ATP-Dependent Proteolytic Activity from Spinach Leaves

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Abstract
Spinach (Spinacia oleracea CV Bloomsdale Long Standing) leaf cytoplasmic starch phosphorylase and rabbit muscle phosphorylase a were inactivated by incubation with partially purified leaf extract in the presence of ATP and Mg2+. The inactivating factor(s) were heat stable and susceptible to protease attack. Phosphorylase inactivation was prevented by incubation in the presence of p-aminobenzamidine and phenylboronic acid, or prolonged treatment with phenylmethylsulfonyl fluoride or leupeptin for the ATP-stimulated inhibitory activity. Mg2+ -dependent inactivation was prevented by incubation with leupeptin, phenylmethylsulfonyl fluoride, p-aminobenzamidine, or 5′-adenylate. ATP-mediated inactivation of phosphorylase was stimulated by Mg2+ with a reduction in the apparent Km for ATP. Casein-degrading activities with the same properties of ATP and/or Mg2+ stimulation, heat stability, and susceptibility to proteinase inhibitors were detected suggesting that phorphorylase inactivation was due to proteolysis. The activity was greatest at about the time of flowering and also appeared to depend on the light regime.