Inhibition of Recombinant Aldose-6-Phosphate Reductase from Peach Leaves by Hexose-Phosphates, Inorganic Phosphate and Oxidants

Glucitol, also known as sorbitol, is a major photosynthetic product in plants from the Rosaceae family. This sugar alcohol is synthesized from glucose-6-phosphate by the combined activities of aldose-6-phosphate reductase (Ald6PRase) and glucitol-6-phosphatase. In this work we show the purification and characterization of recombinant Ald6PRase from peach leaves. The recombinant enzyme was inhibited by glucose-1-phosphate, fructose-6-phosphate, fructose-1,6-bisphosphate and orthophosphate. Oxidizing agents irreversibly inhibited the enzyme and produced protein precipitation. Enzyme thiolation with oxidized glutathione protected the enzyme from insolubilization caused by diamide, while incubation with NADP+ (one of the substrates) completely prevented enzyme precipitation. Our results suggest that Ald6PRase is finely regulated to control carbon partitioning in peach leaves.

Preliminary studies on the inhibition of D-sorbitol-6-phosphate 2–dehydrogenase fromEscherichia coliwith substrate analogues

D-Sorbitol-6-phosphate 2-dehydrogenase catalyzes the NADH-dependent conversion of D-fructose 6-phosphate to D-sorbitol 6-phosphate and improved production and purification of the enzyme from Escherichia coli is reported. Preliminary inhibition studies of the enzyme revealed 5-phospho-D-arabinonohydroxamic acid and 5-phospho-D-arabinonate as new substrate analogue inhibitors of the F6P catalyzed reduction with IC50 values of (40 +/- 1) microM and (48 +/- 3) microM and corresponding Km/IC50 ratio values of 14 and 12, respectively. Furthermore, we report here the phosphomannose isomerase substrate D-mannose 6-phosphate as the best inhibitor of E. coli D-sorbitol-6-phosphate 2-dehydrogenase yet reported with an IC50 = 7.5 +/- 0.4 microM and corresponding Km/IC50 ratio = about 76.

Competitive inhibition of phosphoglucose isomerase of apple leaves by sorbitol 6-phosphate

Apple leaf cytosolic phosphoglucose isomerase (PGI, EC 5.3.1.9) was purified to an apparent homogeneity with a specific activity of 2456 units/mg protein, and chloroplastic PGI was partially purified to a specific activity of 72 units/mg protein to characterize their biochemical properties. These two isoforms showed differential responses to heat treatment; incubation at 50 degrees C for 10 min resulted in a complete loss of the chloroplastic PGI activity, whereas the cytosolic PGI only lost 50% of its activity. Apple cytosolic PGI is a dimeric enzyme with a molecular mass of 66 kDa for each monomer. The activity of both isoforms was strongly inhibited by erythrose 4-phosphate (E4P) with a K(i) of 1.2 and 3.0 microM for the cytosolic PGI and chloroplastic PGI, respectively. Sorbitol 6-phosphate (Sor6P), an intermediate in sorbitol biosynthesis, was found to be a competitive inhibitor for both cytosolic and chloroplastic PGIs with a K(i) of 61 and 40 microM, respectively. PGIs from both spinach and tomato leaves were also inhibited by Sor6P in a similar manner. The possible physiological significance of this finding is discussed.

Effect of intracellular delivery of energy metabolites on intracellular Ca2+ in mouse islets of Langerhans

Regulation of glucose-induced oscillations in intracellular Ca2+ concentration ([Ca2+]i) was investigated by using a novel technique, electroporation from an electrolyte-filled capillary, to deliver energy metabolites to the intracellular compartment of mouse islets. Intracellular application of ATP resulted in a nifedipine-sensitive increase in [Ca2+]i, consistent with a KATP-channel dependent mechanism of Ca2+ influx. [Ca2+]i in islets exposed to 10 mM glucose oscillated with a period of approximately 3 min, often superimposed with faster oscillations. Electroporation of ATP blocked all types of oscillations and elevated [Ca2+]i while delivery of ADP had no effect on oscillations. Intracellular delivery of glucose-6-phosphate or fructose-1,6-bisphosphate tended to transform slow oscillations to fast oscillations. These results demonstrate that modulation of ATP concentrations and glycolytic flux are important in development of slow oscillations.

Differences in regulatory properties between human and rat glucokinase regulatory protein

The inhibition of glucokinase by rat and Xenopus GKRPs (glucokinase regulatory protein) is well documented. We report a comparison of the effects of human and rat GKRPs on glucokinase activity. Human GKRP is a more potent inhibitor of glucokinase than rat GKRP in the absence of fructose 6-phosphate or sorbitol 6-phosphate, and has a higher affinity for these ligands. However, human and rat GKRPs have similar affinities for fructose 1-phosphate and chloride. Residues that are not conserved between the rodent and human proteins affect both the affinity for fructose 6-phosphate and sorbitol 6-phosphate and the inhibitory potency of GKRP on glucokinase in the absence of these ligands.

The charged milieu: a major player in fertilization reactions

In previous studies, we have found that negatively charged, but not uncharged, amino acids and sugars block sea urchin fertilization. These studies were developed from modeling work in non-living systems using derivatized agarose beads that suggested that charge-charge bonding may control at least some adhesive interactions. In the present study, the effects of positively charged, negatively charged and uncharged molecules were examined in the sea urchin sperm-egg system in over 300 individual trials. The results indicate that depending on the specific molecules utilized, both sperm and egg are exquisitely sensitive to charged but not uncharged molecules and to pH changes in sea water caused by some of the charged molecules. It is shown that egg activation, as well as sperm motility and sperm-egg interactions, can be affected by charged molecules. One compound, fructose-1-phosphate blocked fertilization in S. purpuratus sea urchins but not in Lytechinus pictus sea urchins. These findings indicate that charge alone cannot explain all the results. In this case, the presence of a ketone instead of an aldehyde group indicates that species-specific components may control fertilization reactions. The present study is a comprehensive survey of the effects of charge, pH and molecular structure on the fertilization activation continuum in a model system of sea urchins.

Membrane potential-dependent conformational changes in mitochondrially bound hexokinase of brain

Previously characterized monoclonal antibodies (Mabs) were used in a study of Type I hexokinase from rat brain. Based on the relative reactivity of these Mabs with soluble and mitochondrially bound forms, binding to mitochondria was shown to affect specific epitopic regions in both N- and C-terminal halves of the enzyme and to modulate conformational changes induced by binding of the ligands, Glc or ATP. Reactivities with Mabs recognizing epitopes in two defined regions of the N-terminal half and one defined region of the C-terminal half of the mitochondrially bound enzyme were selectively affected by mitochondrial membrane potential, or by addition of oligomycin, carboxyatractyloside, or bongkrekic acid. The Glc-6-P analog, 1 ,5-anhydroglucitol-6-P, was much more effective as a competitive inhibitor against extramitochondrial ATP than against intramitochondrial ATP generated by oxidative phosphorylation. These results provide further insight into the role of hexokinase-mitochondrial interactions in regulation of cerebral glucose metabolism.

