Inhibition of Ribulose 1,5-Diphosphate Carboxylase by 6-Phosphogluconate

6-Phosphogluconate is a much more effective inhibitor of the photosynthetic carboxylation enzyme, ribulose-1, 5-diphosphate carboxylase, than other sugar phosphates and sugar acids of the reductive and oxidative pentose phosphate cycles. The inhibition appears to be noncompetitive with ribulose 1,5-diphosphate. Since 6-phosphogluconate is unique to the oxidative cycle and inhibits at concentrations comparable to those found in vivo, it is proposed that its inhibition of the carboxylase may be a regulatory factor. If so, it would operate during darkness as a different control factor from those factors postulated to activate the carboxylase during photosynthesis.