| The enzyme glucose-6-phosphatase (Glc-6-Pase) (E.C. 3.1.9.3) plays a major role in the maintenance of blood glucose levels. Besides its classical role of hydrolyzing Glc-6-P, Glc-6-Pase also hydrolyzes mannose-6-phosphate (Man-6-P) and phosphorylates glucose via its biosynthetic activity. The enzyme is routinely studied in rat liver microsomes in the presence and absence of an activating/disrupting agent. The activator histone II-A reportedly increases Glc-6-Pase activity without destroying the microsomal membrane (St.-Denis et al. (1995) Biochem. J. 310, 221-224). We examined whether histone II-A disrupts microsomes by using the inhibitor N-bromoacetylethanolamine phosphate (BAEP). This compound inhibits Glc-6-Pase only in native microsomes. We found that BAEP was able to inhibit Glc-6-Pase after treatment with histone II-A, suggesting that histone II-A activates without destroying microsomal membranes.;The low-;The stimulation by amino acids of glycogenesis from glucose has been explained by the regulatory volume decrease mechanism (Meijer et al. (1992) J. Biol. Chem. 267, 5823-5828). It involves amino acid-induced swelling of hepatocytes resulting in loss of chloride ions which leads to deinhibition of glycogen synthase. We studied the effects of amino acids and chloride ion on Glc-6-Pase and correlated our results with those reported by others with glycogen synthase. We found that (1) chloride ion inhibits biosynthetic more extensively than hydrolytic activity of Glc-6-Pase and (2) inhibition by chloride ion and activation due to ionic strength may be important considerations when assessing in vitro Glc-6-Pase activities. We propose that amino acids may play a role in increasing biosynthetic activity of Glc-6-Pase, as well as previously characterized glycogen synthase via the regulatory volume decrease mechanism through diminished chloride ion inhibition. |
