Inhibitory Effect of Physiological Bicarbonate Ion Levels on the Activities of Glucose 6-Phosphate Phosphohydrolase

Rat liver microsomal glucose 6-phosphate phosphohydrolase has been shown to possess two further catalytic activities: the hydrolysis of inorganic pyrophosphate to orthophosphate and a pyrophosphate-glucose phosphotransferase activity. Physiological concentrations of bicarbonate ion (10 to 50 mm) have been found to inhibit all three activities. A detailed kinetic analysis of the nature and extent of this inhibition has been carried out, with both fresh and deoxycholate-treated microsomes. The effect of pH on the degree of inhibition, at constant bicarbonate ion levels, suggests that bicarbonate ion rather than carbon dioxide is the inhibitory species. The inhibition by bicarbonate ion has been found to be competitive with respect to phosphate substrate and noncompetitive with respect to glucose. Measurements at varied substrate levels, when plotted according to the method of Dixon, show that 1/v is a linear function of [I], suggesting that bicarbonate ion is binding at the catalytic active site. The Ki values calculated for the three activities (≅20 mm) show close agreement, providing further support for the belief that a common active site, and possibly a common enzyme-phosphoryl intermediate, is involved in all three activities. It is suggested that the observed inhibition of glucose 6-phosphate phosphohydrolase by bicarbonate ion may explain the observation of Hastings and Longmore that increased bicarbonate-carbon dioxide levels result in increased glycogen accumulation in rat liver slices.