Pathways of glycollate metabolism in the blue-green alga Anabaena cylindrica.

Abstract

1. Exogenous glycollate was assimilated by the blue-green alga Anabaena cylindrica. 2. About 50% of the C-1 carbon of 14C-1-glycollate (i.e.25% of the total carbon) was released as 14CO2 in the dark and also in the light in the presence of DCMU. Most of the 14CO2 released in the light in the absence of DCMU was refixed. 3. Assimilation was almost completely inhibited by α-hydroxy-2-pyridinemethane sulphonic acid, an inhibitor of enzymic glycollate oxidation. Cell extracts catalyzed the oxidation of glycollate to glyoxylate at rates sufficient to account for the in vivo assimilation. 4. Isonicotinylhydrazide, an inhibitor of the conversion of glycine to serine in higher plant/green algae glycollate metabolism, did not significantly affect glycollate metabolism in A. cylindrica. Short-term labelling experiments with 14C-1-glycollate in the light and dark did not show a significant metabolism of 14C via glycine and serine. However, the enzymes for the metabolism of glyoxylate via glycine, serine and hydroxypyruvate to glycerate were demonstrated in cell extracts, although the activity of the enzyme catalyzing the metabolism of serine to hydroxypyruvate was not sufficient to account for the in vivo rate of glycollate assimilation. 5. Cell extracts catalyzed the enzymic condensative decarboxylation of glyoxylate to tartronic semialdehyde and also the enzymic reduction of tartronic semialdehyde to glycerate. The activities in extracts were sufficient to account for the total in vivo photoassimilation of glycollate. The specific activity of malate synthase was insufficient to account for the total photometabolism of glycollate but exceeded the in vivo rate in the dark. 6. On the basis of the inhibitor and kinetic experiments and in terms of the enzymes detected, it appears that in the light glycollate is metabolized mainly via glyoxylate → tatronic semialdehyde → glycerate → 3-phosphoglycerate → (glycolytic pathway) → pyruvate → alanine plus tricarboxylic acid cycle and related compounds. The bulk of the CO2 released in the light is probably refixed via the Calvin cycle. In the dark, the glyoxylate, produced from exogenous glycollate, appears to be metabolized mainly by malate synthase directly to malate.