Regulation of enzymes involved in gluconeogenesis

Abstract

Summary1.A simplified scheme of gluconeogenesis was discussed in terms of the sequence of physiological reactions. The enzymes involved in gluconeogenesis were compared as activities expressed on a uniform basis.2.In studying the effects of cortisone it was shown that the extent of induction of hepatic glucose-6-phosphatase activity was inversely related to the weight of the rats. In contrast, fructose-1,6-diphosphatase activity was increased to the same extent in all weight groups.3.In dose response studies with cortisone, easily measurable enzyme increases were accomplished with doses of 2.5 mg and optimum induction was achieved with a dose of 10 mg/100 g rat. The potency of equal doses of cortisone, hydrocortisone, medrol and triamcinolone was compared and it was found that triamcinolone was the most effective in increasing gluconeogenic enzyme activities and liver glycogen and nitrogen levels.4.In dose response studies with triamcinolone it was found that the minimum effective dose was 0.25 mg and optimum induction was achieved by 1 mg/100 g rat.5.Time sequence studies demonstrated that a single injection of triamcinolone was capable of causing the continued synthesis of gluconeogenic enzymes for 6 days.6.In short-term experiments it was possible to achieve statistically significant rises in 2–4 hr for both gluconeogenic enzymes by the use of triamcinolone. Cortisone was also capable of inducing a rise in fructose1,6-diphosphatase activity in 4 hr, but this hormone failed to increase hepatic glucose-6-phosphatase activity in 24 hr.7.By the use of actinomycin and puromycin blocking of the cortisoneinduced increases, evidence was provided that the corticosteroid-induced increases in hepatic gluconeogenic enzymes were due to de novo protein synthesis.8.Since the markedly increased activities of gluconeogenic enzymes in acute alloxan diabetes were inhibited by ethionine, puromycin, and actinomycin it was concluded that these enzyme increases were also due to de novo synthesis.9.The protein synthetic inhibitors failed to block the increased levels of gluconeogenic enzymes in chronic alloxan diabetes, whereas the inhibitors were effective in animals chronically injected with cortisone. It was suggested that the difference between chronic diabetic and chronic cortisoneinjected rats lies in the fact that the diabetic rats had no insulin.10.It was shown that insulin was able to inhibit the corticosteroidinduced increases in gluconeogenic enzyme activities. In sequence studies when animals were injected with insulin concurrently with triamcinolone administration over a 4-day period there was an immediate and sustained inhibition by insulin of the increases induced by triamcinolone in the activities of gluconeogenic enzymes and in nitrogen content.11.In chronically diabetic rats insulin administration decreased liver glucose-6-phosphatase activity to normal in 6 days and fructose-1,6-diphosphatase activity in 10 days. Thus, insulin was capable of interfering with the synthesis of gluconeogenic enzymes in chronic alloxan diabetes, resulting in an inhibition which could not be accomplished by actinomycin, puromycin or ethionine.12.The presented evidence is treated in the light of a tentative working hypothesis suggesting that insulin may act as the physiological repressor of a sequence of gluconeogenic enzymes in the liver.