Apparent absence of a translocase in the cerebral glucose-6-phosphatase system

In the hepatocyte endoplasmic reticulum, a substrate transporter could provide a means of regulating hydrolysis of glucose-6-phosphate by specifically modulating access of the substrate to the hydrolase. Several characteristics of the cerebral microsomal enzyme suggest that such an hypothesis is untenable in the brain. These are: (a) the inability of the enzyme in either untreated or detergent-disrupted brain microsomes to distinguish between glucose-6-phosphate and mannose-6-phosphate; (b) the close agreement of the apparent Km values for either substrate in intact or disrupted microsomal preparations; (c) the constancy of the latency toward both substrates over a wide concentration range; (d) the inability of nonpenetrating, covalently-linking reagents [e.g., 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS)] to affect the accessibility of the hydrolase to its substrate; (e) the absence of a putative transporter polypeptide, such as that of the liver, in experiments where tritiated H2DIDS, polyacrylamide gel electrophoresis, and radioautography are applied to brain microsomes.