Oxidative Phosphorylation and Glycolate Oxidation by Particles from Spinach Leaves.

It has recently been reported (2) that oxidative phosphorylation occurs when organic acids of the citric acid cycle are supplied as substrates to particles isolated from the cotyledons of etiolated lupine seedlings. The ratios of the number of moles of orthophosphate consumed to the number of atoms of oxygen taken up (P/0 ratios) were found to be as high as those previously reported only for animal tissues (7). Furthermore, Ohmura (13) has shown that particles obtained from green spinach leaves are also capable of carrying out oxidative phosphorylation although he did not determine P/0 ratios. This observation suggested that spinach leaves might provide an active particulate system for further study. Accordingly, such particles have been isolated by a method modified from that of Ohmura and have been found to bring about phosphorylations with high P/O ratios when substrates such as pyruvate, citrate, LS-iSocitrate, a-ketoglutarate, succinate, fumarate, L-malate, and reduced diphosphopyridine nucleotide (DPNH) are used. Glycolic acid oxidase, a riboflavin phosphatelinked enzyme which catalyzes the oxidation of glycolic acid to glyoxylic acid, was isolated some years ago in a highly purified form from extracts of spinach leaves (21) and has recently been crystallized (5). Experiments performed in this laboratory suggest that glycolic acid oxidase plays a role in the respiration of leaves in sunlight (19, 20). Some glycolic acid oxidase activity has now been found to be associated with the particles obtained from spinach leaves, and a search was accordingly made for evidence of oxidative phosphorylation with glycolate as the substrate. No significant activity was found. The effect of glycolate on the oxidation and phosphorylation of other substrates by these particles has also been investigated.