Neurotransmitter-stimulated breakdown of endogenous polyphosphoinositides in post mortem human brain

Membranes from human brain cortex (8-12 h post mortem) were labelled with [3H]inositol, in the presence of CMP, through the back reaction catalysed by PtdIns synthase. The enzyme incorporated [3H]inositol into phosphoinositides at a maximal rate of 419 pmol min-1 mg protein-1. In the absence of CMP, the labelling rate due to the PtdIns headgroup exchanging enzyme was 36 pmol min-1 mg protein-1. Human brain PtdIns synthase showed Kmapp values of 0.49 mM and 18 microM for inositol and CMP, respectively. In the presence of ATP, [3H]polyphosphoinositides formed after [3H]PtdIns were hydrolysed by phospholipase C in a GTP gamma S and neurotransmitter receptor agonist-dependent manner. Production of 3H-inositol phosphates as stimulated by GTP gamma S (350% of basal) was increased by the muscarinic agonists carbachol and oxotremorine-M (600% of basal) and by serotonin (485% of basal). The relative potencies of carbachol and oxotremorine-M were consistent with an action at muscarinic receptors. These results show that coupling between muscarinic and serotonin receptors and phospholipase C is preserved in membranes from post mortem human brain cortex and validate the use of a method involving direct [3H]inositol labelling of a membrane fraction to study the functional state of phospholipase C-coupled receptors in human brain samples.

Characterization of the effects of lithium on phosphatidylinositol (PI) cycle activity in human muscarinic m1 receptor-transfected CHO cells

1. The effects of lithium on [3H]-inositol and [3H]-cytidine incorporation into [3H]-inositol monophosphates ([3H]-InsP1) and [3H]-cytidine monophosphorylphosphatidate ([3H]-CMP-PA), respectively, and inositol 1,4,5-trisphosphate (InsP3) and inositol 1,3,4,5-tetrakisphosphate (InsP4) mass were studied in carbachol-stimulated human m1 muscarinic receptor-transfected Chinese hamster ovary cells (m1 CHO cells). 2. Lithium alone (10 mM) had no appreciable effects on any of the four parameters measured; it was only in carbachol-stimulated cells that the effects of lithium became apparent. 3. In the presence of carbachol (1 mM), lithium (10 mM) caused a relatively rapid (within 5 min) accumulation of [3H]-InsP1 and [3H]-CMP-PA which continued up to about 20-30 min, after which accumulation slowed down. On the other hand, the elevation in InsP3 and InsP4 levels produced by carbachol was not altered by lithium in the short-term and only at later times (> 20-30 min) was the response attenuated, with InsP3 and InsP4 levels approaching basal. 4. The effects of lithium on carbachol-stimulated [3H]-InsP1 and [3H]-CMP-PA accumulation and the attenuation of the carbachol-induced elevation of InsP3 and InsP4 were all dose-dependent, with EC50s in the region of 1 mM. 5. The lithium-induced effects on [3H]-CMP-PA and InsP3 and InsP4 in carbachol-stimulated cells could be reversed, in a dose-dependent manner, by preincubation with exogenous myo-inositol (EC50 = 2-3 mM) but not by the inactive analogue scyllo-inositol, indicating that these effects occur as a consequence of depletion of inositol. 6. The temporal effects of lithium are consistent with lithium inhibiting inositol monophosphatase,causing accumulation of InsP1, resulting in lower free inositol levels. This leads to accumulation of CMP-PA and reduced PI synthesis which, once agonist-linked membrane inositol phospholipids are depleted, produces attenuated InsP3 and InsP4 responses.7. These results in ml CHO cells support the hypothesis that lithium affects the PI cycle cell signalling pathway by depletion of inositol due to inhibition of inositol monophosphatase.

