The relationship of glycolic acid to respiration and photosynthesis in tobacco leaves.

Studies with glycolic acid oxidase and glyoxylic acid reductase isolated from green leaves have led to the suggestion that these enzymes play a part in the oxygen uptake of leaf tissue (l-3). Glycolic acid oxidase is a flavoprotein which is capable of directly oxidizing glycolic acid, a substance rapidly produced during photosynthesis (4, 5). As has been demonstrated in model systems, the oxidase can also couple with glyoxylic acid reductase to oxidize reduced pyridine nucleotides (2). I f glycolic acid oxidase participates to a significant extent in respiration, specific inhibition of the oxidase might be expected to cause an accumulation of the substrate at a rate comparable to the normal rate of oxygen uptake. Such experiments provide a suitable means for evaluating the role of this enzyme in respiration. A series of effective and highly specific competitive inhibitors of glycolic acid oxidase has been described (6, 7). These compounds are all aldehyde bisulfite addition compounds of the general structure R-CHOH-SOaNa, which are cu-hydroxysulfonates and thus are analogues of glycolic acid, CHtOH-COOH. When an excised mature leaf from any of the plant species examined is placed in sunlight with its base in a solution of a suitable cu-hydroxysulfonate, the concentration of glycolic acid in the tissue increases at a rate which is similar on a molar basis to the normal rate of oxygen uptake of the tissue in darkness (7). On the assumption that the glycolic acid which accumulates in the presence of a-hydroxysulfonates would normally have been metabolized by glycolic acid oxidase, it has been concluded that a substantial part of the respiration of mature leaves in sunlight takes place by means of this oxidase. This conclusion is supported by a number of observations that appear in the literature, and which have been discussed (7). Experiments have now been carried out in an attempt to elucidate further the action of the inhibitors of glycolic acid oxidase on the metabolism of the intact tobacco leaf. An cr-hydroxysulfonate has been found which is more effective than other inhibitors that were previously tested. With this compound, cu-hydroxy-2-pyridinemcthanesulfonic acid, the effect of the physiological age of the leaf, as well as of other factors which influence the inhibition of glycolic acid oxidase by a-hydroxysulfonates in tiuo, has also been investigated. In experiments with C1402, it has been found that 50% of the carbon fixed photosynthetically by a tobacco leaf in the presence of an cY-hydroxysulfonate is located in glycolic acid, thus providing further support for the view that glycolic acid oxidase participates in the respiration of leaves in sunlight (7). EXPERIMENTAL