INTERACTION OF PLASTOCYANIN AND P700 IN PSI REACTION CENTER PARTICLES FROM C, REINHARDI AND SPINACH

Highly resolved Photosystem I (PSI) reaction center particles, solubilized from spinach and C. reinhardi chloroplasts with Triton X-100, catalyze very poorly the reduction of P700 by ascorbate at mildly acidic to alkaline pHs (pH 5-8). (Photooxidation of P700 means conversion of P700 to P700+; dark reduction of P700 is the reverse reaction.) The interaction between plastocyanin (PC) and P700 is assessed by measuring the stimulation of the rate of P700 reduction upon addition of PC at submicromolar concentrations. At a ratio of 2.6 PC per P700, about 20- to 25-fold stimulation of the rate of P700 reduction was observed in the presence of MgCl2 or CaCl2. Efficient interaction between PC and P700 occurred at near neutral pH (optimal pH = 6.6-6.8) and exhibited a preferential requirement for divalent metal cations. Monovalent cations are less effective in promoting transfer of electrons from PC to P700. At alkaline pHs (greater than 8.5) an accelerated rate of the PC-independent reduction of P700 was observed. Further, the PC-independent reaction at pH 9.4 showed a strong requirement for divalent cations. In contrast, monovalent cations are only partially effective in promoting the PC-independent reduction of P700 by ascorbate. © 1979.