The muscle fatty acid binding protein of spadefoot toad (Scaphiopus couchii)

Fatty acid binding protein was purified from skeletal muscle of the spadefoot toad (Scaphiopus couchii), an estivating species. While estivating, this animal relies on the fatty acid oxidation for energy. Hence we were interested in the behaviour of fatty acid binding protein under conditions of elevated urea (up to 200 mM) and potassium chloride such as exist during estivation. Also we examined whether there were interactions between glycolytic intermediates and the binding ability of the protein. The amount of bound fatty acid (a fluorescence assay using cis-parinarate) was not affected (P < 0.05) by glucose, fructose 6-phosphate or phosphoenolpyruvate at physiological concentrations. By contrast, glucose 6-phosphate increased the amount of bound cis-parinarate but the apparent dissociation constant was not different from the control. Fructose 1,6-bisphosphate but not fructose 2,6-phosphate decreased cis-parinarate binding by 40%, commensurate with doubling the apparent dissociation constant (1.15-2.62 microM). Urea, guanidinium and trimethylamine N-oxide at 200 mM increased cis-parinarate binding 60% over controls. Urea (1 M) and KCl (200 mM) did not affect cis-parinarate binding compared to controls. The interaction of this fatty acid transporter with fructose 1,6-bisphosphate is discussed in terms of reciprocal interaction with phosphofructokinase since fatty acid is also an inhibitor of phosphofructokinase.

Patterns of phosphoantigen stimulation of human Vgamma9/Vdelta2 T cell clones include Th0 cytokines

This paper examines functional properties of human Vgamma9/Vdelta2 T cell lines and clones generated by in vitro culture with synthetic and natural (mycobacterial) phosphoantigenic molecules. It confirms the broad reactivity of Vgamma9/Vdelta2 T cell lines and clones toward phosphoantigens. Optimal recognition of phosphoantigens by Vgamma9/Vdelta2 T cells required accessory cells to occur, but did not require specialized antigen presenting cells. However, species origin of the APC was irrelevant as proliferation of Vgamma9/Vdelta2 T cells occurred in the presence of syngeneic, allogeneic or xenogeneic APC and was not restricted to APC of particular tissue origin. Moreover antigen uptake and processing was not required for recognition by Vgamma9/ Vdelta2 cells, as evidenced by the ability of fixed APCs to present phosphoantigens. Similarly, the expression of classical MHC class I and class II molecules was not required for phosphoantigen recognition by gammadelta T cells. However, gammadelta T cell clones responded to stimulation by several cytokines including IL-12, IFNgamma and TNFalpha. Finally, Vgamma9/Vdelta2 T cell clones preferentially produced both IFN-gamma and IL-4 in response to PHA or TUBAg stimulation, revealing that a Th0 pattern of cytokine production is frequent among these cells.

A new method for studying the binding of human IgE to CD23 and the inhibition of this binding

CD23, a low-affinity receptor for IgE (Fc epsilonRII), is a type II membrane-bound glycoprotein expressed on many hematopoietic cells, particularly activated B-cells. CD23 binds to IgE at a domain homologous to Ca2+-dependent (C-type) animal lectin. This paper describes a binding assay by which only the specific binding of IgE to CD23 expressed on Epstein-Barr virus (EBV)-transformed B-cell line, L-KT9 cells, can be detected. This assay is useful in the search for CD23 ligands among many chemical compounds, because it is easily carried out and does not require the use of any radiolabeled reagents. Using the assay, we investigated the inhibition of IgE binding to CD23 by fucose-1-phosphate which has been reported to inhibit the binding of sCD23 to IgE [Delespesse, G., Sarfati, M., Wu, C.Y., Fournier, S., Letellier, M., 1992. The low affinity receptor for IgE. Immunol. Rev. 125, 77.] and the binding of CD23 to CD21 [Pochon, S., Graber, P., Yeager, M., Jansen, K., Bernard, A.R., Aubry, T.-P., Bonnefoy, J.-Y., 1992. Demonstration of second ligand for the low affinity receptor for immunoglobulin E (CD23) using recombinant CD23 reconstituted into flourescent liposomes. J. Exp. Med. 176, 389.]. Although both alpha- and beta-L-fucose-l-phosphate/di(cyclohexylammonium) salt decreased the extent of IgE binding to CD23, the inhibitory effects were not due to alpha- or beta-L-fucose-1-phosphate but to cyclohexylamine. The inhibitory effect of cyclohexylamine was dose dependent and the effect was decreased when inhibition tests were carried out in the presence of a 10-fold excess of IgE. These results suggest that cyclohexylamine specifically interacts with the binding of CD23 and IgE.

Affinity labeling of Escherichia coli glucosamine-6-phosphate synthase with a fructose 6-phosphate analog--evidence for proximity between the N-terminal cysteine and the fructose-6-phosphate-binding site

Glucosamine-6-phosphate synthase (GlcNP-synthase) catalyzes the formation of glucosamine 6-phosphate from fructose 6-phosphate using the gamma-amide functionality of glutamine as the nitrogen source. In the absence of glutamine, GlcNP-synthase was recently found to catalyze the formation of glucose 6-phosphate corresponding to a phosphoglucoisomerase-like activity. Here we report active-site directed, irreversible inhibition of Escherichia coli GlcNP-synthase (k(inact) = 0.60 +/- 0.05 min(-1), Kirr = 1.40 +/- 0.20 mM) by anhydro-1,2-hexitol 6-phosphates previously known as irreversible inhibitors of phosphoglucoisomerase. Enzyme inactivation with the tritiated affinity label, followed by tryptic digestion and purification of the radioactive fragments, allowed identification of three peptides. Two of them, accounting for 54% of the recovered radioactivity, are believed to result from the nucleophilic attack of side-chain carboxylates of Glu255 and Glu258 and thiol of Cys300 of the fructose-6-phosphate-binding site on the epoxide functionality of the inhibitor. The major peptide corresponds to derivatization of the N-terminal cysteine from the glutamine-binding site by the inhibitor. These results provide evidence for the close proximity of glutamine and fructose-6-phosphate-binding sites recently suggested by Bearne [Bearne, S. L. (1996) J. Biol. Chem. 271, 3052-3057].

Control of glycolysis in vertebrate skeletal muscle during exercise

The gastrocnemius muscle of the frog (Rana temporaria) has a high capacity for anaerobic glycolysis from glycogen. Glycolytic metabolites and effectors of phosphofructokinase, particularly the hexose bisphosphates, were followed in muscle during exercise (swimming between 5 s and 5 min), recovery (rest for up to 2 h after 5 min of swimming), and repeated exercise (swimming for up to 60 s after 2 h of recovery). Glycogen phosphorylase and phosphofructokinase were swiftly activated with exercise. The hexose bisphosphates followed markedly different time courses. Fructose 1,6-bisphosphate was transiently increased in both exercise and repeated exercise. This appears to be an effect rather than a cause of phosphofructokinase activation. Glucose 1,6-biphosphate was accumulated only while phosphofructokinase was active and was unchanged at other times. Fructose 2,6-biphosphate showed a 10-fold transient increase on exercise in rested frogs, almost disappeared from the muscle during recovery, and did not change during repeated exercise. Fructose 2,6-biphosphate is a potent activator of phosphofructokinase in vitro under near physiological assay conditions, and it may serve this function also in vivo during exercise. Glucose 1,6-biphosphate could be an activator of phosphofructokinase in repeated exercise when fructose 2,6-biphosphate is not available.

