Phosphopyruvate carboxylase activity and carbon dioxide fixation via C4 acids over the division cycle in synchronized Euglena cultures

Summary Phosphoryruvate carboxylase activity was determined in division synchronized Euglena gracilis strain Z cultures. The profile of enzyme activity was essentially that of a peak enzyme; activity increased over the light phase of the cycle, doubling by early dark phase followed by a substantial decline in activity near the end of the dark phase. Dark carbon dioxide fixation did not parallel changes in phosphoryruvate carboxylase activity. The rate of carbon dioxide fixation increased fourfold over the light phase but decreased in the dark phase until it was only double the rate at the beginning of the light phase.Although the specific activity of phosphopyruvate carboxylase was greater than that of ribulose 1–5 diphosphate carboxylase in Euglena cell extracts at all stages over the division cycle C₄ acids were not an early product of carbon dioxide fixation in the light, neither did they ever account for more than a small proportion of the total ¹⁴C present in the soluble fraction of the cells. Phosphopyruvate carboxylase was shown by the non-aqueous localization technique to be present in the cytoplasm in Euglena, and it is concluded that the main function of this enzyme in algal cells is to provide an anaplerotic sequence to the tricarboxylic acid cycle.