Lendzian K, Bassham J A
Biochim Biophys Acta. 1975 Aug 11;396(2):260-75. doi: 10.1016/0005-2728(75)90040-7.
The activity of glucose-6-phosphate dehydrogenase (EC 1.1.1.49) FROM SPINACH CHLOROPLASTS IS STRONGLY REGULATED BY THE RATIO OF NADPH/NADP+, with the extent of this regulation controlled by the concentration of ribulose 1,5-diphosphate. Other metabolites of the reductive pentose phosphate cycle are far less effective in mediating the regulation of the enzyme activity by NADPH/NADP+ ratio. With a ratio of NADPH/NADP+ of 2, and a concentration of ribulose 1,5-diphosphate of 0.6 mM, the activity of the enzyme is completely inhibited. This level of ribulose 1,5-diphosphate is well within the concentration range which has been reported for unicellular green algae photosynthesizing in vivo. Ratios of NADPH/NADP+ of 2.0 have been measured for isolated spinach chloroplasts in the light and under physiological conditions. Since ribulose 1,5-diphosphate is a metabolite unique to the reductive pentose phosphate cycle and inhibits glucose-6-phosphate dehydrogenase in the presence of NADPH/NADP+ ratios found in chloroplasts in the light, it is proposed that regulation of the oxidative pentose phosphate cycle is accomplished in vivo by the levels of ribulose 1,5-diphosphate, NADPH, and NADP+. It already has been shown that several key reactions of the reductive pentose phosphate cycle in chloroplasts are regulated by levels of NADPH/NADP+ or other electron-carrying cofactors, and at least one key-regulated step, the carboxylation reaction is strongly affected by 6-phosphogluconate, the metabolic unique to the oxidative pentose phosphate cycle. Thus there is an interesting inverse regulation system in chloroplasts, in which reduced/oxidized coenzymes provide a general regulatory mechanism. The reductive cycle is activated at high NADPH/NADP+ ratios where the oxidative cycle is inhibited, and ribulose 1,5-diphosphate and 6-phosphogluconate provide further control of the cycles, each regulating the cycle in which it is not a metabolite.
菠菜叶绿体中葡萄糖-6-磷酸脱氢酶（EC 1.1.1.49）的活性受到NADPH/NADP⁺比例的强烈调节，这种调节程度受1,5-二磷酸核酮糖浓度的控制。还原性磷酸戊糖途径的其他代谢物在通过NADPH/NADP⁺比例介导酶活性调节方面的作用要小得多。当NADPH/NADP⁺比例为2且1,5-二磷酸核酮糖浓度为0.6 mM时，该酶的活性被完全抑制。这种1,5-二磷酸核酮糖水平完全处于已报道的单细胞绿藻在体内光合作用时的浓度范围内。在光照和生理条件下，已测得分离的菠菜叶绿体中NADPH/NADP⁺比例为2.0。由于1,5-二磷酸核酮糖是还原性磷酸戊糖途径特有的代谢物，并且在光照下叶绿体中存在的NADPH/NADP⁺比例条件下会抑制葡萄糖-6-磷酸脱氢酶，因此有人提出，在体内，氧化性磷酸戊糖途径的调节是通过1,5-二磷酸核酮糖、NADPH和NADP⁺的水平来实现的。已经表明，叶绿体中还原性磷酸戊糖途径的几个关键反应受NADPH/NADP⁺或其他电子携带辅因子水平的调节，并且至少一个关键调节步骤，即羧化反应受到6-磷酸葡萄糖酸的强烈影响，6-磷酸葡萄糖酸是氧化性磷酸戊糖途径特有的代谢物。因此，叶绿体中存在一个有趣的反向调节系统，其中还原型/氧化型辅酶提供了一种通用的调节机制。在高NADPH/NADP⁺比例下还原性途径被激活，此时氧化性途径被抑制，而1,5-二磷酸核酮糖和6-磷酸葡萄糖酸对这两个途径提供进一步的控制，它们各自调节其不是代谢物的那个途径。