Functional organization of mammalian hexokinases: characterization of chimeric hexokinases constructed from the N- and C-terminal domains of the rat type I and type II isozymes

Chimeric hexokinases consisting of either the N-terminal half of Type I hexokinase fused with the C-terminal half of the Type II isozyme (NICII) or the inverse pair (NIICI), along with the parental isozymes, were expressed in COS-1 cells. The thermal stability of the chimeras was intermediate between that of the highly labile Type II isozyme and the relatively stable Type I hexokinase. In their Kms for substrates, Glc and ATP, the chimeric enzymes were similar to the parental isozyme from which the C-terminal half was derived. Although the Type I and Type II isozymes were similar in their sensitivity to inhibition (competitive vs ATP) by the Glc-6-P analogs, 1,5-anhydroglucitol 6-phosphate (AnGlc-6-P), and Glc-1,6-bisphosphate, the chimeric enzymes differed markedly, with the NIICI chimera being much more sensitive and the NICII chimera much less sensitive than either parental form to these inhibitors. In contrast, the response of the chimeras to Pi, either as an antagonist of inhibition by AnGlc-6-P or, at higher concentrations, as an inhibitor, was correlated with the origin of the N-terminal domain. The results are consistent with the view that catalytic function is associated with the C-terminal domain of the Type I isozyme, with regulatory function--inhibition by Glc-6-P and its analogs and antagonism of this inhibition by Pi--being mediated by the N-terminal domain.

Mechanism of hemolysis of canine erythrocytes induced by L-sorbose

The cause of species difference in the susceptibility of erythrocytes to L-sorbose, and the difference in the hemolytic effect of sorbose on high potassium-containing (HK) and low potassium-containing (LK) canine erythrocytes were examined. L-Sorbose was phosphorylated in canine erythrocytes, but not in human erythrocytes. Furthermore, sorbose-1-phosphate, a metabolite of L-sorbose, strongly inhibited the hexokinase of LK canine erythrocytes, but not that of HK canine erythrocytes. These results strongly indicated that inhibition of hexokinase by sorbose-1-phosphate in LK erythrocytes induced severe glycolytic limitation in these cells, resulting in hemolysis, and that HK erythrocytes are resistant to sorbose-induced hemolysis because these cells have a high hexokinase activity.

Lysines 72, 80 and 213 and aspartic acid 210 of the Lactococcus lactis LacR repressor are involved in the response to the inducer tagatose-6-phosphate leading to induction of lac operon expression

Site-directed mutagenesis of the Lactococcus lactis lacR gene was performed to identify residues in the LacR repressor that are involved in the induction of lacABCDFEGX operon expression by tagatose-6-phosphate. A putative inducer binding domain located near the C-terminus was previously postulated based on homology studies with the Escherichia coli DeoR family of repressors, which all have a phosphorylated sugar as inducer. Residues within this domain and lysine residues that are charge conserved in the DeoR family were changed into alanine or arginine. The production of the LacR mutants K72A, K80A, K80R, D210A, K213A and K213R in the LacR-deficient L.lactis strain NZ3015 resulted in repressed phospho-beta-galactosidase (LacG) activities and decreased growth rates on lactose. Gel mobility shift assays showed that the complex between a DNA fragment carrying the lac operators and LacR mutants K72A, K80A, K213A and D210A did not dissociate in the presence of tagatose-6-phosphate, in contrast to wild type LacR. Other mutations (K62A/K63A, K72R, K73A, K73R, T212A, F214R, R216R and R216K) exhibited no gross effects on inducer response. The results strongly suggest that the lysines at positions 72, 80 and 213 and aspartic acid at position 210 are involved in the induction of lac operon expression by tagatose-6-phosphate.

The regulatory protein of liver glucokinase

Fructose, sorbitol and D-glyceraldehyde stimulate the rate of glucose phosphorylation in isolated hepatocytes. This effect is mediated by fructose 1-phosphate, which releases the inhibition exerted by a regulatory protein on liver glucokinase. In the presence of fructose 6-phosphate, the regulatory protein binds to, and inhibits, liver glucokinase. Fructose 1-phosphate antagonizes this inhibition by causing dissociation of the glucokinase-regulatory protein complex. Both phosphate esters act by binding to the regulatory protein, and by presumably causing changes in its conformation. The regulatory protein behaves as a fully competitive inhibitor. It inhibits liver glucokinase from various species, and rat islet glucokinase, but has no effect on hexokinases from mammalian tissues or from yeast, or on glucokinase from microorganisms. Kinetic studies indicate that the regulatory protein binds to glucokinase at a site distinct from the catalytic site. Several phosphate esters, mainly polyol-phosphates, were found to mimick the effect of fructose 6-phosphate. The most potent is sorbitol 6-phosphate, suggesting that fructose 6-phosphate is recognized by the regulatory protein in its open-chain configuration. Other phosphate esters and Pi have a fructose 1-phosphate-like effect. The stimulatory effect of fructose on glucose phosphorylation is observed not only in isolated hepatocytes but also in the livers of anesthetized rats. This suggests that fructose could be a nutritional signal causing an increase in the hepatic glucose uptake.

Effects of mannose-6-phosphate on receptor-mediated endocytosis of insulin-like growth factor-II

The insulin-like growth factor-II (IGF-II) and lysosomal enzymes containing a mannose-6-phosphate (M6P) recognition site bind to different sites of the same receptor molecule. We have observed that M6P increases the receptor-mediated uptake of IGF-II into IM-9 cells. We now confirm this phenomenon in a different line, the H-35 rat hepatoma cells, and present additional characterization of the underlying mechanisms. When incubated in the presence of radiolabeled IGF-II, H-35 cells accumulated, in a time-dependent fashion, radioactivity that was resistant to removal by trypsin digestion at 15 C, indicating that it was endocytosed. In the presence of 3 mM M6P, endocytosed counts were approximately 2-fold higher after 5 min of incubation, an enhancement that peaked at 10 min, then declined, but was still evident after 40 min (1.5-fold). The rate of release of cell-associated IGF-II, degraded or intact, as measured in a chase experiment, was not affected by M6P. These observations indicate that M6P increased accumulation of IGF-II by accelerating its rate of endocytosis rather than by interfering with IGF-II degradation or with the recycling of intact hormone-receptor complexes to the cell surface. Electrophoresis after affinity cross-linking of labeled cells demonstrated that the enhancement in radioactivity could be located at a molecular size of approximately 250 kDa, corresponding to IGF-II-receptor complexes. Preincubation with M6P did not significantly alter the specific binding of IGF-II to the cell surface of H-35 cells, as measured by a binding assay at 4 C. Finally, pretreatment with cycloheximide for up to 8 h, to remove all newly synthesized lysosomal enzymes bound to the M6P/IGF-II receptor, did not affect IGF-II endocytosis beyond what could be accounted for by some loss of receptor, suggesting that the observed effect of M6P is due to the binding of M6P itself to the receptor and not to displacement of lysosomal enzymes. We conclude that simultaneous occupancy of the M6P/IGF-II receptor by ligands on both binding sites enhances its rate of endocytosis.

