Carbamyl phosphate: glucose phosphotransferase and glucose-6-phosphate phosphohydrolase of nuclear membrane. Interrelationships between membrane integrity, enzymic latency, and catalytic behavior.

The presence of carbamyl-phosphate:glucose phosphotransferase in liver nuclei of five species of mammals and birds is demonstrated. The activity is confined to nuclear membranes and is due exclusively to multifunctional glucose-6-phosphatase-phosphotransferase (D-glucose-6-phosphate phosphohydrolase; EC 3.1.3.9). The nuclear enzyme constitutes approximately 16 to 19 percent of total hepatic glucose-6-phosphatase-phosphotransferase. Carbamyl-phosphate:glucose phosphotransferase and glucose-6-P phosphohydrolase activities of membrane of chicken liver nuclei are shown to be catalytically identical with the maximally activated microsomal enzyme. A correspondence is seen in two-substrate kinetic double reciprocal plots, K-m or apparent K-m values for the various substrates, K-i values for the competitive inhibitors P-i and ATP, and pH-activity profiles. Comparative studies were carried out with various intact, disrupted, and detergent-dispersed membranous preparations by a combination of enzyme kinetic and electron microscopic techniques. It is concluded that (a) intimate interrelationships exists between catalytic behavior of this enzyme and morphological integrity of membranes of which the enzyme is a part; (b) activities of the enzyme of nuclear membrane appear quite available for physiological phosphorylative functions; and (c) interrelationships between membrane morphology and catalytic behavior of this membrane-bound enzyme may well be involved in the bioregulation of this complex, multifunctional enzyme system.