219 J BIOCHEM MOLECULAR TOXICOLOGY Volume 12, Number 4, 1998 Streptozotocin-Induced Diabetes Increases c-Glutamyltranspeptidase Activity but not Expression in Rat Liver John B. Watkins III, 1 James E. Klaunig, 2 , Heather M. Smith, 1 Paul Cornwell, 1 and Ruth A. Sanders 1 1 Medical Sciences Program, Indiana University School of Medicine, Bloomington, IN 47405-4201; 2 Division of Toxicology, Indiana University School of Medicine, Indianapolis, IN 46202 Received 28 July 1997; revised 11 November 1997; accepted 17 November 1997 ABSTRACT: Earlier work describing increased biliary excretion of the acetaminophen-cysteine conjugate ad- vanced the hypothesis that streptozotocin-induced diabetes increases c-glutamyltranspeptidase (GGT) ex- pression in Sprague–Dawley rats. To test this hypoth- esis, rats were divided into control, diabetic, and in- sulin-treated diabetic groups. Diabetes was induced by intravenous injection of 45 mg streptozotocin/kg body weight and was effectively controlled by insulin treat- ment in the appropriate group. Densitometric quanti- fication demonstrated that hepatic GGT activity in di- abetic rats was significantly increased when compared to normal and insulin-treated diabetic controls. His- tochemical staining of liver was greater in female than in male rats, and staining increased in female rat liver as the duration of diabetes lengthened from 30 to 90 days. GGT activity was increased by diabetes in liver canalicular-enriched and basolateral-enriched mem- brane preparations, and it was unchanged in renal brush border-enriched membranes. Total mRNA iso- lated from diabetic and insulin-treated diabetic rat liv- ers did not conclusively demonstrate an elevation of GGT mRNA relative to normal. Western blot analysis showed no differences in the amount of GGT in dia- betic versus normal rat livers. These data indicate that streptozotocin-induced diabetes does not alter the ex- pression of, but does increase the activity of, GGT in liver.  1998 John Wiley & Sons, Inc. J Biochem Toxicol 12: 219–225, 1998 KEY WORDS: Gamma-Glutamyltranspeptidase, Liver, Rats, Insulin-Dependent Diabetes Mellitus, Strepto- zotocin. INTRODUCTION Previous studies have attempted to delineate the changes caused by chemically induced insulin-depen- Address correspondence to John B. Watkins III, Ph.D., Medical Sciences Program, Indiana University School of Medicine, Bloo- mington, IN 47405-4201. Tel.: (812) 855-3201; fax: (812) 855-4436.  1998 John Wiley & Sons, Inc. CCC 1095-6670/98/040219-07 dent diabetes mellitus on hepatobiliary function (1), including numerous changes in bile composition, in bile flow rate, in biotransformation, and in the phar- macokinetic values for many xenobiotics. In particular, the biliary excretion of acetaminophen metabolites is altered in streptozotocin (STZ)-induced diabetic rats (2). Specifically, it was noted that STZ-treated diabetic rats excreted the cysteine conjugate of acetaminophen into bile instead of the glutathione conjugate that was excreted by normal rats and insulin-treated diabetic rats. Furthermore, it was hypothesized that STZ-in- duced diabetes must alter the expression of c-gluta- myltranspeptidase (GGT), the enzyme that catalyzes the conversion of the glutathione conjugate into the cysteine conjugate (2). GGT is a ubiquitous glycoprotein, present in both plants and animals, that catalyzes the hydrolysis of c- glutamyl compounds (including glutathione) and the transfer of the c-glutamyl moiety to certain amino ac- ids and dipeptides (3,4). It consists of two subunits (a heavy chain of 41 kDa and a light chain of 20 kDa), contains five N-glycosylation consensus sequences (5), and is located on the outer surface of the plasma mem- brane of cells expressing it (6,7). Cells having high con- centrations of GGT are generally involved in secretion or absorption (8). Kidney has the highest concentra- tion, pancreas has approximately 20% as much GGT activity as kidney, and seminal vesicles express 2% as much GGT activity as kidney (9). Tissues such as liver have even less GGT activity. In adult mammalian liver, GGT activity is nor- mally low. High activity is found in livers of alcoholics (10); of fasting rats (11); and in prenatal, premalignant, and malignant liver states in rodents (8). Other studies have noted increases in GGT activity in liver and plasma of STZ-induced diabetic rats (12–14). Enzyme expression was once thought to be under control of