Control of Triosephosphate Dehydrogenase in Photosynthesis

Abstract

The kinetics of changes in chlorophyll content and in NAD and NADP dependent triosephosphate dehydrogenases were studied in Euglena gracilis and in Chlamydomonas reinhardi during bleaching and re-greening. In Euglena, the kinetics of changes in DADP-dependent enzyme activity closely paralleled changes in chlorophyll content under all conditions. Changes in DADP-linked activity were less marked in Chlamydomonas and the presence of a reduced C source in the growth medium was as effective in causing a decrease in activity as was bleaching during growth in the dark. The kinetics of changes in the activities of the 2 enzymes during regreening of chlorotic Euglena indicated no conversion of NAD-to NADP-dependent enzyme. The kinetics of changes in activity observed in Chlamydomonas were compatible with such a conversion. The NAD dependent triosephosphate dehydrogenase of the obligately phototrophic bacterium Chromatium was purified extensively. While enzyme from photoorgano-trophically and photolithotrophically grown cells were identical in some respects, the affinities of the 2 enzymes for triose substrates differed. It was found that these Km values and the reactive-SH contents of one form of the enzyme could be varied in vitro by mild oxidation and reduction to approximate those of the other form. It was suggested that Chromatium contains a single NAD-dependent enzyme, the properties of which vary with growth conditions, and may play a role in the regulation of photosynthesis and glycolytic C metabolism.