Modulation of glucose-6-phosphate dehydrogenase by NADPH, NADP(+) and dithiothreitol at variable NADPH/NADP (+) ratios in an illuminated reconstituted Spinach (Spinacia oleracea L.) chloroplast system.

The light-dependent inhibition of glucose-6-phosphate dehydrogenase (EC 1.1.1.49), the key enzyme of the oxidative pentose phosphate cycle, can be gradually abolished in an illuminated reconstituted spinach chloroplast system by increasing the concentration of NADP(+). The inhibition caused by the effector which was produced by the system in the light is characterized as being of the competitive type with respect to NADP(+). The Ki value is 0.070 mM. The addition of NADPH to the illuminated reconstituted system results in a further decrease of the enzyme activity. Again, the type of inhibition is a competitive one, the Ki value being 0.072 mM. In the presence of dithiothreitol (DTT) alone the inhibition is incomplete, non-competitive, and the Ki value is as high as 0.2 mM. From the data presented it is concluded that light modulation of chloroplast glucose-6-phosphate dehydrogenase is mainly achieved by the NADPH/NADP(+) ratio. Disulfide reduction processes induced by a DTT-type compound seem to play a minor physiologic role during illumination.