Glycosylation of gamma-glutamyltransferase is modified by ethanol in H5-6 hepatoma cell line

Gamma glutamyl transferase

Serum gamma-glutamyl transferase (GGT) has been widely used as an index of liver dysfunction and marker of alcohol intake. The last few years have seen improvements in these areas and advances in understanding of its physiological role in counteracting oxidative stress by breaking down extracellular glutathione and making its component amino acids available to the cells. Conditions that increase serum GGT, such as obstructive liver disease, high alcohol consumption, and use of enzyme-inducing drugs, lead to increased free radical production and the threat of glutathione depletion. However, the products of the GGT reaction may themselves lead to increased free radical production, particularly in the presence of iron. There have also been important advances in the definition of the associations between serum GGT and risk of coronary heart disease, Type 2 diabetes, and stroke. People with high serum GGT have higher mortality, partly because of the association between GGT and other risk factors and partly because GGT is an independent predictor of risk. This review aims to summarize the knowledge about GGT's clinical applications, to present information on its physiological roles, consider the results of epidemiological studies, and assess how far these separate areas can be combined into an integrated view.

Prenatal alcohol exposure affects galactosyltransferase activity and glycoconjugates in the Golgi apparatus of fetal rat hepatocytes

Prenatal exposure to alcohol affects the morphological, structural, and functional features of the Golgi apparatus (GA), thus altering the glycosylation process in fetal hepatocytes. To elucidate the cellular mechanisms underlying these alterations, we have studied the effect of alcohol exposure in utero on the activity of liver galactosyltransferase, an enzyme involved in the glycosylation process, and on the hepatic glycoprotein sugar composition. For this, livers from 21-day-old fetuses obtained from control and ethanol-fed rats were used. Galactosyltransferase (GT) activity was determined in isolated GA cis and trans fractions. Colloidal gold-labeled lectin cytochemistry was used to analyze sugar residues in hepatocytes at the subcellular level. Finally, the integrity of the GA after alcohol treatment was assessed by electron microscopy and by evaluating the distribution of the Golgi beta-COP, a protein involved in vesicular trafficking. Prenatal alcohol exposure induces a significant increase in both liver weight and total protein content in the trans Golgi. Moreover, this treatment decreases the activity of galactosyltransferase, increases alpha-L-Fuc residues, and reduces the number of alpha-Man, GlcNAc(beta1,4,GlcNAc)1,2, GalNAc alpha1,3GalNAc, alpha-GalNAc, and a-Gal residues. Alcohol exposure also causes the Golgi cisternae to disappear in about 30% of the hepatocytes, and reduces 75% the number of anti-Golgi beta-COP protein binding sites. Our results suggest that the decrease in galactosyltransferase activity, the alterations in the oligosaccharide chain composition, and the reduction in the amount of Golgi beta-COP protein could be involved in the alterations in the glycosylation process, as well as in the accumulation of hepatic proteins observed after prenatal alcohol exposure. These alterations could contribute, therefore, to the alcohol-induced injury in the developing liver.

A precursor form of human kidney gamma-glutamyl transferase in normal and cancerous tissues, and its possible post-translational modification

We have found three molecular forms of human gamma-glutamyl transferase (GGT) in normal and renal cell carcinomatous tissues, and also have reported the marked differences in the sugar chains of GGTs between normal and cancerous tissues by serial lectin affinity chromatographies. In this study, the peptide maps of three purified GGTs (79 kDa, 50 kDa and 25 kDa) obtained by lysylendopeptidase digestion, the subcellular localization of GGTs, and the sugar chains of GGTs were compared between normal and cancerous tissues. According to the results, the total peptide bands of the digested 79 kDa component represented the sum of those of the digested 50 and 25 kDa components on 12.5% SDS-PAGE. In addition, C-terminal and N-terminal amino-acid sequences of the 79 kDa protein were the same as the sequences of light and heavy subunits, respectively, suggesting that the 79 kDa component is of the precursor form of the 50 kDa mature heavy and 25 kDa light subunits, respectively. On the other hand, the GGT activity in renal cell carcinomatous tissues was significantly increased in the microsomal fraction and decreased in the soluble fraction compared with that of normal tissues. Meanwhile, the sugar moiety of GGTs in the respective subcellular fractions was obviously different between normal and cancerous tissues. In particular, a reduced multiantennary complex type sugar chain and an elevated high-mannose or hybrid-type sugar chain in the microsomal fraction were observed in the GGT in cancerous tissues.