Hydrogen peroxide synthesis in isolated spinach chloroplast lamellae : an analysis of the mehler reaction in the presence of NADP reduction and ATP formation

Abstract

Light-dependent O(2) reduction concomitant with O(2) evolution, ATP formation, and NADP reduction were determined in isolated spinach (Spinacia oleracea L. var. America) chloroplast lamellae fortified with NADP and ferredoxin. These reactions were investigated in the presence or absence of catalase, providing a tool to estimate the reduction of O(2) to H(2)O(2) (Mehler reaction) concomitant with NADP reduction. In the presence of 250 micromolar O(2), O(2) photoreduction, simultaneous with NADP photoreduction, was dependent upon light intensity, ferredoxin, Mn(2+), NADP, and the extent of coupling of phosphorylation to electron flow.In the presence of an uncoupling concentration of NH(4) (+), saturating light intensity (>500 watts/square meter), saturating ferredoxin (10 micromolarity) rate-limiting to saturating NADP (0.2-0.9 millimolarity), and Mn(2+) (50-1000 micromolarity), the maxium rates of O(2) reduction were 13-25 micromoles/milligram chlorophyll per hour, while concomitant rates of O(2) evolution and NADP reduction were 69 to 96 and 134 to 192 micromoles/milligram chlorophyll per hour, respectively. Catalase did not affect the rate of NADPH or ATP formation but decreased the NADPH:O(2) ratios from 2.3-2.8 to 1.9-2.1 in the presence of rate-limiting as well as saturating concentrations of NADP.Photosynthetic electron flow at a rate of 31 micromoles O(2) evolved/milligram chlorophyll per hour was coupled to the synthesis of 91 micromoles ATP/milligram chlorophyll per hour, while the concomitant rate of O(2) reduction was 0.6 micromoles/milligram chlorophyll per hour and was calculated to be associated with an apparent ATP formation of only 2 micromoles/milligram chlorophyll per hour. Thus, electron flow from H(2)O to O(2) did not result in ATP formation significantly above that produced during NADP reduction.