-54-kDa proteins in the liver microsomes from both tissues are UDP-glucuronosyltransferase and that [beta-32P]5-N3UDP-GlcA represents a new alternative approach in the study of UDP-glucuronosyltransferase and other UDP-GlcA-utilizing enzymes.

Partial Purification, Photoaffinity Labeling, and Properties of Mung Bean UDP-Glucose:Dolicholphosphate Glucosyltransferase

UDP-glucose:dolichylphosphate glucosyltransferase has been purified 734-fold from Triton X-100 solubilized mung bean (Phaseolus aureus) microsomes. The partially purified enzyme has broad pH optima of activity from 6.0 to 7.0 and is maximally stimulated with 10 millimolar MgCl(2). The K(m) for UDP-glucose was determined as 27 micromolar, and the K(m) for dolichol-P was 2 micromolar. Using the UDP-glucose photoaffinity analog, 5-azido-UDP-glucose, a polypeptide of 39 kilodaltons on sodium dodecyl sulfate-polyacrylamide gels was identified as the catalytic subunit of the enzyme. Photoinsertion into this 39-kilodalton polypeptide with [(32)P]5-azido-UDP-glucose was saturable, and was maximally protected with the native substrate UDP-glucose. 5-Azido-UDP-glucose behaves competitively with UDP-glucose in enzyme assays, and upon photolysis inhibits activity in proportion to its concentration. This study represents the first subunit identification of a plant glycosyltransferase involved in the biosynthesis of the lipid-linked oligosaccharides that are precursors of N-linked glycoproteins.

Purification and photoaffinity labeling of sucrose phosphate synthase from spinach leaves

Sucrose phosphate synthase (SPS) was isolated from spinach leaves by precipitation with polyethylene glycol, ion-exchange and hydrophobic interaction chromatography, and rate zonal centrifugation. The enzyme was purified more than 600-fold to a specific activity of 57 mumol/min/mg protein. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that a 120-kDa polypeptide was enriched through purification and was the major polypeptide in the final SPS preparation. The 120-kDa polypeptide was photoaffinity labeled with the substrate analog, 5-azidouridine [beta-32P]5'-diphosphate-glucose ([beta-32P]5-N3UDP-Glc). Covalent incorporation of 5-N3UDP-Glc into the 120-kDa polypeptide exhibited an apparent Kd of 74 microM, similar to the apparent Ki for inhibition of SPS activity by unphotolyzed 5-N3UDP-Glc. Competition experiments showed that photolabeling of the 120-kDa polypeptide by 5-N3UDP-Glc was reduced in the presence of UDP-Glc, exhibiting an apparent Ki value that was similar to the apparent Km (UDP-Glc) of 2.9 mM for the purified enzyme. The relative molecular mass of the SPS holoenzyme was 253,000, and the isoelectric point of the 120-kDa subunit was 5.2. The data confirmed the identity of the 120-kDa polypeptide as the SPS subunit, established the structure of the active enzyme as a dimer, and demonstrated active-site labeling of SPS by a photoaffinity analog of the substrate.

Resolution of phosphoglucomatase and the 62-kDA acceptor for the glucosylphosphotransferase

The radioactive, photoactivatable labeling probe [beta-32P]5-azidouridine 5'-diphosphoglucose has recently been shown to label a 62-kDa protein in crude homogenates and in partially purified enzyme preparations without photoactivation. Here, we report that a portion of this radioactivity is due to labeling of phosphoglucomutase by contaminating levels of [32P]alpha Glc-1-P initially present at less than 1% of the total 32P. This conclusion is based in part on the ability of excess unlabeled alpha Glc-1-P and Glc-6-P, but not UDP-Glc, to block the labeling. In addition, the labeled protein in liver homogenates had a tryptic peptide pattern similar to that of authentic phosphoglucomutase. These findings, however, raised a second question. Assays for the UDP-Glc: glycoprotein glucosyl phosphotransferase (Glc phosphotransferase) have utilized [beta-32P]UDP-Glc and have resulted in the labeling of a small number of acceptors, including one of approximately 62 kDa. Despite the fact that these assays had routinely been performed in the presence of 1 mM alpha Glc-1-P, the coincidence in molecular weights led to these further studies. We conclude that the acceptor of approximately 62 kDa is distinct from phosphoglucomutase. This conclusion is based on differences in the time courses of incorporation, the specificity of blocking agents, the presence of covalently linked glucose, the products of acid hydrolysis and of beta-elimination, and isoelectric points.