Glyceraldehyde 3-phosphate dehydrogenase of Scenedesmus obliquus: Promotion of NADPH-dependent activity and antagonisation by NAD

Abstract

The glyceraldehyde 3-phosphate dehydrogenase activity of extracts from heterotrophic Scenedesmus obliquus was linked predominantly to NADH. However, on DEAE-cellulose chromatography the enzyme was eluted by a gradient of phosphate in a form characterized by high NADPH-dependent glyceraldehyde 3-phosphate dehydrogenase activity. This interconversion of enzyme forms could be prevented by the presence of NAD during DEAE-cellulose chromatography. High concentrations of phosphate stimulated the NADPH-dependent activity of the purified enzyme at the expense of activity linked to NADH and these changes were associated with depolymerization of a hexadecamer to a tetramer. The effect of phosphate on the rates of increase in NADPH-dependent activity and of a decrease in activity linked to NADH was cooperative with a Hill coefficient of 3.2. The inversely related changes in coenzyme specificity were inhibited to the same extent by NAD and the response to this ligand was anticooperative. These findings imply a strictly inverse proportional relationship between the rates of change of NADH and NADPH-linked activity. In the presence of dithiothreitol, low concentrations of phosphate promoted NADPH-dependent activity by stabilising the unstable tetrameric form produced from the hexadecamer by the thiol. These phenomena are discussed in relation to a general mechanism for the in vivo promotion of NADPH-dependent glyceraldehyde 3-phosphate dehydrogenase activity.