Regulation of glucokinase by a fructose-1-phosphate-sensitive protein in pancreatic islets

In the post-microsomal supernatant of pancreatic islets, prepared from fasted or fed rats, D-fructose 1-phosphate increased the activity of glucokinase by 20-30% as measured in the presence of D-glucose 6-phosphate and D-fructose 6-phosphate. Such an activation was less marked than that found in liver extracts. The islet cytosol was also found to inhibit purified liver glucokinase, and this effect was antagonized by D-fructose 1-phosphate. In the presence of hexose 6-phosphates, partially purified islet glucokinase was inhibited by the hepatic glucokinase regulatory protein in a D-fructose-1-phosphate-sensitive manner. In intact islets, D-glyceraldehyde stimulated the generation of 14C-labelled D-fructose 1-phosphate from D-[U-14C]glucose and increased the production of 3H2O from D-[5-3H]glucose. These findings suggest that the activity of glucokinase in islet cells may be regulated by a protein mediating the antagonistic effects of D-fructose 6-phosphate and D-fructose 1-phosphate in a manner qualitatively similar to that operating in hepatocytes, but with lower efficiency.

Mannose-6-phosphate stimulates proliferation of neuronal precursor cells

The mitogenic signal function of mannose-6-phosphate (Man-6-P)/insulin-like growth factor II (IGF-II) receptors was studied in neuronal precursor cells from developing rat brain (E15). About 30% of the cellular Man-6-P/IGF-II receptors were present on the cell surface. Man-6-P and IGF-II stimulated DNA synthesis twofold and their effects were additive. Antibody 3637 to the Man-6-P/IGF-II receptor blocked the response to Man-6-P but not that to IGF-II. Other phosphorylated hexoses were also active. Fructose-1-phosphate was equally potent with Man-6-P, whereas glucose-6-phosphate was 5 times less potent. We conclude that Man-6-P-containing proteins and IGF-II act as mitogens in developing brain by interaction with the Man-6-P/IGF-II receptor and the IGF-I receptor, respectively.

A bisubstrate analog inhibitor for alpha(1----2)-fucosyltransferase

Porcine submaxillary beta-galactoside alpha(1----2)-fucosyltransferase is known to transfer a fucosyl residue from guanosine 5'-diphosphofucose (GDP-fucose) to the 2-OH group of beta-D-galactopyranosides with inversion of configuration at the fucopyranosyl anomeric carbon. A bisubstrate analog (1) of the postulated transition-state for this reaction, which has O-2 of phenyl beta-D-galactopyranoside attached to the terminal phosphorous of GDP through a flexible ethylene bridge, has been chemically synthesized and evaluated as an inhibitor of this enzyme. Compound 1 was found to be a competitive inhibitor with respect to both GDP-fucose and phenyl beta-D-galactopyranoside for both the membrane-bound and soluble forms of the fucosyltransferase. It was also a competitive inhibitor with respect to the alternate acceptor beta DGal(1----3)beta DGlcNAcO(CH2)8-COOMe. The Ki values were in the range 2.3-16 microM. Compound 1 is the first example of a bisubstrate analog inhibitor for a glycosyltransferase which binds to both the acceptor and donor recognition sites of the enzyme. The potential of a bisubstrate analog strategy for the production of specific glycosyltransferase inhibitors is discussed.

Inhibition by lysosomal enzymes and mannose-6-phosphate of the phosphorylation of the lysosomal enzyme binding receptor protein from monkey brain

The lysosomal enzymes beta-glucuronidase and alpha-L-fucosidase and mannose-6-phosphate inhibited the phosphorylation of the lysosomal enzyme binding receptor protein prepared from monkey brain. Inhibition of both serine and tyrosine phosphorylation was observed. A non-lysosomal glycoprotein enzyme butyrylcholinesterase, mannose or glucose did not inhibit phosphorylation. Tyrosine phosphorylation of histone by the receptor protein was also inhibited by the lysosomal enzymes and mannose-6-phosphate.

Mannose 6-phosphate potentiates insulin-like growth factor II-stimulated inositol trisphosphate production in proximal tubular basolateral membranes

To ascertain whether mannose 6-phosphate affects insulin-like growth factor (IGF) II stimulation of phospholipase C activity in the basolateral membrane of the renal proximal tubular cell, we determined the effect of mannose 6-phosphate on IGF II-stimulated production of inositol trisphosphate (Ins-P3) in isolated basolateral membranes. Production of Ins-P3 measured in the presence of 10(-10), 10(-9), or 10(-8) M rat IGF II was potentiated approximately 2-fold by inclusion of 5 mM mannose 6-phosphate in incubations. Mannose 6-phosphate had no effect on Ins-P3 production in the absence of IGF II. Neither mannose 1-phosphate, mannose, glucose 6-phosphate, nor fructose 1-phosphate exerted similar potentiation. Enhancement of IGF II-stimulated Ins-P3 production required concentrations on the order of several millimolar mannose 6-phosphate. Total and specific binding of 10(-10) M 125I-IGF II to basolateral membranes was significantly increased by 5 mM mannose 6-phosphate. However, there was no significant effect on total or specific binding of 10(-9) or 10(-8) M 125I-IGF II. Our findings suggest that mannose 6-phosphate potentiates stimulation of phospholipase C by IGF II in the basolateral membrane of the renal proximal tubular cell and that potentiation is mediated via a mechanism in addition to enhanced binding of IGF II. Such potentiation could reflect a role for the mannose 6-phosphate moiety as a modulator of IGF II "signal" transmission in vivo.

