The molecular identity of mammalian phosphopentomutase has not yet been established unequivocally. That of glucose-1,6-bisphosphate synthase, the enzyme that synthesizes a cofactor for phosphomutases and putative regulator of glycolysis, is completely unknown. In the present work, we have purified phosphopentomutase from human erythrocytes and found it to copurify with a 68-kDa polypeptide that was identified by mass spectrometry as phosphoglucomutase 2 (PGM2), a protein of the ␣-D-phosphohexomutase family and sharing about 20% identity with mammalian phosphoglucomutase 1. Data base searches indicated that vertebrate genomes contained, in addition to PGM2, a homologue (PGM2L1, for PGM2-like 1) sharing about 60% sequence identity with this protein. Both PGM2 and PGM2L1 were overexpressed in Escherichia coli, purified, and their properties were studied. Using catalytic efficiency as a criterion, PGM2 acted more than 10-fold better as a phosphopentomutase (both on deoxyribose 1-phosphate and on ribose 1-phosphate) than as a phosphoglucomutase. PGM2L1 showed only low (<5%) phosphopentomutase and phosphoglucomutase activities compared with PGM2, but was about 5-20-fold better than the latter enzyme in catalyzing the 1,3-bisphosphoglycerate-dependent synthesis of glucose 1,6-bisphosphate and other aldosebisphosphates. Furthermore, quantitative real-time PCR analysis indicated that PGM2L1 was mainly expressed in brain where glucose-1,6-bisphosphate synthase activity was previously shown to be particularly high. We conclude that mammalian phosphopentomutase and glucose-1,6-bisphosphate synthase correspond to two closely related proteins, PGM2 and PGM2L1, encoded by two genes that separated early in vertebrate evolution.Phosphopentomutase catalyzes the conversion of the nucleoside breakdown products ribose 1-phosphate and deoxyribose 1-phosphate to the corresponding 5-phosphopentoses. Most bacterial phosphopentomutases characterized so far belong to the same protein family as alkaline phosphatases, sulfatases, and cofactor-independent bisphosphoglycerate mutases (1), though the enzyme from Thermococcus kodakaraensis belongs to the same protein family as mammalian phosphoglucomutase 1 (PGM1), 5 the enzyme that catalyzes the interconversion of glucose 1-phosphate and glucose 6-phosphate (2).The molecular identity of the mammalian phosphopentomutase is still not ascertained. Biochemical characterization of phosphoglucomutase (PGM) isozymes indicated that one of them, designated PGM2 in man (PGM1 in mouse) was more active as a phosphopentomutase than as a phosphoglucomutase, whereas mammalian PGM1 (equivalent to PGM2 in mouse) has a phosphopentomutase activity representing only about 0.2% of its phosphoglucomutase activity (3). Phosphopentomutase has been purified from rat liver to near homogeneity and shown to coelute with a polypeptide of 32.5 kDa (4), the sequence of which was not determined. Analysis of cell hybrids indicated that the PGM2 locus is on human chromosome 4p14-q12 (5). A putative protein sharing Ϸ20% identit...

Glucose 1,6-bisphosphate (Glc-1,6-P 2 ) concentration in brain is much higher than what is required for the functioning of phosphoglucomutase, suggesting that this compound has a role other than as a cofactor of phosphomutases. In cell-free systems, Glc-1,6-P 2 is formed from 1,3-bisphosphoglycerate and Glc-6-P by two related enzymes: PGM2L1 (phosphoglucomutase 2-like 1) and, to a lesser extent, PGM2 (phosphoglucomutase 2). It is hydrolyzed by the IMP-stimulated brain Glc-1,6-bisphosphatase of still unknown identity. Our aim was to test whether Glc-1,6-bisphosphatase corresponds to the phosphomannomutase PMM1, an enzyme of mysterious physiological function sharing several properties with Glc-1,6-bisphosphatase. We show that IMP, but not other nucleotides, stimulated by >100-fold (K a ≈ 20 M) the intrinsic Glc-1,6-bisphosphatase activity of recombinant PMM1 while inhibiting its phosphoglucomutase activity. No such effects were observed with PMM2, an enzyme paralogous to PMM1 that physiologically acts as a phosphomannomutase in mammals. Transfection of HEK293T cells with PGM2L1, but not the related enzyme PGM2, caused an ≈20-fold increase in the concentration of Glc-1,6-P 2 . Transfection with PMM1 caused a profound decrease (>5-fold) in Glc-1,6-P 2 in cells that were or were not cotransfected with PGM2L1. Furthermore, the concentration of Glc-1,6-P 2 in wildtype mouse brain decreased with time after ischemia, whereas it did not change in PMM1-deficient mouse brain. Taken together, these data show that PMM1 corresponds to the IMP-stimulated Glc-1,6-bisphosphatase and that this enzyme is responsible for the degradation of Glc-1,6-P 2 in brain. In addition, the role of PGM2L1 as the enzyme responsible for the synthesis of the elevated concentrations of Glc-1,6-P 2 in brain is established.Glucose 1,6-bisphosphate (Glc-1,6-P 2 ), 2 a well known cofactor for phosphoglucomutase and other sugar phosphomutases (1), is ubiquitously present in tissues. Its concentration is particularly elevated in brain, where it reaches values of Ͼ100 M (2), i.e. Ͼ1000-fold higher than the concentrations required to stimulate phosphoglucomutase. Glc-1,6-P 2 has been proposed to be an effector for several enzymes. Phosphofructokinase (3, 4) and liver pyruvate kinase (5) are both stimulated by this compound, whereas low K m hexokinases (6 -8), 6-phosphogluconate dehydrogenase (9), and fructose-1,6-bisphosphatase (10) are inhibited. These effects have been demonstrated in vitro, but under conditions that are not necessarily physiologically relevant. In addition, the occurrence of this regulation in intact cells has not been demonstrated.Glc-1,6-P 2 is synthesized from 1,3-bisphosphoglycerate and glucose 1-phosphate or glucose 6-phosphate by Glc-1,6-P 2 synthase, an enzyme particularly abundant in brain (11) and recently identified as PGM2L1 (phosphoglucomutase 2-like 1) (12). In vitro, the related enzyme PGM2 (phosphoglucomutase 2), which shares ϳ60% identity with PGM2L1 and acts mainly as a phosphopentomutase, also catalyzes the synthesis of...

