RESUMO
Quinate:NAD(+) 3-oxidoreductase (EC 1.1.1.24) from carrot cell suspension cultures has previously been shown to be activated by phosphorylation and inactivated by dephosphorylation. Here it is shown that the reactivation of the inactivated quinate:NAD(+) oxidoreductase is an enzyme-mediated process that requires ATP and protein kinase activity. The reactivation is completely inhibited by EGTA and can be restored by the addition of Ca(2+). Cyclic AMP at concentrations up to 5 muM did not have any effect on the reactivation either with or without EGTA in the medium. Calmodulin-depleted fractions containing quinate:NAD(+) oxidoreductase were obtained by passage of the crude extracts through an affinity column of 2-chloro-10-(3-aminopropyl)phenothiazine coupled to Sepharose 4B. The enzyme in this calmodulin-deficient fraction could be inactivated but not reactivated even in the presence of ATP and Ca(2+). However, addition of bovine brain calmodulin completely restored the activity of the enzyme. Half-maximal activation occurred at 130 nM calmodulin. We conclude from these data that the quinate:NAD(+) oxidoreductase is activated by a Ca(2+) - and calmodulin-dependent plant protein kinase.
RESUMO
Quinate: NAD(+) oxidoreductase (EC 1.1.1.24) from carrot cells was deactivated by incubating partially purified extract with MgCl2 at 30°C. The deactivation process was prevented by adding fluoride, a phosphatase inhibitor. Once inactivated, the enzyme could recover its initial activity on incubation with ATP-Mg either in combination with or not in combination with an exogenous protein kinase. (32)PO4 was incorporated into the purified enzyme when the cell cultures were supplemented with labeled phosphate in vivo. Moreover, (32)P from [γ-(32)P]ATP was incorporated into the reductase when the enzyme was reactivated in the presence of protein kinase. From these results, it is concluded that the activation-inactivation process is due to phosphorylation-dephosphorylation of quinate:NAD(+) oxidoreductase.