Rat brain hexokinase: further studies on the specificity of the hexose and hexose 6-phosphate binding sites

Based on the lack of correlation between the ability of various hexoses to serve as substrate and the ability of the corresponding hexose 6-phosphates to inhibit brain hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1), R. K. Crane and A. Sols (1954, J. Biol. Chem. 210, 597-606) proposed that this enzyme possesses two discrete sites capable of binding hexose moieties, one serving as the substrate binding site and a second, regulatory in function, to which inhibitory 6-phosphates bind. Subsequent work has provided further experimental support for this proposal. The pioneering work by Crane and Sols focused primarily on the specificity of these sites with respect to requirements for orientation of hydroxyl substituents at the various positions of the pyranose ring. The present study explores additional aspects of the specificity of these sites, namely, the effect of substitution of a sulfur atom in place of the oxygen in the pyranose ring on ability to serve as substrate or inhibitor, and the effect of modification in charge of the substituent at the 6-position on inhibitory effectiveness. 5-Thioglucose is a linear competitive (versus glucose) inhibitor of rat brain hexokinase, with a Ki of about 0.2 mM, and is a linear mixed inhibitor (versus ATP), with Ki values in this same range. 5-Thioglucose is not, however, readily phosphorylated by brain hexokinase. Thus, although 5-thioglucose binds with moderate affinity to the glucose binding site, it is not effectively used as a substrate of the enzyme. Inhibition of brain hexokinase by glucose 6-phosphate or its analogs has been found to require a dianionic substituent at the 6-position. The 6-fluorophosphate derivative and glucose 6-sulfate are poor inhibitors of the enzyme, and the Ki for inhibition by 1,5-anhydroglucitol 6-phosphate increases markedly at pH values below the pK of the 6-phosphate group, indicating that the monoanionic form is ineffective as an inhibitor. In contrast to the detrimental effect that substitution of the oxygen atom in the pyranose ring with a sulfur has on ability to serve as substrate, 5-thio analogs are considerably more effective as inhibitors, the Ki for inhibition by 5-thioglucose 6-phosphate being 10-fold lower than that seen with glucose 6-phosphate. This effect of the heteroatom substitution can partially offset the decreased inhibition resulting from monoanionic character at the 6-position, but the 6-fluorophosphate derivative of 5-thioglucose 6-phosphate still inhibits with a Ki about 1000-fold greater than that seen with 5-thioglucose 6-phosphate.(ABSTRACT TRUNCATED AT 400 WORDS)

Endocytosis of receptor-bound insulin-like growth factor II is enhanced by mannose-6-phosphate in IM9 cells

The insulin-like growth factor II (IGF-II), and glycoproteins containing mannose 6-phosphate (M6P), bind to two different sites of the same receptor molecule (Morgan et al, Nature 329:301, 1987). To study the interactions between the two ligands on their common receptor in intact cells, we examined the effect of free M6P on IGF-II binding and endocytosis in the IM9 human lymphoblastoid cell line. M6P, up to a 3 mM concentration, had no effect on the binding of IGF-II to the cell surface receptor of intact IM9 cells at 4 degrees C. By contrast, when IM9 cells were incubated with 125I-IGF-II at 37 degrees C, 1 mM M6P increased cell-associated radioactivity by twofold. The increase was resistant to acid wash at 4 degrees C, and therefore assumed to represent endocytosed IGF-II. Acid-washable radioactivity was no different, confirming that, in intact cells, M6P does not affect IGF-II surface binding. In addition, preincubation of cells with M6P at 37 degrees C for up to 3 hours did not change the abundance of receptor on the cell surface, as measured by a subsequent 4 degrees C binding assay. We conclude that M6P causes a shift of IGF-II-occupied receptors form the cell surface to intracellular locations without affecting surface binding of this ligand in IM9 cells. The effect could be produced by the binding of M6P itself, or by the displacement of endogenous phosphomannosylated ligands.

Characterization of yeast fructose-2,6-bisphosphate 6-phosphatase

To obtain information on the biological significance of yeast fructose-2,6-bisphosphate 6-phosphatase, kinetic data of the purified enzyme [(1987) Eur. J. Biochem. 164, 27-30] have been measured. Maximal activity was found between pH 6 and 7, the apparent Michaelis constant with fructose 2,6-bisphosphate was 7.2 microM at pH 6.0 and 79 microM at pH 7.0. Concentrations required for 50% inhibition of the enzyme at pH 6.0 were 8 microM Fru2P, 45 microM G1c6P, 80 microM Fru6P and 200 microM inorganic phosphate. The known intracellular steady-state level of about 10 microM fructose 2,6-bisphosphate in the presence of glucose is likely to be the result of a balance between the rapid synthesis of fructose 2,6-bisphosphate catalyzed by 6-phosphofructose 2-kinase and a fructose 2,6-bisphosphate degrading activity. The biological function of fructose-2,6-bisphosphate 6-phosphatase with an apparent Michaelis constant between 7 and 79 microM fructose 2,6-bisphosphate at pH 6-7 is therefore suggested to participate in the maintenance of a steady-state level of fructose 2,6-bisphosphate in a concentration range that fits well with the Michaelis constant of the enzyme.

Endocytosis of receptor-bound insulin-like growth factor II is enhanced by mannose-6-phosphate in IM9 cells

The insulin-like growth factor II (IGF-II), and glycoproteins containing mannose 6-phosphate (M6P), bind to two different sites of the same receptor molecule (Morgan et al, Nature 329:301, 1987). To study the interactions between the two ligands on their common receptor in intact cells, we examined the effect of free M6P on IGF-II binding and endocytosis in the IM9 human lymphoblastoid cell line. M6P, up to a 3 mM concentration, had no effect on the binding of IGF-II to the cell surface receptor of intact IM9 cells at 4 degrees C. By contrast, when IM9 cells were incubated with 125I-IGF-II at 37 degrees C, 1mM M6P increased cell-associated radioactivity by twofold. The increase was resistant to acid wash at 4 degrees C, and therefore assumed to represent endocytosed IGF-II. Acid-washable radioactivity was no different, confirming that, in intact cells, M6P does not affect IGF-II surface binding. In addition, preincubation of cells with M6P at 37 degrees C for up to 3 hours did not change the abundance of receptor on the cell surface, as measured by a subsequent 4 degrees C binding assay. We conclude that M6P causes a shift of IGF-II-occupied receptors form the cell surface to intracellular locations without affecting surface binding of this ligand in IM9 cells. The effect could be produced by the binding of M6P itself, or by the displacement of endogenous phosphomannosylated ligands.

Interactions of the receptor for insulin-like growth factor II with mannose-6-phosphate and antibodies to the mannose-6-phosphate receptor

Recently, the sequence of the human receptor for insulin-like growth factor II (IGF-II) was found to be 80% identical [Morgan et al., (1987) Nature 329, 301-307] to the sequence of a partial clone of the bovine cation-independent mannose-6-phosphate receptor [Lobel et al., (1987) Proc. Natl. Acad. Sci. USA 84, 2233-2237]. In the present study, the purified receptor for insulin-like growth factor II (IGF-II) was found to react with two different polyclonal antibodies to the purified mannose-6-phosphate receptor. Moreover, mannose-6-phosphate was found to stimulate the binding of labeled IGF-II to the IGF-II receptor by two-fold. This effect had the same specificity and affinity as the reported binding of mannose-6-phosphate to its receptor; mannose-1-phosphate and mannose had no effect on the binding of labeled IGF-II to its receptor, and the half-maximally effective concentration of mannose-6-phosphate was 0.3 mM. Also, mannose-6-phosphate did not affect labeled IGF-II binding to the insulin receptor. These results support the hypothesis that a single protein of Mr-250,000 binds both IGF-II and mannose-6-phosphate. Furthermore, they indicate that mannose-6-phosphate can modulate the interaction of IGF-II to its receptor.

