Interactions between magnesium ions, pH, glucose-6-phosphate, and NADPH/NADP(+) ratios in the modulation of chloroplast glucose-6-phosphate dehydrogenase in vitro.

Glucose-6-phosphate dehydrogenase (EC 1.1.1.49) from spinach chloroplasts is strongly affected by interactions between Mg(2+), proton, and substrate concentrations. Mg(2+) activates the enzyme to different degrees; however, it is not essential for enzyme activity. The Mg(2+)-dependent activation follows a maximum curve, magnitude and position of the maximum being dependent on pH and NADPH/NADP(+) ratios. At a ratio of zero and pH 7.2, maximum activity is observed at 10 mM Mg(2+). Increasing the NADPH/NADP(+) ratio up to 1.7 (a ratio measured in the stroma during a light period), maximum activity is shifted to much lower Mg(2+) concentrations. At pH 8.2 (corresponding to the pH of the stroma in the light) and at a high NADPH/NADP(+) ratio, enzyme activity is not affected by the Mg(2+) ion. The results are discussed in relation to dark-light-dark regulation of the oxidative pentose phosphate cycle in spinach chloroplasts.