EFFECTS OF IONIC-STRENGTH AND CHLORIDE-ION ON ACTIVITIES OF THE GLUCOSE-6-PHOSPHATASE SYSTEM - REGULATION OF THE BIOSYNTHETIC ACTIVITY OF GLUCOSE-6-PHOSPHATASE BY CHLORIDE-ION INHIBITION DEINHIBITION/

Certain amino acids stimulate glycogenesis from glucose. The regulator y volume decrease mechanism explaining these effects was defined by Me ijer ct al. (1992, J. Biol. Chem. 267, 5823-5828). It involves amino a cid-induced swelling of hepatocytes resulting in loss of chloride ions which leads to deinhibition of glycogen synthase phosphatase. This re sults in enhanced conversion of the inactive to active form of glycoge n synthase and thus enhanced glycogen synthesis. We have studied the e ffects of amino acids and chloride ion on the glucose-6-phosphatase sy stem (Glc-6-Pase) with rat liver microsomal preparations, and correlat ed our results with those reported by others with glycogen synthase. G lc-6-Pase activities are increased by elevated ionic strength varied b y increasing the concentration of various buffers or charged amino aci ds but are not affected by changes in osmolarity, varied with disaccha rides or uncharged amino acids. With undisrupted microsomes, chloride ion competitively inhibits carbamyl phosphate: glucose phosphotransfer ase (K-i,Cl-(CP,t,UM) = 19 mM) more extensively than Glc-6-P phosphohy drolase (K-i,Cl-(G6P,h,UM) = 117 mM). Inhibition by chloride ion and a ctivation due to ionic strength may be important considerations when a ssessing in vitro Glc-6-Pase activities where an attempt is made to re plicate physiologic conditions. Further we propose that amino acids ma y play a role in increasing biosynthetic activity of Glc-6-Pase, as we ll as previously characterized glycogen synthase (Meijer ct al., op. c it.), via the regulatory volume decrease mechanism through diminished chloride ion inhibition. Reduced concentration of chloride ion will (1 ) deinhibit the biosynthetic activity of Glc-6-Pase, while still inhib iting Glc-6-P hydrolysis, leading to an increased cellular concentrati on of Glc-6-P (an important glycogenic intermediate as well as alloste ric activator of glycogen synthase) and (2) increase the active form o f glycogen synthase by deinhibiting glycogen synthase phosphatase both through the previously defined mechanism (see above) and via Glc-6-P- enhanced conversion of glycogen synthase from its inactive to active f orm. We propose that the biosynthetic activity of Glc-6-Pase may act i n concert with glycogen synthase during amino acid-induced glycogenesi s from glucose. (C) 1998 Academic Press.