Computer modelling studies of ribonuclease A-pyrimidine nucleotide complexes

Different modes of binding of pyrimidine monophosphates 2'-UMP, 3'-UMP, 2'-CMP and 3'-CMP to ribonuclease (RNase) A are studied by energy minimization in torsion angle and subsequently in Cartesian coordinate space. The results are analysed in the light of primary binding sites. The hydrogen bonding pattern brings out roles for amino acids such as Asn44 and Ser123 apart from the well known active site residues viz., His12,Lys41,Thr45 and His119. Amino acid segments 43-45 and 119-121 seem to be guiding the ligand binding by forming a pocket. Many of the active site charged residues display considerable movement upon nucleotide binding.

[A new antileukemic drug, cytarabine ocfosfate]

Cytarabine ocfosfate (commercial name: Starasid) is a prodrug having stearyl group attached to phosphoric acid at 5' position of arabinose moiety of cytosine arabinoside (Ara-C). This drug is given orally. The mode of action is in the inhibition of DNA synthesis after conversion to Ara-CTP as in Ara-C. The drug is metabolized in the liver, producing the intermediate metabolite, C-C3PCA which is converted to Ara-C gradually. This property results in the maintenance of relatively long time the blood Ara-C levels. This was proved to be active clinically against acute leukemia and MDS.

Comparative effects of lithium on the phosphoinositide cycle in rat cerebral cortex, hippocampus, and striatum

The effects of lithium on muscarinic cholinoceptor-stimulated phosphoinositide turnover have been investigated in rat hippocampal, striatal, and cerebral cortical slices using [3H]inositol or [3H]cytidine prelabelling and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] mass determination methods. Carbachol addition resulted in maintained increases in Ins(1,4,5)P3 and Ins(1,3,4,5)P4 mass levels in hippocampus and cerebral cortex, whereas in striatal slices these responses declined significantly over a 30-min incubation period. Carbachol-stimulated Ins(1,4,5)P3 and Ins(1,3,4,5)P4 accumulations were inhibited by lithium in all brain regions studied in a time- and concentration-dependent manner. For example, in hippocampal slices significant inhibitory effects of LiCl were observed at times > 10 min after agonist challenge; IC50 values for inhibition of agonist-stimulated Ins(1,4,5)P3 and Ins(1,3,4,5)P4 accumulations by lithium were 0.22 +/- 0.09 and 0.33 +/- 0.13 mM, respectively. [3H]CMP-phosphatidate accumulation increased in all brain regions when slices were stimulated by agonist and lithium. The ability of myo-inositol to reverse these effects, as well as lithium-suppressed Ins(1,4,5)P3 accumulation, implicates myo-inositol depletion in the action of lithium in the hippocampus and cortex at least. The results of this study suggest that although significant differences in the magnitude and time courses of changes in inositol (poly)phosphate metabolites occur in different brain regions, lithium evokes qualitatively similar enhancements of [3H]inositol monophosphate and [3H]-CMP-phosphatidate levels and inhibitions of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 accumulations. However, the inability of striatal slices to sustain carbachol-stimulated inositol polyphosphate accumulation in the absence of lithium and the inability to reverse effects with myo-inositol may indicate differences in phosphoinositide signalling in this brain region.

Characterization of poly C preferential ribonuclease from chicken liver

Poly C preferential RNase previously reported by Levy and Karpetsky [J. Biol. Chem. 255, 2153-2159 (1980)] and Miura et al. [Chem. Pharm. Bull. 32, 4053-4060 (1984)] was extensively purified from chicken liver to homogeneity as determined by SDS-PAGE (RNase CL2). The poly C preference over poly U was slightly higher than that of bovine pancreatic RNase A. However, the kinetic constants for 8 dinucleoside phosphates, CpY and UpY (Y = one of A, G, U, and C) as substrates showed that RNase CL2 was preferential for cytidylic acid, but less so than RNase A, and the influence of Y base on the rate of hydrolysis of CpY or UpY was less marked than in the case of RNase A. The primary structure of RNase CL2 was determined. The molecular weight calculated from the sequence was 13,420. Comparison of the amino acid sequence of RNase CL2 with those of other vertebrate RNases showed that RNase CL2 is a member of the RNase A family, but is not a non-secretory RNase. It retains 3 disulfide bridges of RNase A, but Cys65-Cys72 of RNase A is missing. As for the active site, the amino acid residues of the P0 and P1 sites of RNase A are completely conserved. Among the B1 site components, Thr45 (RNase A numbering) is conserved, but Phe120 and Ser123 are substituted by Leu and Thr, respectively. Among the B2 site residues, Gln69, Asn71, and Glu111 are substituted by other amino acids.

