On the mechanism of the light-induced activation of the NADP-dependent glyceraldehyde phosphate dehydrogenase

Abstract

1. 1. The effects of some metabolites on the activity of the NADP- and NAD-dependent glyceraldehyde phosphate dehydrogenase from spinach chloroplasts have been studied. 2. 2. NADP + enhanced the activity of the NADP-dependent dehydrogenase while the NAD-dependent form was slightly inhibited. NADP + could not increase the activity of the NADP-dependent enzyme in addition to the activation by NADPH. The NAD-dependent dehydrogenase was not influenced by NAD + or NADH. 3. 3. ATP was similarly effective in activating the NADP-dependent dehydrogenase as was found earlier for NADPH. The activation as a function of ATP concentration is described by a sigmoid curve indicating a cooperative mechanism. A combination of ATP and NADPH activated the NADP-dependent enzyme no more than did ATP or NADPH alone. The NAD-dependent form was slightly activated by ATP. ADP, phosphoglyceric acid, and glyceraldehyde phosphate did not modify the activity. AMP inhibited the NADP-dependent enzyme. 4. 4. 50 mM MgCl 2 activated the NADP-dependent enzyme while higher concentrations had an inhibitory effect. The activation was about 1/10 that of NADPH. The activations by 50 mM MgCl 2 and NADPH were additive. 5. 5. The results give further evidence for a cooperative mechanism of the activation. The known activation of the NADP-dependent enzyme in vivo, being closely connected with the light reactions of photosynthesis, is explained by an allosteric activation with NADPH and/or ATP as effectors. This is discussed in respect to the regulation of the reductive pentose phosphate cycle.