Investigations on the Possible Involvement of Phospholipids in the Glucose‐6‐Phosphate Transport System of Rat‐Liver Microsomal Glucose‐6‐Phosphatase

Abstract

Interrelationships between the catalytic behavior of glucose-6-phosphatase and the structure of rat-liver microsomal membranes were investigated. Rabbit anti-microsomal serum completely inhibited G-6-P hydrolysis in detergent-modified microsomes but showed no inhibitory effect on the enzyme activity of intact or mechanically disrupted vesicles. Controlled proteolysis of intact microsomes using carboxypeptidase A and/or aminopeptidase M largely denatured enzymes situated on the outer surface of the microsomal vesicles such as monodehydroascorbate reductase and cytochrome c reductase. It did not effect the glucose-6-phosphatase activity at all, which remained in a latent state within the membrane. Temperature studies on glucose-6-phosphatase have revealed that only the enzyme activity of intact microsomes exhibited a nonlinear Arrhenius plot, whereas detergent-modified microsomes showed a linear temperature response. Treatment of microsomes with phospholipase C and toluene-2,4-diisocyanate resulted in an apparent loss of about 65% and 85% of the original glucose-6-phosphatase activity and was closely correlated with hydrolysis and chemical modification of phosphatidylethanolamine, respectively. These apparent inactivations could be reversed by addition of Triton X-114 alone without any phospholipid supplementation. Glucose-6-phosphatase is apparently buried within the microsomal membrane, not exposed on either side. Phospholipids may be involved in the G-6-P transport mechanism.