title: Regulation of glucose-6-phosphate dehydrogenase in spinach chloroplasts by ribulose 1,5-diphosphate and NADPH/NADP+ ratios.
abstract: 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.
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology; D002736:Chloroplasts / Q000201:enzymology*; D004305:Dose-Response Relationship, Drug; D005636:Fructosephosphates / Q000494:pharmacology; D005942:Gluconates / Q000494:pharmacology; D005954:Glucosephosphate Dehydrogenase / Q000037:antagonists & inhibitors*; D007700:Kinetics; D009243:NAD / Q000494:pharmacology*; D009249:NADP / Q000494:pharmacology*; D010428:Pentosephosphates / Q000494:pharmacology*; D010710:Phosphates / Q000494:pharmacology; D010944:Plants / Q000201:enzymology; D012267:Ribosemonophosphates / Q000494:pharmacology; D012274:Ribulosephosphates / Q000494:pharmacology*
chemical_list: D005636:Fructosephosphates; D005942:Gluconates; D010428:Pentosephosphates; D010710:Phosphates; D012267:Ribosemonophosphates; D012274:Ribulosephosphates; D009243:NAD; D009249:NADP; D000255:Adenosine Triphosphate; D005954:Glucosephosphate Dehydrogenase