| Popis: |
The relative abilities of a variety of phosphoanhydrides, phosphate esters, and mixed phosphate anhydrides (acyl phosphate compounds) to function (a) as substrates for hydrolase and phosphotransferase activities of rat liver microsomal d-glucose 6-phosphate phosphohydrolase (EC 3.1.3.9) and (b) as inhibitors of glucose 6-phosphate phosphohydrolase activity have been considered as a function of reaction pH. Competitive inhibitions of glucose 6-phosphate phosphohydrolase activity by adenosine 5'-triphosphate, inorganic pyrophosphate, mannose 6-phosphate, and carbamyl phosphate have been observed over the entire range of hydrogen ion concentrations studied, pH 5 to 8. In contrast, hydrolytic and synthetic activities with pyrophosphate and adenosine 5'-triphosphate have been observed only at neutral and acid pH, while glucose 6-phosphate, mannose 6-phosphate, carbamyl phosphate (and several other acyl phosphate anhydrides) are effective substrates at alkaline as well as lower pH values. Under conditions where both parameters may be determined, Km values for various compounds functioning as substrates for phosphohydrolase and phosphotransferase activities agree well with Ki values for these compounds acting as competitive inhibitors. Activity-pH profiles for both hydrolase and phosphotransferase (with glucose as acceptor molecule) activities are, with each substrate, essentially identical. It is concluded from these observations (a) that all these compounds may bind to a common enzymic active site at all pH values to produce binary enzyme-phosphoryl compound complexes, as indicated by inhibition data; (b) that assay pH does not discriminate between hydrolase and phosphotransferase activity of the enzyme, per se, since both types of activity are demonstrable at alkaline, as well as lower pH values with certain tested compounds; and (c) that pH discriminant effects (reactivity, or inhibition at high compared with lower pH values, or both) thus are dependent upon the nature of the phosphate substrate rather than the type of reaction involved. It is concluded, from these observations and a consideration of relevant earlier studies, that discrimination by assay pH with respect to the modes of reactivity of various phosphate compounds (ability to function as substrates and inhibitors as contrast with ability to function only as inhibitors) is manifest at an intermediate step in the over-all reaction mechanism, that involving the dissociation of binary enzyme-phosphoryl compound complexes to produce a phosphorylenzyme intermediate. The possible physiological implications of these observations are discussed briefly in terms of roles and regulation at pH 7 of activities of glucose 6-phosphatase, which may be under rigid metabolite control. |
