The effects of ethanol on the glycosylation of human transferrin

Abstract

Appearance of a hyposialylated transferrin fraction in the plasma during chronic alcohol exposure is a well-known phenomenon, and it represents the best available marker of chronic alcohol consumption. The mechanisms of its appearance are still not well understood and are extremely complex, involving biosynthesis and catabolism alterations, although the only structural abnormality described corresponds to the loss of an entire glycan chain. We analyzed and compared the oligosaccharides present on the different isoforms of purified transferrin isolated from control and patients with severe alcohol abuse by fluorescent carbohydrate electrophoresis and matrix-assisted laser desorption ionization mass spectrometry. Our data indicate that the major modification observed is the loss of an entire oligosaccharide chain; we also demonstrate that there is a modification of terminal sialylation. Carbohydrate-deficient transferrin (CDT) is the result of multiple alterations of glycosylation. These results give a partial explanation to the poor sensitivity of the measurement of CDT and its controversial use as a marker of chronic alcohol consumption.

Disialo–trisialo bridging of transferrin is due to increased branching and fucosylation of the carbohydrate moiety

“…In addition minor peaks representing fucosylated glycans were also present. This glycosylation pattern is consisten with previous studies [10,26,27]. Mass spectrometric analysis of oligosaccharides released from a TGP-1 sample showed, in addition to ions corresponding to the mono-and disialylated biantennary glycans, a number of ions corresponding to triantennary glycans (Fig.…”

“…These isoforms lack one or both of the carbohydrate chains [9,10] and are collectively denoted carbohydrate deficient transferrin (CDT). Among several laboratory tests routinely used in clinical and forensic medicine, the percentage of CDT of total transferrin (% CDT) is considered the most specific biochemical marker for detecting chronic alcohol abuse and for monitoring abstinence during treatment [6,11,12].…”

Alcohol abuse and glycoconjugate metabolism

“…The synthesis of asparagine-linked oligosaccharides of glycoproteins involves long-chain polyprenol dolichol which acts as a glycosyl carrier in the process of protein N-glycosylation [29,30]. The synthesis of O-glycosidically linked saccharide chains requires individual nucleotide-activated monosaccharides added by site-specific glycosyltransferases in a stepwise manner [26].…”

“…Dolichol has been found to be very sensitive to attack by free radicals [33], thus its function is easily declined during alcohol-induced oxidative stress. Acetaldehyde (the main metabolite of alcohol oxidation) has been reported to bind covalently to proteins of Golgi membranes (including to glycosyltransferases), decreasing terminal glycosylation at the level of the Golgi apparatus [9,10,29,30]. An increase in NADH/NAD + ratio during drinking decreases ATP production by glycolytic pathway and citric acid cycle.…”

“…After heavy alcohol drinking, an abnormal glycosylation of glycoproteins and glycolipids (due to decreased activity of glycosyltransferases including galactosyltransferase, sialyltransferase, N-acetylglucosaminyltransferase, and increased activity of sialidase) and decreased secretion (due to inhibited polymerization of tubulin) of glycoproteins and glycolipids, have been reported [28,40,41]. Carbohydrate-deficient transferrin (CDT) has been widely investigated as one of the most valuable markers for monitoring chronic alcohol abuse [9,10,30]. CDT is a mixture of the asialo, monosialo or disialo isoforms of transferrin, the N-glycosylated protein.…”