Phosphate requirement for the light activation of ribulose- 1,5-biphosphate carboxylase in intact spinach chloroplasts

Although the fxation of CO* during photosynthesis is a dark reaction, it is generally accepted that some of the enzymes of the reductive COZ fixation cycle are indirectly regulated by light. Fructose and sedoheptulose bisphosphatase and ribulose bisphosphate carboxylase have been identified as major regulatory sites in the reductive CO2 fixation cycle [l-3]. Ribulosebisphosphate carboxylase (EC 4.1 .1.39) converted from an inactive into an active form by reaction with COZ and Mg2+, this activation being enhanced by a pH-shift from pH 7.0-8.5 [4-61. The activation is a relatively slow process with a half-time in the range of l-3 min. The activated enzyme is stable enough to be assayed for 90 s without change of activity [4] . A number of effecters such as 3-phosphoglycerate j 6-phosphogluconate, fructose-l ,6bisphosphate and NADPH have been found to increase the activation of the enzyme especially at pH values below 8.0 [7-91. Using a rapid procedure for the lysis of chloroplasts and assay of the carboxylase activity, the factors