GLYCOLLATE FORMATION DURING PHOTORESPIRATION OF ACETATE BY CHLORELLA

WhenChlorella pyrenoidosa photoassimilates 3H-14C-acetate the glycollic acid formed shows a high 3H/14C ratio, the only other compounds showing similar ratios being glycerate and serine. The 3H/14C ratio of glycollate was unaffected by the TCA cycle inhibitors MFA, diethylmalonate and arsenite showing that 3H in glycollate does not result from the oxidation of acetate via the TCA cycle, the resulting NADP3H2 or NAD3H2 being used for the reduction of the glycollate precursor. Although DCMU decreased the 3H/14C ratio, complete inhibition of glycollate labelling was not observed with 10-6 M DCMU, at which concentration complete inhibition of the Hill reaction is achieved. Although the 3H/14C ratio was unaltered, total dpm of both 14C and 3H in glycollate were increased by INH. The 3H/14C ratios of glycerate and serine were decreased by INH, as were the total dpm of 3H and 14C incorporated into these compounds. Thus, INH inhibits the further metabolism of glycollate to glycerate and serine. The effect of INH on incorporation of 14C-I-acetate into various cell fractions was investigated. The incorporation of 14C into polysaccharide and lipid was decreased, while the incorporation of 14C into the water-soluble fraction of cells and therelease of 14CO2 were little affected. Although glycollic acid was an early product of acetate photoassimilation in Chlorella pyrenoidosa, glycollate excretion does not take place under a wide range of environmental conditions shown to favour glycollate excretion by other algae. However, small amounts of labelled glycollate were detected in the supernatant from the cells during the photoassimilation of 3H-14C-acetate, but this glycollate did not show the high 3H/14C ratio of glycollate present within the cell. The failure of Chlorella pyrenoidosa to excrete appreciable amounts of glycollate when photoassimilating acetate or carbon dioxide was considered to result from the presence of glycollate oxidase (EC 1.1.3.1) which allowed the further metabolism of glycollate. Besides glycollate oxidase, glyoxylate reductase was also demonstrated in Chlorella pyrenoidosa so that glycollate could function in hydrogen transfer during the photoassimilation of acetate. © 1962 Oxford University Press.