Modulation of quinate: NAD(+) oxidoreductase activity through reversible phosphorylation in carrot cell suspensions

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.