ABSENCE OF GLUCOSE-UPTAKE BY LIVER-MICROSOMES - AN EXPLANATION FOR THE COMPLETE LATENCY OF GLUCOSE-DEHYDROGENASE

The permeability of rat liver microsomes to glucose was investigated i n relation to the hexose-6-phosphate dehydrogenase system (EC 1.1.1.47 ). It was found that glucose-6-phosphate dehydrogenase activity could be assayed with NADP as coenzyme in both untreated and detergent-treat ed microsomes. However, when glucose was used as substrate, activity w as only measurable in detergent-treated microsomes. Moreover, radioact ive glucose added to microsomes in a variety of experimental condition s was never taken up by the vesicles. Our results indicate that NADP ( or NAD) availability is probably not the reason for the absence of glu cose dehydrogenase activity in untreated microsomes but rather membran e impermeability to glucose would account for the complete latency obs erved. This finding calls for a reevaluation of glucose transport in r elation to other enzymes of the endoplasmic reticulum, such as glucose -6-phosphatase. (C) 1994 Academic Press, Inc.