Ontogeny of the catalytic subunit and putative glucose-6-phosphate transporter proteins of the rat microsomal liver glucose-6-phosphatase system

The catalytic subunit (p36) and putative glucose-6-phosphate (G6P) transpor ter (p46) protein levels of the rat glucose-6-phosphatase (G6Pase) system w ere studied in relation to G6Pase hydrolytic activity and G6P uptake in liv er microsomes during the fetal to neonatal period. The mean G6P hydrolytic activity in liver microsomes increased significantly from the 20th to 21st day of gestation (from 6 to 22 mU/mg protein) and was further enhanced by 3 -fold 6 hours after birth, with a maximal activity at 1 day of age (112 mU/ mg protein). In contrast, G6P uptake into the vesicles was undetectable bef ore birth, appeared after day 1 (656 pmol/mg protein), and decreased after day 2 (about 330 pmol/mg protein). Immunoblot analysis showed that the mean p36 protein level was low (<1.6 arbitrary units [AU]) during gestation, in creased sharply (to about 4.0 AU) during the first day, and remained stable afterward. Unlike p36, p46 protein was present before birth at values comp arable to those postpartum. P46 increased from 3.2 AU at 20 days to 4.6 AU at 21 days of gestation, and decreased transiently after birth. These resul ts show that (1) G6Pase hydrolytic activity before birth can occur without detectable G6P uptake function; (2) the presence of the putative G6P transp orter protein is not sufficient to elicit G6P uptake; and (3) full G6Pase a ctivity requires optimal expression of both p36 and p46 proteins. These dat a are discussed in relation to the function of G6Pase. Copyright (C) 2000 b y W.B. Saunders Company.