A 5' exoribonuclease from cytoplasmic extracts of mouse sarcoma 180 ascites cells

An exonuclease that appears to represent the predominant nuclease activity in cytoplasmic extracts of sarcoma 180 ascites cells has been partially purified and characterized. The enzyme attacks RNA chains in a 5' to 3' direction, and releases 5'-mononucleotides. The initial cleavage, however, can occur at either the first, second and probably third phosphodiester linkage in some RNAs. The enzyme attacks transcripts terminated with a 5'-triphosphate more slowly than those with a 5' monophosphate, and releases a compound larger than GTP from transcripts that begin with a pppG. Capped transcripts are cleaved at least as readily as those with a 5'-P, yielding a compound larger than 7mGpppGm. The occurrence of an such an exonuclease capable of attacking capped RNAs would make it possible for mammalian cells to initiate mRNA degradation by a 5' exonucleolytic mechanism.

Transcription-driven site-specific DNA recombination in vitro

Transcription of a topologically relaxed, circular DNA triggers recombination between two directly repeated res sites by gamma delta resolvase in vitro. This activation of recombination depends on the res site-to-site distance and the orientation of sites with respect to the direction of RNA polymerase tracking. In addition to functioning as a site-specific recombinase, gamma delta resolvase acts as a site-specific topoisomerase and increases the topological linking number of templates during transcription. The data suggest that the link between transcription and recombination could be negative DNA supercoiling that transiently builds up on a relatively short DNA segment in the wake of an advancing RNA polymerase. Surprisingly, transcription-driven recombination is not inhibited by the presence of large amounts of eukaryotic topoisomerase type I, indicating that site-specific recombination can override relaxation by diffusible topoisomerases. This in vitro system might therefore serve as a model for some transcription-directed recombination events observed in vivo.

Elevated phosphatidyl-CMP is not the source of diacylglycerol accumulation induced by lithium in NG108-15 cells

Previous studies have shown that in the neuroblastoma x glioma hybrid cell line NG108-15 lithium is able to induce an increase in diacylglycerol levels. This effect was shown to be enhanced by the presence of bradykinin. Another striking effect of lithium was a marked gain in the level of the liponucleotide phosphatidyl-CMP. Increased phosphatidyl-CMP levels were detected in the presence of lithium alone but were considerably more pronounced in the presence of both lithium and bradykinin. These results are consistent with the inhibitory action of lithium on key enzymes of the degradation pathway of inositol phosphates, resulting in a decrease in cellular inositol content and in an elevation in levels of phosphorylated inositols. Comparison of the mass of the inositol phosphates and diacylglycerol showed that the lithium-induced diacylglycerol levels were substantially greater than would be expected from phosphoinositide hydrolysis alone. One possible reason for the increase in the level of diacylglycerol through the action of lithium is the reversal of the reaction for the formation of phosphatidyl-CMP. The resulting phosphatidic acid would then need to be further dephosphorylated to diacylglycerol. The lithium-induced elevation of phosphatidyl-CMP was prevented by addition of myo-inositol (10-30 mM), suggesting that the increase in liponucleotide level was due to depletion of cellular inositol. Under the same conditions the elevated diacylglycerol concentration remained unchanged. Consequently, phosphatidyl-CMP is not its source, and diacylglycerol may arise through an effect of lithium on the degradation of phospholipids other than phosphoinositides. The action of phospholipase C or D on phosphatidylcholine is the most likely mechanism.

