Regulation of C₄ Photosynthesis A Role for Pyruvate in Regulating Pyruvate,P_i Dikinase Activity in vivo

Abstract

Results are presented in support of the view that pyruvate levels are critical in the light-dark regulation of pyruvate,P_I dikinase in vivo. In an in vitro system containing purified Zea mays pyruvate,P_I dikinase, increasing pyruvate reduced both the initial rate and the final extent of ADP-mediated inactivation of the enzyme. Pyruvate also reversed the inhibitory effect of ADP on the initial rate of P_I-mediated activation of pyruvate,P*i dikinase. With intact Zea mays leaves, inactivation of pyruvate,P_I dikinase in darkened leaves was prevented by anaerobic conditions or reduced temperature; both treatments resulted in elevated steady-state levels of pyruvate. Added pyruvate was shown to substantially increase the level of active pyruvate,P_I dikinase in chloroplasts incubated with P_I in the dark but not in the light. The likely physiological significance of these results is discussed.