Inhibition of hexokinase activity by a fructose 2,6-bisphosphate-dependent cytosolic protein from liver

Mammalian and yeast hexokinases were found to be reversibly inhibited by fructose 2,6-bisphosphate, an effect requiring the presence of a cytosolic protein factor. Experimental evidence suggests that this factor (inhibitor) is a regulatory protein, the interactions of which with hexokinases are modulated by fructose 2,6-bisphosphate. The Vmax of hexokinase D was decreased, and no changes on other kinetic parameters were observed. The inhibitor was present in fresh liver cytosol filtered through Sephadex G-25 and was partially isolated by negative absorption on DEAE-cellulose followed by ammonium sulfate fractionation. The inhibitor was also present in brain and kidney, but not in muscle. A molecular mass of 200,000 was determined by gel filtration. The inhibition was dependent on the concentrations of both the inhibitory protein and fructose 2,6-bisphosphate. No delay in fructose 2,6-bisphosphate inhibition was observed. Several other hexose phosphates were tested and were not effective. In the presence of amounts of inhibitor sufficient to produce complete inhibition of hexokinase D, the concentration of fructose 2,6-bisphosphate required to produce 50% inhibition was about 0.5 microM. The inhibitor was unstable and was stabilized by the presence of fructose 2,6-bisphosphate.

Hexose phosphates as regulators of hepatic glycogen synthase phosphatases

The activity of glycogen synthase phosphatase from smooth endoplasmic reticulum of liver was stimulated markedly by galactose-6- and fructose-6-phosphates and to a lesser extent by glucose-1- and 2-deoxyglucose-6-phosphates. The synthase phosphatase of liver cytosol showed strong activation by glucose-1-, glucose-6- and fructose-6-phosphates and smaller activation by galactose-6- and 2-deoxyglucose-6-phosphates. Kinetic analysis showed that the activators did not affect the Km for glycogen synthase D, for either enzyme. The mechanism of activation of the two phosphatases by hexose phosphates appears to be by combination of the activator at a specific activator site on the enzyme rather than by substrate modulation. It is concluded that certain hexose phosphates, particularly fructose-6-phosphate and glucose-1-phosphate, can function as regulators of hepatic synthase phosphatase activity, and that this may explain the ability of elevated blood glucose to increase both glycogen synthase I activity and glycogen synthesis in the liver.

The effect of mannose 6-phosphate on the turnover of the proteoglycans in the extracellular matrix of human fibroblasts

Human fibroblasts (SL66) were cultured in medium containing 35SO2-4 to label the glycosaminoglycans (GAGs). The cells were then detached from the culture dish to leave radioactively-labeled components of the extracellular matrix, hereafter termed 35S-labeled substrate-attached material. When unlabeled SL66 fibroblasts were plated onto this 35S-labeled substrate-attached material, the cells mediated two distinct events: (a) release of radioactivity from the substrate-attached material into the medium; (b) degradation of certain glycosaminoglycans into radioactive components of very low molecular weight including free radioactive sulfate. In the presence of mannose 6-phosphate, however, the degradation of the substrate-attached material by SL66 cells was partially inhibited. Analyses of this effect in terms of the dose-response curve, saccharide specificity, ammonium chloride sensitivity, and the requirement for cells suggest that both an intracellular compartment and the mannose 6-phosphate receptor that binds lysosomal enzymes at the cell surface may play important roles in the turnover and degradation of certain proteoglycans in substrate-attached material.

Isolation of a mutation resulting in constitutive synthesis of L-fucose catabolic enzymes

A ribitol-positive transductant of Escherichia coli K-12, JM2112, was used to facilitate the isolation and identification of mutations affecting the L-fucose catabolic pathway. Analysis of L-fucose-negative mutants of JM2112 enabled us to confirm that L-fucose-1-phosphate is the apparent inducer of the fucose catabolic enzymes. Plating of an L-fuculokinase-negative mutant of JM2112 on D-arabinose yielded an isolate containing a second fucose mutation which resulted in the constitutive synthesis of L-fucose permease, isomerase, and kinase. This constitutive mutation differs from the constitutive mutation described by Chen et al. (J. Bacteriol. 159:725-729, 1984) in that it is tightly linked to the fucose genes and appears to be located in the gene believed to code for the positive activator of the L-fucose genes.

Anomeric and other substrate specificity studies with myo-inositol-1-P synthase

L-myo-Inositol-1-phosphate synthase has been found to have at least a 5-fold preference for the beta-anomer of its natural substrate D-Glc-6-P. The alpha-anomer appears to be an inhibitor of the reaction and may be converted to product as well. As well as showing an enzymatic preference for the equatorial C-1 hydroxyl of D-Glc-6-P, our results suggest that it is the pyranose form of D-Glc-6-P that binds to the enzyme and that ring-opening is an enzymatic step. We have also found D-2-dGlc-6-P, D-2-F-2-dGlc-6-P, and D-Man-6-P each to be both competitive inhibitors and substrates that are converted to inositol phosphates by the synthase. D-Allose-6-P is a weak inhibitor of the enzyme, but not a substrate. D-Gal-6-P is neither substrate nor inhibitor. Thus the specificity of the synthase with respect to single position epimers of D-Glc-6-P increases in the order C1 less than C2 much less than C3 less than C4.

Studies on natural, antibody-dependent, and interleukin-2-activated killer-cell activity of a patient with mucolipidosis III as a test of the mannose-6-phosphate lytic acceptor hypothesis

The natural (NK), antibody-dependent (K), and interleukin-2-generated (LAK) killer-cell activity of a patient with mucolipidosis III (ML III; an autosomal recessive defect in UDP-N-acetylglucosamine: lysosomal enzyme N-acetylglucosamine-1-phosphotransferase) was studied to determine whether or not the defect in the phosphorylation of lysosomal enzyme mannose residues resulted in a failure of target-cell lysis, as would be predicted from recent studies showing NK inhibition by phosphorylated sugars, especially mannose-6-phosphate. The patient was studied in parallel with normal donors known to be at the high and low extremes of NK activity. The following results were obtained: NK activity was markedly elevated against K562, Molt-4, human fibroblasts, HL-60, and MeWo to levels approximately one to two times that of our previous highest donor and five times the mean of normal donors previously tested. Interleukin-2-generated killer-cell activity and antibody-dependent cell-mediated cytotoxicity against antibody-coated P815 cells were normal and increased, respectively. HNK-1-positive cells were normal in frequency (7.3 +/- 1.7%), while lytic conjugates were proportional to activity (3.9 +/- 0.6 vs 2.7 +/- 0.4% for the "high" donor), and this was attributable to an increased proportion of lytic cells. The addition of fresh serum from the ML III patient had no effect on the NK activity of normal donors and the effects of preincubation with interferon (enhancement), monensin (inhibition), fructose-6-phosphate (inhibition), and mannose-6-phosphate (inhibition) were identical to those seen using lymphocytes from normal donors. Studies on the NK activity of the parents and two normal female siblings showed that the father's NK activity was high, the mother's was low, and both siblings' was intermediate but low. The data obtained suggest that the inability of lymphocytes to phosphorylate lysosomal enzyme mannose residues had no effect on NK-, K-, or LAK-cell function and that the mechanism of target-cell lysis is independent of either a mannose-6-phosphate-bearing lytic moiety or a mannose-6-phosphate-dependent ligand mechanism.

