The role of glycolic acid oxidase in the respiration of leaves.

It has also been shown that glycolic acid oxidase, together with the pyridine nucleotide-linked glyoxylic acid reductase, catalyzes the oxidation of reduced pyridine nucleotides by molecular oxygen in model systems (2). In a previous report it was shown that bisulfite addition compounds of aldehydes are effective and highly specific competitive inhibitors of glycolic acid oxidase (8). These compounds are salts of a-hydroxysulfonic acids of the general formula R-CHOH-SOzNa. The enzyme is also inhibited by sodium bisulfite itself because the glyoxylate produced interacts rapidly to form an cr-hydroxysulfonate. The inhibitory action of a-hydroxysulfonates has now provided a means for evaluating the extent of the participation of glycolic acid oxidase in the respiration of green leaves under various physiological conditions. If this enzyme functions to a significant degree within the living leaf, inhibition of it would be expected to result in the accumulation of glycolic acid at a rate comparable to that at which oxygen is normally consumed. It has now been found that, when excised green leaves are permitted to take up a-hydroxysulfonates by the technique which has been commonly used in biochemical studies in this laboratory (9), the concentration of glycolic acid rapidly increases to more than 10 times the normal level. This accumulation takes place only in sunlight under conditions such that one would expect glycolic acid oxidase to be inhibited. The results of these experiments are in concordance with the observations of Benson and Calvin (10) and Wilson and Calvin (11) that glycolic acid arises from an early photosynthetic intermediate, and also suggest that a large part of the respiration of green leaves in sunlight takes place through glycolic acid oxidase system.