[The influence of starch on the activity of pyrophosphatase from isolated spinach chloroplasts]

The activity of the inorganic pyrophosphatase from isolated spinach (Spinacia oleracea L.) chloroplasts is strongly dependent upon the addition of magnesium ions. Since the complex of the bivalent ion with inorganic pyrophosphate is the real substrate, a definite Mg(2+)/Na4P2O7-ratio is required for maximum activity. When the activity was measured in particle-free extracts from chloroplasts, this ratio was shown to be approximately 3. However, an increase up to 10 was observed in the presence of thylakoid membranes. Furthermore, the kinetics in the presence of broken chloroplasts becomes sigmoidal.The altered kinetics have been shown to be due to starch located in the thylakoids. The inhibitory effect is caused by amylose alone but not by amylopectin., Detailed kinetic analysis of the inhibition showed no influence of amylose on the Hill-coefficient. Since ethylenediaminetetracetic acid was shown to have similar effects as amylose, starch might regulate the pyrophosphatase activity by binding Mg(2+)-ions.