Lichtinduzierte, reversible Aktivitätssteigerung der NADP‐abhängigen Glycerinaldehyd‐3‐phosphat‐Dehydrogenase in Chloroplasten

1 Irradiation of fully developed green leaves as well as irradiation of isolated chloroplasts causes a rapid, reversible increase in the activity of NADP‐linked phosphoglyceraldehyde dehydrogenase. This increase is closely connected with the functioning of the non‐cyclic electron flow of photosynthesis. It is accompanied by a decrease in the activity of the NAD‐linked enzyme. 2 The experiments presented here show that it is not the kinase but the dehydrogenase which is the rate limiting factor in the over‐all reaction measured up to date. 3 The activation of the “dark‐enzyme” (= basic enzyme) resulting in the “light‐enzyme” (= activated enzyme) is not due to an unspecific reduction of S‐S‐groups in the reactive center. Dithiothreitol only restores the activity lost during the isolation procedure of the chloroplasts. 4 NADPH rather than dithiothreitol causes a specific activation of the basic enzyme. However the basic enzyme must be present in the completely reduced form to undergo maximal activation. Incubation with dithiothreitol and NADPH thus results in an activation rate as high as that caused by light in vivo.5 Activation of the NADP‐linked dehydrogenase could also be attained by NADPH after its partial purification by ammonium sulfate fractionation. 6 The activation rate by incubation with different concentrations of NADPH shows a sigmoidal type of curve. 7 There are characteristic differences in sensitivity between the activity of the basic enzyme and its ability to be activated at different temperatures between 52°–60°. 8 The results indicate that the action of NADPH may be an allosteric one, causing a transition from a NAD‐ (or possibly a NAD + NADP)‐dependent to a NADP‐dependent form of the enzyme. A diagram for a feed‐forward activation of the NADP‐linked dehydrogenase which possibly takes part in regulation of CO 2 ‐fixation is presented.