Association of glycolate oxidation with photosynthetic electron transport in plant and algal chloroplasts

Photosynthetic carbon metabolism is initiated by ribulose-bisphosphate carboxylase/oxygenase (Rubisco), which uses both CO2 and O-2 as substrates. One 2-phosphoglycolate (P-glycolate) molecule is produced for each Oz molecule fixed, P-glycolate has been considered to be metabolized exclusively via the oxidative photosynthetic carbon cycle, This paper reports an additional pathway for P-glycolate and glycolate metabolism in the chloroplasts. Light-dependent glycolate or P-glycolate oxidation by osmotically shocked chloroplasts from the algae Dunaliella or spinach leaves was measured by three electron accepters, methyl viologen (MV), potassium ferricyanide, or dichloroindophenol, Glycolate oxidation was assayed with 3-(3,4)-dichlcsrophenyl)-1,l-dimethylurea (DCMU) as oxygen uptake in the presence of MV at a rate of 9 mu mol per mg of chlorophyll per h. Washed thylakoids from spinach leaves oxidized glycolate at a rate of 22 mu mol per mg of chlorophyll per h, This light-dependent oxidation was inhibited completely by SHAM, an inhibitor of quinone oxidoreductase, and 75% by 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), which inhibits electron transfer from plastoquinone to the cytochrome b(6)f complex, SHAM stimulated severalfold glycolate excretion by algal cells, Dunaliella or Chlamydomonas, and by isolated Dunaliella chloroplasts. Glycolate and P-glycolate were oxidized about equally well to glyoxylate and phosphate, On the basis of results of inhibitor action, the possible site which accepts electrons from glycolate or P-glycolate is a quinone after the DCMU site but before the DBMIB site, This glycolate oxidation is a light-dependent, SHAM-sensitive, glycolate-quinone oxidoreductase system that is associated with photosynthetic electron transport in the chloroplasts.