A novel form of 6-phosphofructo-2-kinase was partially purified from spinach (Spinacia oleracea L.) leaves. As isolated, the new enzyme form possessed little or no enzymatic activity. However, pretreatment with 2 mM Mg-ATP dramatically increased the activities of 6-phosphofructo-2-kinase and fructose-2,6-bisphosphatase. Activation only occurred in the presence of magnesium plus ATP; either alone was ineffective. The ATP activation was reversed by treatment with alkaline phosphatase and could be completely restored by subsequent incubation with Mg-ATP. Thus, protein phosphorylation appears to be the mechanism involved. This is the first evidence that higher plants contain an interconvertible form of the enzyme.