Catalytic properties of the CMP-N-acetylneuraminic acid hydroxylase from the starfish Asterias rubens: comparison with the mammalian enzyme

The biosynthesis of N-glycolylneuraminic acid (Neu5Gc) was investigated in cell-free extracts of the starfish Asterias rubens, which is one of the evolutionarily least-advanced species known to possess Neu5Gc-containing glycoconjugates. As in higher animals, Neu5Gc is synthesised in Asterias rubens by the action of a CMP-Neu5Ac hydroxylase. Enzyme activity was detected in all starfish tissues tested, the turnover being the greatest in the gonads. The enzyme from this tissue has a temperature optimum between 25 and 33 degrees C and a pH optimum between pH 6.0 and 6.4. This hydroxylase exhibits many characteristics in common with the mammalian enzyme. For example, the enzyme is extracted in a predominantly soluble form. Oxygen and a reduced pyridine nucleotide are necessary for activity, with NADH being the most effective cofactor. Furthermore, the activation of the hydroxylase by exogenously added iron salts and the potent inhibitory effects of several iron ligands point to the involvement of a non-haem iron cofactor. The enzyme has a high affinity for the substrate CMP-Neu5Ac, the apparent Km being 18 microM. In contrast to the mammalian enzyme, the hydroxylase from Asterias rubens is not inhibited by increased ionic strength and cannot be activated by non-ionic detergents. Moreover, the CMP-Neu5Ac turnover increased linearily with increasing protein concentration. In accordance with other enzymes in starfish, seasonal changes in the CMP-Neu5Ac hydroxylase activity were also observed.

Synthesis of CMP-deaminoneuraminic acid (CMP-KDN) using the CTP:CMP-3-deoxynonulosonate cytidylyltransferase from rainbow trout testis. Identification and characterization of a CMP-KDN synthetase

The sugar nucleotide, cytidine 5'-(3-deoxy-D-glycero-D-galacto-2-nonulosonic phosphate) (CMP-KDN) is expected to serve as a donor of KDN residues in the synthesis of KDN-containing glycoconjugates. We report here the identification and characterization of CMP-KDN synthetase, a novel enzyme responsible for synthesis of CMP-KDN from KDN and CTP. The enzyme was partially purified from the testis of rainbow trout (Oncorhynchus mykiss), where KDN gangliosides were first discovered (Yu, S., Kitajima, K., Inoue, S., and Inoue, Y. (1991) J. Biol. Chem. 266, 21929-21935), and used to synthesize CMP-[14C]KDN, which was characterized by 1H NMR. Vmax/Km studies showed that KDN was a preferred nonulosonic acid substrate compared to N-acetylneuraminic acid (Neu5Ac) or N-glycolylneuraminic acid (Neu5Gc) (4.4 x 10(-3) min-1 for KDN versus 2.3 and 1.8 x 10(-3) min-1 for Neu5Ac and Neu5Gc, respectively). CMP-KDN synthetase activity was maximal at pH 9-10 and at 25 degrees C. The presence of either Mg2+ or Mn2+ was essential for CMP-KDN synthetase activity. 25 mM Mg2+ stimulated formation of CMP-KDN more than 10-fold, yet only stimulated formation of CMP-Neu5Ac and CMP-Neu5Gc 4-fold, relative to 1 mM Mg2+. A kinetic study using mixed substrates showed that both CMP-KDN and CMP-Neu5Ac synthetase activities in the partially purified enzyme were due to the same active site of a single enzyme. In contrast, Neu5Ac and Neu5Gc were the preferred nonulosonic acid substrates for the calf brain CMP-sialic acid synthetase. Thus, mammalian CMP-sialic acid synthetases recognizes similar, yet distinctively different, substrate specificity determinants. Thus, the trout testis enzyme was considered to synthesize activated sugar nucleotides required for synthesis of both (KDN)GM3 and (Neu5Ac)GM3. The expression of CMP-KDN synthetase was shown to be temporally correlated with development and to parallel the developmental expression of (KDN)GM3 in sperm.