Natural killer cell-mediated cytotoxicity does not depend on recognition of mannose 6-phosphate residues

Interaction of mannose 6-phosphate-specific receptors with their ligands has been suggested to be essential for natural killer cell (NK)-mediated cytotoxicity. Indeed, mannose 6-phosphate-specific receptors and ligands bearing mannose 6-phosphate residues are demonstrable on human peripheral blood leukocytes with NK activity as well as on K-562 NK target cells, allowing at least in principle such an interaction. It can also be shown that NK activity of human peripheral blood leukocytes is inhibited by mannose 6-phosphate. The following observations, however, exclude an essential role of the mannose 6-phosphate receptor-ligand system in NK cell-mediated cytotoxicity. 1) NK cytotoxicity is sensitive to a broad range of structurally unrelated sugar phosphates. 2) NK activity is normal in patients with I cell disease (mucolipidosis II), which due to a genetic defect are unable to synthesize the ligands for the mannose 6-phosphate-specific receptor. 3) NK cytotoxicity is not inhibited by an antiserum against the mannose 6-phosphate receptor, which blocks the receptor function.

Fructose 2,6-bisphosphate, a potent regulator of carbohydrate metabolism, inhibits trehalose phosphorylase from protist Euglena gracilis

Partially purified trehalose phosphorylase (EC 2.4.1.64) from Euglena gracilis SM-ZK was inhibited by fructose 2,6-bisphosphate in both synthetic and degradative directions. Ki value for trehalose phosphorolysis was 1.2 microM and that for trehalose synthesis was 0.5 microM. Functions of fructose 2,6-bisphosphate in Euglena, particularly in the regulative mechanism of the two reserve carbohydrates, paramylon and trehalose, are discussed.

2,5-Anhydro-1-deoxy-1-phosphono-D-altritol, an isosteric analogue of alpha-D-ribofuranose 1-phosphate

Condensation of tetramethyl methylenebisphosphonate with 2,3-O-isopropylidene-5-O-trityl-D-ribose gave a mixture of 2,5-anhydro-1-deoxy-1-(diethoxyphosphinyl)-2,3 -O-isopropylidene-5-O-trityl-D-altritol and -allitol. Separation of the isomers and deprotection gave 2,5-anhydro-1-deoxy-1-phosphono-D-altritol and -allitol. The former is the stable isosteric methylenephosphonate analogue of alpha-D-ribose 1-phosphate, the ribose donor in nucleoside phosphorylase catalyzed nucleoside biosynthetic reactions. It did not, however, inhibit purine nucleoside phosphorylase at concentrations of 6 mM.

Glucose-stimulated protein phosphorylation in the pancreatic islet

A glucose-dependent phosphorylation of a 68kDa islet-cell protein was observed in islet-cell homogenates. In the presence of [gamma-32P]ATP the protein was phosphorylated only in the presence of alpha-D-glucose; other sugars were ineffective. Activation of the phosphorylation was half-maximal at 0.34 mM-glucose, 7 microM-ATP and 0.3 mM-Mg2+. Although the addition of glucose 6-phosphate in this design did not stimulate phosphorylation of the islet-cell protein, addition of glucose 6-phosphate to the radioactively labelled 68kDa protein rapidly removed (chased) the 32P label. The addition of presynthesized glucose 6-[32P]phosphate phosphorylated the 68kDa band in the islet-cell homogenate and also phosphorylated purified skeletal-muscle phosphoglucomutase. Phosphoglucomutase labelled thus by 32P was indistinguishable from the islet-cell phosphoprotein on electrophoretic gels. The 32P incorporated into both the islet-cell protein and the purified skeletal-muscle phosphoglucomutase was chased similarly by hexose phosphates. The purified phosphoglucomutase could also be phosphorylated by cyclic AMP-dependent protein kinase or by a mannoheptulose-insensitive process by the islet-cell cytosol. The phosphoenzyme formed thus was also dephosphorylated by D-glucose 6-phosphate and alpha-D-glucose 1-phosphate, suggesting that this may be a mechanism for generation of glucose 1,6-bisphosphate.

Identification and activation of storage protein receptor of Sarcophaga peregrina fat body by 20-hydroxyecdysone

Previous work showed that 20-hydroxyecdysone activates the fat body of Sarcophaga peregrina larvae to incorporate storage protein selectively from the hemolymph. In this study, storage protein receptors of the fat body membrane which were induced on pupation or on treatment of larval fat body with 20-hydroxyecdysone in vitro were identified. The binding of storage protein to its receptor required divalent cations, especially Ca2+, and the binding was very sensitive to pH. The storage protein receptor was inactivated when the fat body membrane was treated with trypsin. The storage protein receptor is probably a protein and it may be synthesized de novo or a cryptic form may be converted to the active form when the concentration of 20-hydroxyecdysone in the hemolymph reaches a physiological level.

Further characterization of the heat-stable factor in the alpha-hydroxylation and oxidation of lignoceric acid in brain: effect of acidic amino acids and hexose-phosphates on brain fatty acid metabolism

Lignoceric acid and other very long-chain fatty acids are converted to alpha-hydroxy fatty acids and ceramide in brain. These fatty acids are also oxidized and produce glutamic acid and other water-soluble products. All of these metabolic conversions are catalyzed by a rat brain particulate fraction and require NADPH, heat-labile factor, and heat-stable factor. The heat-stable factor was prepared from calf cerebellum. Glucose 6-phosphate and N-acetylaspartic acid have previously been identified as active components of the heat-stable factor. We report in this manuscript that glutamic acid, glutamine, aspartic acid, and gamma-aminobutyric acid as well as inorganic phosphate and adenosine nucleotides are also active components of the heat-stable factor. When the amino acids, glucose 6-phosphate, AMP, and phosphoric acid were combined, full activity of the heat-stable factor for the formation of cerebronate (by alpha-hydroxylation) and glutamate (presumably by beta-oxidation) from lignoceric acid was recovered. The role of the acidic amino acids in the metabolic conversion of lignoceric acid in brain appears to be their conversion to the corresponding alpha-keto acids and then incorporation into the TCA cycle. Glucose 6-phosphate is also likely to be involved in the TCA cycle through the Emden-Meyerhof pathway. Inorganic phosphate and AMP seemingly are used to produce ATP. However, the addition of up to 20 mM ATP alone did not replace the heat-stable factor.

