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Articles

Measurement and Preservation of the in Vivo Activation of Ribulose 1,5-Bisphosphate Carboxylase in Leaf Extracts ¹

Department of Plant Sciences, University of Arizona, Tucson, Arizona 85721, Department of Biochemistry, University of Arizona, Tucson, Arizona 85721

Photosynthetic carbon fixation is regulated in the chloroplast by the amount of ribulose 1,5-bisphosphate carboxylase which is activated. The activated carboxylase was preserved in detached leaves (barley, maize, soybean, spinach, wheat) for 90 min when stored on ice. With leaf extracts stored at 2°C, the amount of activated enzyme, representing that originally in the leaf, as well as the fully activated enzyme, formed by incubation of leaf extracts with Mg²⁺ and bicarbonate, both slowly declined in activity. However, for each activity this decline was proportional such that the ratio (percent activation) appeared constant. No change was observed in activation of the enzyme during the brief time of leaf homogenization. Optimal conditions (Mg²⁺, incubation time) for measurement of leaf activation of ribulose bisphosphate carboxylase vary depending on the plant.

3-Phosphonoproprionate, a positive effector of the purified ribulose bisphosphate carboxylase and a metabolically inert analog of 2-phosphoglycolate, was used to examine what metabolic effectors might do to enzyme activation during leaf homogenization and preparation of the extract at 2° C. Activation under these conditions was not altered by 3-phosphonoproprionate. When 3-phosphonoproprionate was brushed on attached leaves or taken up by the transpiration stream of detached leaves, a considerable increase in activation of the carboxylase was measured.

¹ Supported by the Science and Education Administration of the United States Department of Agriculture under Grant 5901-0410-8-0114-0 from the Competitive Research Grants Office, by the Monsanto Agricultural Products Co., and by Grant PCM 77-26284 from the National Science Foundation. This is University of Arizona Agricultural Experiment Station Paper No. 3535