Incorporation of cytosine arabinoside monophosphate into DNA at internucleotide linkages by human DNA polymerase alpha

The incorporation of cytosine arabinoside monophosphate (araCMP) into DNA at internucleotide linkages by DNA polymerase alpha (DNA pol alpha) has been investigated by using oligonucleotide primed DNA templates. The products of reactions catalyzed by DNA pol alpha in vitro were analyzed on polyacrylamide gels to measure insertion of araCMP, extension from an araCMP 3' terminus, and binding of the enzyme to an araCMP 3' terminus. The results show that insertion of araCMP opposite dGMP in the DNA template is about 3-fold less efficient than insertion of dCMP. Extension from an araCMP 3' terminus by addition of the next complementary nucleotide is approximately 2000-fold less efficient than extension from a correctly base-paired 3' terminus. In the absence of the second substrate, dNTP, DNA pol alpha binds with approximately equal affinities to DNA templates that contain oligonucleotide primers with araCMP or dCMP positioned at the 3' terminus. In the presence of dNTP, the enzyme extends the araCMP 3' terminus or dissociates, but it is not trapped at the araCMP 3' terminus in a nonproductive ternary complex as is observed at the ddCMP 3' terminus. To determine if slow phosphodiester bond formation contributes to the observed extension rate from the araCMP 3' terminus by DNA pol alpha, oligonucleotide primers with araCMP positioned at the 3' terminus were elongated by addition of the alpha-phosphorothioate analogue of the next complementary nucleotide. The rate of extension from araCMP by addition of 2'-deoxyadenosine 5'-O-phosphorothioate (dAMP alpha S) was 6-fold slower than by addition of dAMP, indicating that bond formation is partially rate limiting in the extension reaction. Thus, inefficient extension from the araCMP 3' terminus is the major determinant contributing to the low incorporation frequency of araCMP into DNA by DNA pol alpha, and this inefficiency can be attributed, in part, to slower phosphodiester bond formation at the araCMP 3' terminus.

Two-dimensional differential scanning calorimetry: simultaneous resolution of intrinsic protein structural energetics and ligand binding interactions by global linkage analysis

A general theoretical development for the design and analysis of two-dimensional thermal stability surfaces of proteins is presented. The surfaces are generated from multiple excess heat capacity profiles (<delta Cp> vs T) obtained at varying concentrations of an interacting ligand. The energetics of both the intrinsic protein stability and the protein-ligand interaction are simultaneously resolved by employing statistical thermodynamic models in global linkage analysis. This formalism allows resolution of the intrinsic protein folding-unfolding parameters (enthalpy, entropy, and heat capacity changes) as well as the ligand interaction parameters (binding stoichiometry, enthalpy, entropy, and heat capacity changes). The theory has been applied to the case of ribonuclease A and its interaction with cytidine-2'-monophosphate. The accuracy of the thermodynamic parameters obtained by this approach compares within error with those parameters that can be obtained by direct measurements.

Retroviral nucleocapsid protein specifically recognizes the base and the ribose of mononucleotides and mononucleotide components

The interaction of the retroviral nucleocapsid protein (NC) with nucleic acids forms the basis of its varied roles in the replication cycle, which include binding and condensing the viral RNA within the virion, stimulation of the early steps in reverse transcription, and dissociation from RNA in the replication complex. As part of an investigation of the NC binding site and of the forces that drive its interaction with nucleic acids, the relative affinities of NC from avian myeloblastosis virus were determined for a series of mononucleotides and mononucleotide components using a competitive displacement assay utilizing the extrinsic fluorescent probe bis-ANS [Secnik, J., Wang, Q., Chang, C.-M., & Jentoft, J.E. (1990) Biochemistry 29, 7991-7997]. The estimated binding affinities were unexpectedly similar for nucleotides, nucleosides, and bases (Ka greater than 10(6) M-1). AMP, UMP, GMP, and CMP bound to NC with essentially equal affinity, indicating that NC does not discriminate between bases. This is consistent with its role in coating, condensing, and packaging the RNA within virions. Nucleosides, bases, riboses, and ribose phosphate bind to NC with 1000-fold higher affinity than inorganic phosphate, indicating that the NC binding site includes elements that recognize nucleotide base and ribose components in addition to phosphate ions. However, the binding affinities of components are not additive, i.e., the Kapp values for adenine and deoxyribose are very similar to that for deoxyadenosine, indicating that the interaction between the NC subsite and the base and the sugar components is complex. The stoichiometry of the complex between bis-ANS and NC was established to be NC.(bis-ANS)3.(ABSTRACT TRUNCATED AT 250 WORDS)

