PARTICIPATION OF MITOCHONDRIAL METABOLISM IN PHOTORESPIRATION - RECONSTITUTED SYSTEM OF PEROXISOMES AND MITOCHONDRIA FROM SPINACH LEAVES

In this study the interplay of mitochondria and peroxisomes in photore spiration was simulated in a reconstituted system of isolated mitochon dria and peroxisomes from spinach (Spinacia oleracea L.) leaves. The m itochondria oxidizing glycine produced serine, which was reduced in th e peroxisomes to glycerate. The required reducing equivalents were pro vided by the mitochondria via the malate-oxaloacetate (OAA) shuttle, i n which OAA was reduced in the mitochondrial matrix by NADH generated during glycine oxidation. The rate of peroxisomal glycerate formation, as compared with peroxisomal protein, resembled the corresponding rat e required during leaf photosynthesis under ambient conditions. When t he reconstituted system produced glycerate at this rate, the malate-to -OAA ratio was in equilibrium with a ratio of NADH/NAD of 8.8 x 10(-3) . This low ratio is in the same range as the ratio of NADH/NAD in the cytosol of mesophyll cells of intact illuminated spinach leaves, as we had estimated earlier. This result demonstrates that in the photoresp iratory cycle a transfer of redox equivalents from the mitochondria to peroxisomes, as postulated from separate experiments with isolated mi tochondria and peroxisomes, can indeed operate under conditions of the very low reductive state of the NADH/NAD system prevailing in the cyt osol of mesophyll cells in a leaf during photosynthesis.