Abstract
The effects of fasting on hydrolytic and synthetic activities of liver microsomal glucose 6-phosphate phosphohydrolase (EC 3.1.3.9) have been investigated. Differential responses of the two types of activity were observed. Glucose 6-phosphate and inorganic pyrophosphate phosphohydrolase activity levels were elevated after a 48-hour fast, while inorganic pyrophosphate-glucose phosphotransferase activity responded relatively slightly, based on assays carried out in the absence of detergent. However, when homogenates were treated with deoxycholate prior to assay, parallel increases in all three activities were obtained. This latter observation constitutes additional substantiation for the multifunctional nature of microsomal glucose 6-phosphatase. The differential response patterns, and modifying effects of detergent thereon, were observed regardless of the hydrogen ion concentration in assay mixtures in the pH range 4.5 to 7. Kinetic analyses revealed that the differential response patterns were not due simply to fasting-induced changes in Km values, for the noted differences persisted when activity levels were expressed on the basis of Vmax values extrapolated to infinite concentrations of all substrates. Phosphotransferase activity of enzyme newly synthesized or newly activated in response to fasting appears to be blocked at the level of phosphoryl transfer from phosphoryl-enzyme intermediate to glucose. The observed differential patterns of response are consistent with the established hydrolytic role for the enzyme in fasted animals, and contrast with previously observed modes of response of these activities to glucocorticoid administration and experimental diabetes.
Published Version
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