Fluorescent CMP-sialic acids as a tool to study the specificity of the CMP-sialic acid carrier and the glycoconjugate sialylation in permeabilized cells

The specificity of the Golgi carrier for CMP-sialic-acids and the lumenal sialylation of glycoconjugates in mechanically permeabilized cells (semi-intact CHO 15B cells) was studied with CMP-activated fluorescent sialic acids as sensitive markers. Semi-intact cells represent a well-established cellular model for studies on the constitutive secretion pathway because the perforated plasma membrane allows membrane-impermeable CMP-sialic-acids to gain access to cellular organelles. The subcellular structures of semi-intact cells remain morphologically intact and hence synthetic CMP-sialic-acids can be assayed as substrates for the corresponding Golgi sugar-nucleotide transporter. The results prove that the CMP-sialic-acid carrier is able to translocate fluorescent CMP-glycosides, despite the bulky fluoresceinyl residue located at position C5 or C9 of the sialic-acid moiety; the data suggest a slightly higher affinity of the carrier for the C9-substituted CMP-glycoside, whereas the affinity of cellular sialyltransferases is fourfold higher for CMP-5-N-fluoresceinylaminoacetylneuraminic acid (5-FTIUNeuAc; 5-N-fluoresceinylaminoneuraminic acid). Using CMP-9-fluoresceinylthioureido-N-acetylneuraminic acid (CMP-9-FTIUNeuAc), an easy and sensitive fluorometric assay was established for the lumenal sialylation in semi-intact cells. Cellular proteins and gangliosides are both labelled by covalent incorporation of the fluorescent N-acetylneuraminic acid analogue. The assay allows rapid screening for small biomolecules or proteins that influence cellular sialyl transport and sialyl transfer; the lumenal fluorescence incorporation does not require ATP or cytosolic compounds. The suitability of fluorescent CMP-glycosides as markers for intracellular sialylation, proven in this paper, introduces the use of synthetic sialic acids for visualisation of cellular sialic acid pathways by fluorescence microscopy. Based on the data presented here, specific CMP-N-acetylneuraminic-acid analogues can be produced and used for the characterization of the Golgi CMP-sialic-acid carrier.

Enzymes of pyrimidine metabolism in the musculus complexus of the chick during development

The pattern of cytidylate and uridylate phosphatase, uridine phosphorylase, cytidine and cytosine deaminase activities has been studied in M. complexus during chick development. The comparison of these enzyme activities with thigh muscles ones has shown that quantitative and temporal changes occur, in parallel with the unusual pre-natal and early post-natal development of M. complexus. The results suggest that during the first period of incubation, UMP might follow the anabolic pathway UMP-UTP, which leads to cytidine nucleotides, while approaching the hatching, the catabolic pathway should prevail. In addition, immediately after hatching, pyrimidine metabolism is especially supported by cytidine nucleotides.

Excision repair of UV-damaged plasmid DNA in Xenopus oocytes is mediated by DNA polymerase alpha (and/or delta)

We studied DNA repair by injecting plasmids containing random pyrimidine dimers into Xenopus oocytes. We demonstrated excision repair by recovering plasmids and analyzing them with T4 UV endonuclease treatment and alkaline agarose gel electrophoresis. The mechanism for excision repair of these plasmids appears to be processive, rather than distributive, since repair occurs in 'all or none' fashion. At less than 4-5 dimers/plasmid, nearly all repair occurs within 4-6 hours (approximately 10(10) dimers repaired per oocyte); the oocyte, therefore, has abundant repair activity. Specific antibodies and inhibitors were used to determine enzymes involved in repair. We conclude that DNA polymerase alpha (and/or delta) is required because repair is inhibited by antibodies to human DNA polymerase alpha, as well as by aphidicolin, an inhibitor of polymerases alpha (and/or delta). Repair was not inhibited by hydroxyurea, cytosine beta-D-arabinofuranoside, or inhibitors of topoisomerase II (novobiocin). Oocyte repair does not activate semi-conservative DNA replication, nor is protein synthesis required. Photoreactivation cannot account for repair because dimer removal is independent of exogenous light.

