Kirkman H N, Gaetani G F, Clemons E H
J Biol Chem. 1986 Mar 25;261(9):4039-45.
Glucose-6-phosphate dehydrogenase catalyzes the initial and rate-limiting step of the pathway that is the principal source of NADPH in many cells. Earlier studies of cells from several species indicated that the intracellular enzyme is under severe and unexplained restraint or inhibition. Moreover, the intracellular enzyme of human erythrocytes exhibits sigmoid kinetics, whereas the purified enzyme exhibits only classical kinetics. We here report that most of the NADP in the human erythrocyte is bound by soluble proteins. In addition, the fraction of unbound NADP that is in the oxidized form, [NADP+]/[NADP], varies in a sigmoid manner relative to the fraction of bound NADP that is in the oxidized form. These features of intracellular binding of NADP: 1) account for the previously unexplained inhibition and sigmoid kinetics of glucose-6-phosphate dehydrogenase within human erythrocytes and 2) represent a system in which activity of a rate-limiting enzyme is largely determined by the binding and release of substrate and product by intracellular proteins other than the enzyme itself.
葡萄糖-6-磷酸脱氢酶催化该途径的起始步骤和限速步骤，此途径是许多细胞中NADPH的主要来源。早期对多个物种细胞的研究表明，细胞内的这种酶受到严重且无法解释的限制或抑制。此外，人类红细胞中的细胞内酶表现出S形动力学，而纯化后的酶仅表现出经典动力学。我们在此报告，人类红细胞中的大多数NADP与可溶性蛋白质结合。此外，未结合的以氧化形式存在的NADP部分，即[NADP+]/[NADP]，相对于以氧化形式存在的结合NADP部分呈S形变化。NADP细胞内结合的这些特征：1）解释了人类红细胞内葡萄糖-6-磷酸脱氢酶先前无法解释的抑制作用和S形动力学，2）代表了一种系统，其中限速酶的活性在很大程度上由该酶本身以外的细胞内蛋白质对底物和产物的结合与释放所决定。