The effect of mannose6-phosphate on the turnover of cell surface glycosaminoglycans

Human fibroblasts (SL66) were cultured in medium containing 35SO4(2-) to label the glycosaminoglycans (GAGs). After washing, the labeled cells were chased in the presence or absence of mannose6-phosphate (M6P) and the GAGs were analyzed in terms of three arbitrary fractions: 1, Extracellular (soluble medium), 35S radioactivity higher in cultures without M6P than in cultures with M6P. 2, Pericellular (cell surface-associated), 35S radioactivity lower in cultures without M6P than in cultures with M6P. 3, Intracellular (residue within the intact cell), no difference in 35S radioactivity between the two sets of cultures. In addition, when the 35S-labeled GAGs from corresponding cellular compartments derived from cultures with and without M6P were digested with pronase and chondroitin ABC lyase, and then compared by chromatography on Sepharose CL-6B, distinct molecular differences in both the extracellular and pericellular fractions were observed. Several lines of evidence indicate that the effect of M6P on the turnover of 35S-labeled GAGs in our assay system reflects disruption of cell surface lysosomal enzyme activity. For example, when the experiment was performed with I cells, which lack enzymes carrying the M6P marker, no difference was seen in cultures with or without M6P. The addition of lysosomal enzymes derived from normal human fibroblasts to 35SO4-labeled I cells, however, resulted in the turnover of pericellular GAGs and this effect was inhibited by M6P. These results suggest that one possible function of cell surface receptors recognizing the M6P moiety of lysosomal enzymes is to anchor certain of these enzymes proximate to their substrates at the cell surface.

The predominant secreted protein of transformed murine fibroblasts carries the lysosomal mannose 6-phosphate recognition marker

We have found that the major excreted protein (MEP) of transformed mouse fibroblasts, a phosphoglycoprotein of Mr = 35,000, carries the mannose 6-phosphate recognition marker. MEP secreted by Kirsten virus-transformed NIH 3T3 cells binds to a purified preparation of lysosomal enzyme phosphomannosyl receptor, and this binding is specifically inhibited by mannose 6-phosphate. 32Pi introduced into MEP by metabolic labeling of intact cells is exclusively associated with asparagine-linked oligosaccharides as indicated by sensitivity to endohexosaminidase H. Labeling studies utilizing [2-3H]mannose indicate that approximately one-fifth of the mannose residues of MEP are phosphorylated. Comparative studies of the synthesis, secretion, and uptake of MEP and the lysosomal enzyme beta-galactosidase indicate that MEP made by Kirsten virus-transformed NIH 3T3 cells is not handled in the same manner as are other lysosomal enzymes. MEP may be an unusual lysosomal protein, or both.

Adsorptive pinocytosis of phosphorylated oligosaccharides by human fibroblasts

Adsorptive pinocytosis of lysosomal enzymes by human fibroblasts depends on phosphomannosyl recognition markers on the enzymes and on high affinity receptors on the cell surface. To define the role of phosphorylated oligosaccharides in enzyme recognition, we studied the pinocytosis of [2-3H]mannose-labeled oligosaccharides purified from glycoproteins secreted by fibroblasts. Uptake of the oligosaccharides was inhibited 97% by 2 mM mannose-6-phosphate, 33% by 2 mM glucose 6-phosphate, and 5% or less by 2 mM alpha-methylmannoside, mannose, galactose, or L-fucose. The oligosaccharides were separated into neutral and five anionic species by chromatography on quaternary aminoethyl-Sephadex, characterized, and compared for susceptibility to pinocytosis. Treatment of the phosphorylated oligosaccharides with alkaline phosphatase before or after mild acid hydrolysis demonstrated that they contained one or two phosphates in phosphodiester linkage (covered) or phosphomonoester linkage (uncovered), or two phosphates, one in monoester linkage and one in diester linkage. Neutral oligosaccharides and those with one covered phosphate were not taken up by fibroblasts. Species with one uncovered phosphate or two covered phosphates showed low but detectable uptake. Oligosaccharides isolated as species with two uncovered phosphates, or those converted to this form by mild acid hydrolysis, were taken up 30-fold greater than the lower uptake forms during a 12-h incubation. Thus, oligosaccharides with two uncovered phosphates were far better ligands for the phosphomannosyl receptor than other oligosaccharides on acid hydrolases secreted by fibroblasts and initial rates of uptake of these oligosaccharides were comparable to those reported for several "high uptake" lysosomal enzymes.

Mannose 6-, fructose 1-, and fructose 6-phosphates inhibit human natural cell-mediated cytotoxicity

In vitro human natural cell-mediated cytotoxicity (NCMC) to K-562, Molt-4, and F-265 cells is inhibited in a dose-dependent manner by mannose 6-phosphate, fructose 1-phosphate and fructose 6-phosphate. This inhibition is not observed with mannose, glucose, fucose, glucose 6-phosphate, mannose 1-phosphate, galactose 1-phosphate, or galactose 6-phosphate. Preincubation of the effector cells, obtained from fresh whole blood, with mannose-6-phosphate, fructose-1-phosphate, or fructose-6-phosphate did not inhibit cytotoxicity, which indicated that these hexose phosphates are not nonspecifically toxic towards the effector lymphocytes. Mannose-6-phosphate and the stereochemically similar fructose-1-phosphate are more potent inhibitors than fructose-6-phosphate in terms of concentration required and time of onset of effect. Inhibition of cytotoxicity by mannose-6-phosphate varied with target cell type: F-265 is protected at much lower concentrations of mannose-6-phosphate (less than 1 mM) than is either Molt-4 or K-562. The inhibition of NCMC is also observed with the inhibitors of lysosomal function, NH4Cl, and chloroquine. The presence of a functional mannose-6-phosphate receptor on target cells was demonstrated: (i) Gelonin, a seed protein that inactivates the eukaryotic ribosome but is nontoxic to intact cells, was covalently linked to monophosphopentamannose, and this conjugate ws toxic to both K-562 and F-265 target cells, the latter being by far the more sensitive; and (ii) chloroquine, NH4Cl, and mannose-6-phosphate all inhibited the toxicity of gelonin-monophosphopentamannose. These results suggest either that a cytolytic lymphokine contains a hexose phosphate residue and may be taken up by target cells through the lysosomal/mannose 6-phosphate pathway or that such a residue is involved in target cell-effector cell recognition.

Insulin action in intact mouse diaphragm. I. Activation of glycogen synthase through stimulation of sugar transport and phosphorylation

The incubation of intact mouse diaphragms with insulin caused a dose and time dependent increase in the independent activity of glycogen synthase in tissue extracts. 2-deoxyglucose (2-10 mM) alone markedly stimulated the conversion of glycogen synthase to the independent activity under conditions in which tissue ATP concentrations were not affected. The incubation of diaphragms with both insulin and 2-deoxyglucose resulted in a greater than additive effect. Insulin stimulated the uptake of 2-deoxyglucose into mouse diaphragms, accumulating as 2-deoxyglucose-6-phosphate. The accumulation of 2-deoxyglucose-6-phosphate correlated well with the increase in the independent activity of glycogen synthase and with the activation of glycogen synthase phosphatase in tissue extracts. The uptake of 3-0 methyl glucose was also markedly stimulated by insulin, without affecting the activity of glycogen synthase. Both glucose-6-phosphate and 2-deoxyglucose-6-phosphate stimulated the activation of endogenous glycogen synthase phosphatase activity in muscle homogenates. We conclude that insulin, in addition to its effects in the absence of exogenous sugars, increases the independent activity of glycogen synthase through increased sugar transport resulting in increased concentrations of sugar-phosphates which promote the activity of glycogen synthase phosphatase.