Study of strong to ultratight protein interactions using differential scanning calorimetry

Data from differential scanning calorimetry (DSC) may be used to estimate very large binding constants that cannot be conveniently measured by more conventional equilibrium techniques. Thermodynamic models have been formulated to describe interacting systems that involve either one thermal transition (protein-ligand) or two thermal transitions (protein-protein) and either 1:1 or higher binding stoichiometry. Methods are described for obtaining binding constants and heats of binding by two different methods: calculation or simulation fitting of data. Extensive DSC data on 2'CMP binding to RNase are presented and analyzed by the two methods. It is found that the methods agree when binding sites are completely saturated, but substantial errors arise in the calculation method when site saturation is incomplete and the transition of liganded molecules overlaps that of unliganded molecules. This arises primarily from an inability to determine TM (i.e., the temperature where concentrations of folded and unfolded protein are equal) under weak-binding conditions. Results from simulation show that the binding constants and heats of binding from the DSC method agree quantitatively with corresponding estimates obtained from equilibrium methods when extrapolated to the same temperature. It was also found from the DSC data that the binding constant decreases with increasing concentration of ligand, which might arise from nonideality effects associated with dimerization of 2'CMP. Simulations show that the DSC method is capable of estimating binding constants for ultratight interactions up to perhaps 10(40) M-1 or higher, while most equilibrium methods fail well below 10(10) M-1. DSC data from the literature on a number of interacting systems (trypsin-soybean trypsin inhibitor, trypsin-ovomucoid, trypsin-pancreatic trypsin inhibitor, chymotrypsin-subtilisin inhibitor, subtilisin BPN-subtilisin inhibitor, RNase S protein-RNase S peptide, avidin-biotin, ovotransferrin-Fe3+, superoxide dismutase-Zn2+, alkaline phosphatase-Zn2+, and assembly of regulatory and catalytic subunits of aspartate transcarbamoylase) were analyzed by simulation fitting or by calculation. Apparent single-site binding constants ranged from ca. 10(5) to 10(20) M-1, while the interaction constant for assembly of aspartate transcarbamoylase was estimated as 10(37) in molarity units. For most of these systems, the DSC interaction constants compared favorably with other literature estimates, for some it did not for reasons unknown, while for still others this represented the first estimate. Simulations show that for proteins having two binding sites for the same ligand within a single cooperative unit, ligand rearrangement will occur spontaneously during a DSC scan as the transition temperature of the unliganded protein is approached.(ABSTRACT TRUNCATED AT 400 WORDS)

Participation of the peroxisomal beta-oxidation system in the chain-shortening of PCA16, a metabolite of the cytosine arabinoside prodrug, YNKO1, in rat liver

When PCA16, a metabolite of the cytosine arabinoside prodrug YNKO1, was incubated with isolated rat hepatocytes, time-dependent H2O2 generation was found. When the hepatocytes obtained from clofibrate-treated rat liver were used as an enzyme source, PCA16-dependent production of H2O2 was increased by around 6-fold. The activity of peroxisomal beta-oxidation for PCA16 assayed by H2O2 generation was 3-fold higher than that for palmitic acid, whereas the activity of mitochondrial beta-oxidation for PCA16 assayed by ketone body production was much less than that for palmitic acid. A subcellular distribution study revealed that the distribution of the activities of beta-oxidation and fatty acyl-CoA oxidase for PCA16-CoA coincided with those of cyanide-insensitive palmitoyl-CoA-dependent beta-oxidation and catalase, a marker enzyme of peroxisomes. The profile of the cofactor requirement for beta-oxidation of PCA16-CoA in isolated peroxisomes was similar to that for palmitoyl-CoA oxidation, and the reaction was not inhibited by KCN. The formation of CoA derivative prior to beta-oxidation reaction was essential. HPLC analysis of metabolites after incubation of PCA16-CoA with isolated peroxisomes demonstrated the production of four metabolites, two of which were identified as PCA14 and PCA12 by fast atomic bombardment-mass spectrometry. These results indicate that peroxisomal beta-oxidation participates in the shortening of the alkyl-side chain of PCA16 and plays an important role in the formation of antileukemic cytosine arabinoside from YNKO1.

