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PubMed1976

Stabilization of glucosaminephosphate synthase from rat liver by hexose 6-phosphates. Properties and interconversion of two molecular forms.

Glucosaminephosphate synthase (glucosaminephosphate isomerase (glutamine-forming), EC 5.3.1.19) prepared from rat liver by extraction in the presence of glucose 6-phosphate (Glc-6-P) followed by precipitation with (NH4)2SO4 is susceptible to digestion by trypsin. This enzyme, designated form A, can be converted to tryptic-insusceptible form B upon incubation with Glc-6-P or fructose 6-phosphate (Fru-6-P) at 37 degrees C. The two forms also differ in the degree of activation by dithiothreitol, the degree of inhibition by methyl-glyoxal and the behavior on DEAE-Sephadex and Sephadex G-200 column chromatography. During purification with DEAE-Sephadex followed by hydroxyapatite, form B is converted to form A if Fru-6-P is absent and form A to form B if Fru-6-P is present. The two forms are therefore intercovertible. Under the conditions of purification, form B is more stable than form A, since the purity and yield of the final product are greater with form B than with form A. These findings suggest that the two forms of glucosaminephosphate synthase differ conformationally and that the equilibrium position depends on the concentration of Fru-6-P. Glc-6-P is effective only when it gives rise to Fru-6-P by mediation of glucose-phosphate isomerase.

Animals↗
PubMed1978

3-Hexulosephosphate synthase from Methylomonas aminofaciens 77a. Purification, properties and kinetics.

3-Hexulosephosphate synthase (D-arabino-3-hexulose 6-phosphate formaldehyde lyase) was purified from an obligate methylotroph, Methylomonas aminofaciens, to homogeneity as judged by polyacrylamide gel electrophoresis and analytical ultracentrifugation. The molecular weight was determined to be 45 000-47 000 by sedimentation velocity and gel filtration. The enzyme appears to be composed of two identical subunits (Mr = 23 000). A bivalent cation is required for the activation and stabilization of the enzyme. The enzyme is specific for formaldehyde and D-ribulose 5-phosphate. The optimum pH is 8.0 (isoelectric point, pH 5.1) and the optimum temperature is 45 degrees C. Initial velocity studies are consistent with a sequential mechanism. The Michaelis constants are 0.29 mM for formaldehyde and 0.059 mM for D-ribulose 5-phosphate.

Aldehyde-Lyases↗
PubMed1981

A novel enzyme, tagatose kinase, from Mycobacterium butyricum.

A novel enzyme catalyzing the phosphorylation of D-tagatose to D-tagatose 6-phosphate with ATP has been identified in extracts of dulcitol-grown Mycobacterium butyricum. The enzyme was purified 100-fold with 29% recovery. It required Mg2+, Mn2+ or Fe2+ and showed maximum activity at pH 7.5. The molecular weight as determined by Sephadex G-100 filtration amounted to 63 000. The apparent Michaelis constants for D-tagatose and ATP were 0.8 and 1.0 mM, respectively. The enzyme preparations were not very sensitive to SH group inhibitors and heavy metals but rapidly lost activity on heating above 50 degrees C.

Drug Stability↗
PubMed1988

13-cis-retinoic acid stimulates in vitro mannose 6-phosphate hydrolysis and inhibits retinol esterification and benzo[a]pyrene hydroxylation by rat-liver microsomes.

13-cis-Retinoic acid, a drug used at high doses in the treatment of recalcitrant acne, increased the permeability of rat-liver microsomal membranes to mannose 6-phosphate in vitro, as indicated by an increase in mannose-6-phosphatase activity. At the same concentrations, four other amphiphiles, including all-trans-retinoic acid, were much less effective. 13-cis-Retinoic acid also inhibited retinol esterification and benzo[a]pyrene hydroxylation in microsome preparations in vitro. Although the molecular mechanism and the reversibility of these effects have not yet been studied, the interaction of 13-cis-retinoic acid with cell membranes may well be involved in both its therapeutic and toxic manifestations.

Animals↗
PubMed1975

Microheterogeneity in yeast invertase.

Yeast external invertase (EC 3.2.1.25), a glycoenzyme consisting of equal parts by weight of protein and mannan, has been found to contain covalently bound phosphate. Three preparations (from two yeast strains) had mannose/PO4 ratios of 31-35, equivalent to 24-27 PO4 residues per mol of enzyme, while a fourth had only 7 PO4 residues per mol. From one of the high-PO4 enzymes, approx. 69% of the phosphorus was recovered as mannose 6-phosphate. No correlation was found between invertase activity and phosphorus content. The PO4 contents of the invertases exceeded those of the cell wall mannans from the respective yeasts. Thus, contamination of the invertases by cell wall phosphomannan is unlikely. Electrofocusing of the low-PO4 invertase yielded four components with pI values from 3.96 to 4.40, and yeast internal invertase (a mannan- and PO4-free, cytoplasmic isozyme) was isoelectric at approx. pH 4.5. The high-PO4 invertase was considerably more heterogeneous, with two major species of pI 3.65 and 3.32 and a highly acidic component of pI smaller than 2.7; however, the mannose/PO4 ratio of each species was approximately the same. PO4-gradient elution from hydroxyapatite resolved the high-PO4 invertase into five isozymes of increasing acidity and mannan content. Since the mannose/PO4 ratios of these invertase species are constant, the increase in the mannan/protein (and, therefore PO4/protein ratio is apparently responsible for the microheterogeneity of phosphoinvertase.23

Chromatography↗