The synthesis of N-acetylneuraminate (Neu5Ac), the main form of sialic acid, proceeds in vertebrates through the condensation of N-acetylmannosamine 6-phosphate and phosphoenolpyruvate to Neu5Ac-9-phosphate, followed by the dephosphorylation of the latter by a specific phosphatase. The sequence encoding Neu5Ac-9-phosphate phosphatase (Neu5Ac-9-Pase; E.C. 3.1.3.29) has not been determined until now. In this work, we have purified Neu5Ac-9-Pase more than 1000-fold from rat liver. Its dependency on Mg2+ and the fact that it was inhibited by vanadate and Ca2+ suggested that it belonged to the haloacid dehalogenase family of phosphatases. Trypsin digestion and mass spectrometry analysis of a polypeptide of about 30 kDa that co-eluted with the activity in the last purification step indicated the presence of a protein designated "haloacid dehalogenase-like hydrolase domain containing 4." The human ortholog of this protein is encoded by a 2-exon gene present on chromosome 20p11. The human protein was overexpressed in Escherichia coli as a fusion protein with a polyHis tag and purified to homogeneity. The recombinant enzyme displayed a >230-fold higher catalytic efficiency on Neu5Ac-9-phosphate than on its second best substrate. Its properties were similar to those of the enzyme purified from rat liver. Neu5Ac inhibited the enzymatic activity by 50% at 15 mM, indicating that no significant inhibition is exerted at physiological concentrations of Neu5Ac. The identification of the gene encoding Neu5Ac-9-Pase will facilitate studies aimed at testing its potential implication in unexplained forms of glycosylation deficiency.

Fructosamine-3-kinase (FN3K) is a recently described protein-repair enzyme responsible for the removal of fructosamines, which are the products of a spontaneous reaction of glucose with amines. We show here that, compared with glucose, glucose 6-phosphate (Glu-6-P) reacted 3-6-fold more rapidly with proteins and 8-fold more rapidly with N-alpha-t-Boc-lysine, being therefore a more significant intracellular glycating agent than glucose in skeletal muscle and heart. Fructosamine 6-phosphates, which result from the reaction of amines with Glu-6-P, were not substrates for FN3K. However, a phosphatase that dephosphorylates protein-bound fructosamine 6-phosphates was found to be present in rat tissues. This enzyme was purified to near homogeneity from skeletal muscle and was identified as magnesium-dependent phosphatase-1 (MDP-1), an enzyme of the haloacid dehalogenase family with a putative protein-tyrosine phosphatase function. Human recombinant MDP-1 acted on protein-bound fructosamine 6-phosphates with a catalytic efficiency >10-fold higher than those observed with its next best substrates (arabinose 5-phosphate and free fructoselysine 6-phosphate) and >100-fold higher than with protein-phosphotyrosine. It had no detectable activity on fructosamine 3-phosphates. MDP-1 dephosphorylated up to approximately 75% of the fructosamine 6-phosphates that are present on lysozyme after incubation of this protein with Glu-6-P. Furthermore, lysozyme glycated with Glu-6-P was converted by MDP-1 to a substrate for FN3K. We conclude that MDP-1 may act physiologically in conjunction with FN3K to free proteins from the glycation products derived from Glu-6-P.