Enzymic formation of glycolate in Chromatium. Role of superoxide radical in a transketolase-type mechanism.
Chromatophores prepared from Chromatium exhibit a light-dependent O2 uptake in the presence of reduced 2,6-dichlorophenolindophenol, the maximum rate observed being 10.8 micronmol (mg of Bchl)-1 h-1 (air-saturated condition). As it was found that the uptake of O2 was markedly inhibited by superoxide dismutase, it is suggested that molecular oxygen is subject to light-dependent monovalent reduction, resulting in the formation of the superoxide anion radical (O2-). By coupling baker's yeast transketolase with illuminated chromatophore preparations, it was demonstrated that [U-14C]-fructose 6-phosphate (6-P) is oxidatively split to produce glycolate, and that the reaction was markedly inhibited by superoxide dismutase and less strongly by catalase. A coupled system containing yeast transketolase and xanthine plus xanthine oxidase showed a similar oxidative formation of glycolate from [U-14C] fructose 6-P. It is thus suggested that photogenerated O2- serves as an oxidant in the transketolase-catalyzed formation of glycolate from the alpha, beta-dihydroxyethyl (C2) thiamine pyrophosphate complex, whereas H2O2 is not an efficient oxidant. The rate of glycolate formation in vitro utilizing O2- does not account for the in vivo rate of glycolate photosynthesis in Chromatium cells exposed to an O2 atmosphere (10 micronmol (mg of Bchl)-1 h-1). However, the enhancement of glycolate formation by the autoxidizable electron acceptor methyl viologen in Chromatium cells in O2, as well as the strong suppression by 1,2-dihydroxybenzene-3,5-disulfonic acid (Tiron), an O2- scavenger, suggest that O2- is involved in the light-dependent formation of glycolate in vivo.
D014013:1,2-Dihydroxybenzene-3,5-Disulfonic Acid Disodium Salt / Q000494:pharmacology; D020130:Bacterial Chromatophores / Q000201:enzymology; D002245:Carbon Dioxide / Q000378:metabolism; D002844:Chromatium / Q000201:enzymology* / Q000378:metabolism; D005609:Free Radicals; D006016:Glycolates / Q000378:metabolism*; D007700:Kinetics; D008027:Light; D010101:Oxygen Consumption; D010269:Paraquat / Q000494:pharmacology; D010788:Photosynthesis; D012441:Saccharomyces cerevisiae / Q000201:enzymology; D013045:Species Specificity; D013481:Superoxides; D014174:Transketolase / Q000378:metabolism*
D005609:Free Radicals; D006016:Glycolates; D013481:Superoxides; D002245:Carbon Dioxide; D014013:1,2-Dihydroxybenzene-3,5-Disulfonic Acid Disodium Salt; D014174:Transketolase; D010269:Paraquat