Phosphofructokinase Regulation in Some Transplantable Thyroid Tumors

Some kinetic, molecular, and rate-limiting properties of partially purified phosphofructokinase from normal rat thyroid and three transplantable rat thyroid tumors have been studied, and interesting differences have been found. Citrate inhibition of tumor phosphofructokinase and its reversal by cyclic adenosine 3′:5′-monophosphate were lower than those of the normal enzyme. A direct relationship between tumor growth rate and the decrease of citrate inhibition was observed. The diethylaminoethyl cellulose affinity properties of the rat thyroid phosphofructokinase were slightly but consistently different from those of the three tumors; differences observed among the tumor enzymes with regard to these properties were likewise related to their respective growth rates. When the amounts of lactate, pyruvate, and glycerol 3-phosphate produced from each of the substrates of the glycolytic pathway were measured, it was found that, in contrast to results obtained with normal thyroid extracts, no rate-limiting function was associated with phosphofructokinase activity in tumor extracts. It was concluded that the modulation of phosphofructokinase by its allosteric effectors was similar in all three tumors, although different from that of the enzyme from normal rat thyroid; it was proportional to tumor growth rate and probably related to molecular and regulatory modifications.