Regulation and properties of purified glucose-6-phosphate dehydrogenase from rat brain

Glucose-6-phosphate dehydrogenase from rat brain was purified 13,000 fold to a specific activity of 480 units/mg protein. The molecular weight was 121 kDa. The kinetics of brain glucose-6-phosphate dehydrogenase are compatible with a model involving two possible states of the enzyme with a low and high affinity for the substrate D-glucose-6-phosphate. NADP+ and ADP offered protection against p-chloromercuribenzoate inhibition. NADPH is a powerful competitive inhibitor with respect to NADP+. The apparent Ki for NADPH inhibition was lower than the Km for NADP+. ADP inhibited the enzyme competitively with respect to NADP+. ATP inhibited the enzyme non-competitively with respect to NADP+, whereas kinetics of mixed inhibition was observed with respect to substrate D-glucose-6-phosphate. The interplay between NADP+ and NADPH leading to enzyme activation or inhibition according to their relative or absolute concentrations as well as the control of enzyme activity by the adenine nucleotide system may contribute a refined mechanism for the regulation of glucose-6-phosphate dehydrogenase and therefore the pentose phosphate pathway in brain.