Kinetic properties and tissular distribution of mammalian phosphomannomutase isozymes

Human tissues contain two types of phosphomannomutase, PMM1 and PMM2. Mutat ions in the PMM2 gene are responsible for the most common form of carbohydr ate-deficient glycoprotein syndrome [Matthijs, Schollen, Pardon, Veiga-da-C unha, Jaeken, Cassiman and Van Schaftingen (1997) Nat. Genet. 19, 88-92]. T he protein encoded by this gene has now been produced in Escherichia coli a nd purified to homogeneity, and its properties have been compared with thos e of recombinant human PMM1. PMM2 converts mannose 1-phosphate into mannose 6-phosphate about 20 times more rapidly than glucose 1-phosphate to glucos e 6-phosphate, whereas PMM1 displays identical V-max values with both subst rates. The K(a)values for both mannose 1,6-bisphosphate and glucose 1,6-bis phosphate are significantly lower in the case of PMM2 than in the case of P MM1. Like PMM1, PMM2 forms a phosphoenzyme with the chemical characteristic s of an acyl-phosphate. PMM1 and PMM2 hydrolyse different hexose bisphospha tes (glucose 1,6-bisphosphate, mannose 1,6-bisphosphate, fructose 1,6-bisph osphate) at maximal rates of approximate to 3.5 and 0.3% of their PMM activ ity, respectively. Fructose 1,6-bisphosphate does not activate PMM2 but cau ses a time-dependent stimulation of PMM1 due to the progressive formation o f mannose 1,6-bisphosphate from fructose 1,6-bisphosphate and mannose 1-pho sphate. Experiments with specific antibodies, kinetic studies and Northern blots indicated that PMM2 is the only detectable isozyme in most rat tissue s except brain and lung, where PMM1 accounts for about 66 and 13%, of the t otal activities, respectively.