Different development changes in latency for two functions of a single membrane bound enzyme Glusoce-6-phophatase activities as a function of age

Abstract

1. (1). The capacity for the synthesis of glucose 6-phosphate from PP i and glucose as well as for glucose-6- P hydrolysis, catalyzed by rat liver microsomal glucose-6-phosphatase, increases rapidly from low prenatal levels to a maximum between the second and fifth day, then slowly decreases to reach adult levels. When measured in enzyme preparations optimally activated by hydroxyl ions, the maximum neonatal activities were 4–5-fold higher than in adult animals and several-fold higher than had previously been observed for the unactivated enzyme. 2. (2). The latencies of two catalytic activities associated with the same membrane-bound enzyme show strikingly different age-related changes. The latency of PP i-glucose phosphotransferase activity reaches high levels (60–80% latent) soon after birth and remains high throughout life, while the latency of glucose-6- P phosphohydrolase decreases with age, The phosphohydrolase is 2—3 times more talent in th liver of the neonatal animal than in the adult. 3. (3). Then well established neonatal overshoot of liver glucose-6-phosophatase is almost entirely due to changes in the enzymes in the rough microsomal membranes. The enzyme activity in the rough membrane reaches a maximum and then decreases after day 2, while that in the smooth membrane is still slowly increasing. Despite the great differences in absolute specific activities and in the pattern of early enzyme development between the rough and smooth microsome, enzyme latency in the two subfractions remains parallel, glucose-6-P phosphohydrolase being only slightly more latent, while PP i-glucose phosphotransferase is much more latent in smooth than in rough membranes througout life. 4. (4). Kidney glucose-6-P phosphohydrolase and PP i-glucose phosphotransferase activities were found to change in a parallel fashion with age, showing a small neonatal peak between days 2 and 7 before rising the adult levels. Kidney phosphotransferase activity, like that of liver, remained highly latent throughout life. In contrast to liver, the glucose-6- P phosphohydrolase of kidney did not show a characteristics decreases in latency with age and in the adult remained appreciably more latent than in liver. 5. (5). An improved method was device for the separation of smooth microsomes from liver homogenates.