Insulin: Inducer of phosphofructokinase: The integrative action of insulin at the enzyme biosynthetic level

Abstract

An understanding of the action of hormones as coordinators of metabolic events is facilitated by the concept which suggests that certain hormones exert their effect through influencing whole genome units governing the biosynthesis of functionally related, key, rate-limiting enzymes (1). It was concluded that the key gluconeogenic enzymes in liver are produced on such a functional genome unit, and the evidence was summarized regarding the action of glucocorticoid hormone as inducer and insulin as suppressor of the biosynthesis of these enzymes (2, 3). The key enzymes for glycolysis, glucokinase, phosphofructokinase, and pyruvate kinase, are thought to be produced on another functional genome unit (1,3,4). Similar to the properties of gluconeogenic enzymes, these key glycolytic enzymes also have low activities and govern one-way reactions; however, they operate in the opposite direction to the function of the key gluconeogenic enzymes. A simultaneous, but antagonistic action on the key gluconeogenic and glycolytic enzymes may well determine the direction of overall metabolic flow for gluconeogenesis or glycolysis. It follows from the functional genome unit concept that the three key glycolytic enzymes should respond similarly to hormonal regulatory influences, and this concept was put to the experimental test. The behavior of glucokinase had already been investigated extensively by Weinhouse (5) and other workers (6–8), and the following behavioral patterns was established. Liver glucokinase decreased in diabetes or in starvation; insulin treatment in diabetic rats and refeeding of starved animals returned it to normal. The insulin- or refeeding-induced rise in glucokinase activity was inhibited by blockers of protein synthesis, and the increase was attributed to de novo biosynthesis. Glucokinase was not affected by treatment with adrenal cortical hormone. First, in the experimental study of this concept hepatic pyruvate kinase was examined. This enzyme markedly decreased in alloxan diabetes and insulin restored it to normal (4). The insulin-induced rise was blocked by injection of inhibitors of protein synthesis, ethionine or actinomycin, and it was concluded that the enzyme increase was due to de novo enzyme biosynthesis (4). Furthermore, pyruvate kinase activity markedly decreased in starvation and rapidly returned to normal on refeeding (9). This enzyme increase was blocked also by inhibitors of protein synthesis (Weber, G., and N. B. Stamm, To be published). Pyruvate kinase activity was not affected by injection of the fluorinated steroid, triamcinolone (4). Thus, the evidence indicated that the behavior of hepatic pyruvate kinase paralleled that of glucokinase, which is in line with the prediction of the functional genome unit concept. Our next step was to extend these studies to the biosynthetic regulation of hepatic phosphofructokinase. The intricate control of this mammalian enzyme activity in heart (10, 11), skeletal muscle (12, 13), brain (14), and liver (15) has been the topic of research; however, the regulation of biosynthesis of phosphofructokinase in liver has not been the subject of much exploration as yet. The present results show that the biosynthetic control of hepatic phosphofructokinase is similar to the regulatory pattern observed for the other two key glycolytic enzymes. This evidence is now outlined.