Rates of synthesis and source of glycolate in intact chloroplasts

Abstract

Intact chloroplasts capable of high rates of CO2 assimilation completely oxidized 3-phosphoglycerate and dihydroxyacetone phosphate to glycolate when CO2 concentrations were low. Bicarbonate was converted first into products of the Calvin cycle and then into glycolate. Under high oxygen and at high pH values CO2 fixation and glycolate formation ceased before bicarbonate was exhausted. This is interpreted as the consequence of a depletion of ribulose diphosphate (RuDP) at the oxygen compensation point, where oxygen consumption by glycolate formation and oxygen evolution by phosphoglycerate reduction balance each other. Depletion of RuDP by glycolate formation is proposed to play a role in the Warburg effect. The maximum rate of glycolate synthesis observed with dihydroxyacetone phosphate as substrate was 35 μmol mg(-1) chlorophyll h(-1) at 20°C. This may not reflect the maximum capacity of chloroplasts for glycolate synthesis. Dithiothreitol and catalase, which prevent accumulation of oxygen radicals or H2O2 during carbon assimilation, increased glycolate formation. H2O2 was inhibitory. Other inhibitors of glycolate formation were glyceraldehyde and carbonylcyanide p-trifluoro-methoxphenylhydrazone. From the sensitivity of glycolate synthesis to uncoupling and the ATP requirement of RuDP formation it is concluded that glycolate originated from RuDP. Different induction periods of carbon fixation and glycolyte formation suggested that glycolate synthesis is not only regulated by the ratio of oxygen to CO2 but also by another factor.