Parallel changes in vivo in microsomal inorganic pyrophosphatase, pyrophosphate-glucose phosphotransferase and glucose 6-phosphatase activities

Abstract

1. 1. Three enzymatic activities, glucose 6-phosphatase (EC 3.1.3.9), microsomal inorganic pyrophosphatase and inorganic pyrophosphate-glucose phosphotransferase, have been measured in liver and kidney preparations from normal rats and from rats subjected to fasting and to a number of endocrine changes. 2. 2. The three activities, measured in either whole homogenates or microsomal fractions of liver, were found to change in an exactly parallel fashion in all cases studied. The enzymes were comparably elevated after cortisone administration, approximately doubled after an 18-h fast, and increased about 4-fold in alloxan-diabetic rats. Insulin administration or adrenalectomy, in diabetic rats, reduced hepatic enzymatic activities to approximately normal levels. The level of the three activities was somewhat low in livers from rats in late pregnancy and nearly absent in 18- to 21-day-old fetal livers. 3. 3. Comparable levels of the three activities were present in kidney and liver homogenates from normal rats, but the kidney enzyme was somewhat less responsive to changes in vivo than was the liver enzyme. 4. 4. In each of the metabolic states studied, changes in the inorganic pyrophosphate-glucose phosphotransferase and inorganic pyrophosphatase activities were parallel to changes in glucose 6-phosphatase activity. The constant ratios of the three enzymatic activities, under a variety of physiological conditions, provide further evidence for our earlier conclusion, based on kinetic evidence, that the three enzymatic activities are catalyzed by a single microsomal enzyme.