Effects of 1-beta-D-arabinofuranosylcytosine on DNA replication intermediates monitored by pH-step alkaline elution

The pH-step alkaline elution method enables the isolation and quantification of nascent DNA (nDNA) replication intermediates, including Okazaki fragments, short length nDNA from replicon origins, longer lengths of nascent but subgenomic length nDNA (molecular weight, 20-30 x 10(6)), and full (or genomic) length nDNA (L. C. Erickson et al., Chromosoma, 74: 125-139, 1979). We utilized this technique to study, in HL-60 cells, the effects of 1-beta-D-arabinofuranosylcytosine (ara-C) on the formation of these replication intermediates and the kinetics of transit of radiolabel from [3H]thymidine ([3H]dThd) or [3H]-ara-C through these nDNA fragments and into full length nDNA. In the continuous presence of [3H]-ara-C (4 microM), the majority of radiolabel (greater than 85%) remained in the nascent subgenomic fractions, with 30-50% remaining in Okazaki fragments. These proportions did not change substantially with increasing time of exposure to [3H]-ara-C (from 1 to 24 h), although the total amount of [3H]-ara-C incorporated into DNA continued to increase with increasing time of exposure. In contrast, when cells were exposed to [3H]-ara-C for 1 h, placed in drug-free medium, and studied by the pH-step method at various times thereafter, the transit of radiolabel through progressively larger nDNA intermediates and into full length nDNA was rapid and equal to that of [3H]dThd in cells not exposed to drug. The observed elution of [3H]-ara-C in the subgenomic-length DNA fragments was not due to ara-C-induced breaks in template (parental) DNA and subsequent incorporation of [3H]-ara-C into the template strand, since ara-C treatment of cells prelabeled with [14C]dThd failed to cause substantial elution of the 14C label at the various pH steps used. In studies of the effects of ara-C on [3H]dThd incorporation into nDNA, concentrations of 1 to 10 microM ara-C inhibited total incorporation of radiolabel into DNA by greater than 90% and incorporation into full length nDNA by greater than 97%. In contrast, these concentrations of ara-C failed to decrease the amount of [3H]dThd incorporated into Okazaki fragments or other non-mitochondrial low molecular weight nDNA, compared to control. These studies demonstrate that, in HL-60 cells, ara-C causes profound inhibition of nascent chain elongation, does not cause chain termination, and does not inhibit initiation. In fact, ara-C may stimulate initiation, leading credence to recent theories proposing endoreduplication or reinitiation as consequences of ara-C incorporation into DNA.

The difference spectra of bovine kidney RNase K2 induced upon binding with nucleotides markedly differ from those of bovine pancreatic RNase

The difference spectra obtained upon the addition of nucleotides to bovine kidney RNase, which shows 40% sequence homology with bovine pancreatic RNase, are markedly different from those of bovine pancreatic RNase. As one of the factors which possibly contribute to this difference, we examined the effect of the substitution of Phe120 in bovine pancreatic RNase by Leu in RNase K2 on the difference spectra.

On the inhibition of deoxycytidylate hydroxymethylase by 5-fluoro-2'-deoxycytidine 5'-monophosphate

Studies were performed to determine whether 5-fluoro-2'-deoxycytidine 5'-monophosphate (FdCMP) is an inhibitor of deoxycytidylate hydroxymethylase and whether it could form an isolable covalent complex with the enzyme and the cofactor, 5,10-methyl-enetetrahydrofolate. The results showed that although FdCMP is a competitive inhibitor of dCMP hydroxymethylase, it does not cause time-dependent inhibition of the enzyme in the presence of cofactor. Further, although uv difference spectral evidence was found for FdCMP-cofactor-enzyme complex, the complex was not sufficiently stable to isolate on nitrocellulose filters. We conclude that FdCMP is not a mechanism-based inhibitor of dCMP hydroxymethylase.