Biliary catabolism of glutathione and differential reabsorption of its amino acid constituents

Abstract

Biliary excretion of glutathione, free amino acids, and total amino acids (after acid hydrolysis) was measured in hepatic bile collected from guinea pigs, rabbits, and dogs anesthetized with pentobarbital sodium. In controls, the concentration of glutathione in bile was less than 20 microM in all three species. However, when hepatic gamma-glutamyltransferase activity was decreased by retrograde intrabiliary infusion of the irreversible inhibitor acivicin (AT-125; 20 mumol/kg), there was a marked increase in biliary glutathione excretion (in mumol glutathione equivalents.kg body wt-1.h-1) from 0.10 +/- 0.04 to 2.2 +/- 0.6 in guinea pigs, from 0.014 +/- 0.013 to 2.5 +/- 1.9 in rabbits, and from an undetectable level (less than 0.001) to 0.11 +/- 0.05 in dogs. Amino acid analysis of bile revealed that the concentration of glutathione's constituent amino acids (free glutamate, cystine, and glycine) in control bile samples from these three species were quite low and were not affected by AT-125. However, acid hydrolyzates of these same bile samples revealed an unusually high degree of amino acid conjugation. Glutamate (0.06-0.5 mM), cystine (0.2-1.1 mM), and glycine (1.7-2.8 mM) constituted the overwhelming majority of total amino acids in hydrolyzed bile from controls. After AT-125, concentrations of total glutamate and cystine were elevated in hydrolyzed bile, while concentrations of all other amino acids remained the same. Thus glutathione is avidly secreted into bile in the guinea pig, rabbit, and dog but is almost quantitatively broken down within the biliary tree. Subsequently, the glutamate and cysteine moieties derived from catabolism of glutathione must be partially reabsorbed either as peptides, conjugates, or free amino acids.(ABSTRACT TRUNCATED AT 250 WORDS)

H+-peptide cotransport in the human bile duct epithelium cell line SK-ChA-1

“…Very little is known about the presence of small peptides in bile fluid. Glutathione is secreted into bile and is almost completely broken down (6). The authors of this study suggested that the resulting products are reabsorbed either as peptides, conjugates, or free amino acids.…”

Distribution of -glutamyl transpeptidase in the mouse epididymis and its response to acivicin

“…Previously acivicin has been used to inhibit -GT under in-vivo conditions in the rat kidney (Scott & Curthoys, 1987) and brain (Hill et al, 1985) and during perfusion in bile (Ballatori et al, 1988). These studies have resulted in glutathionuria and an accumulation of tissue GSH with a decrease of free L-glutamic acid levels.…”

Physiology of Cholangiocytes

“…Glutathione, which is secreted by hepatocytes, is degraded in bile by -glutamyltranspeptidase, an enzyme found abundantly on the canalicular hepatocyte and ductal cholangiocyte apical plasma membranes (116). Functional studies suggest that cholangiocytes absorb biliary glutamate, cysteine, and glycine by Na + -dependent and independent transport mechanisms that are distinct from those in hepatocytes (26,42). Absorption of these amino acids by cholangiocytes may conserve them for glutathione resynthesis and thus be involved in "bile salt-independent" canalicular bile secretion (183); in addition, it may generate osmotic gradients of 3 to 12 mOsm that favor water absorption and thereby be directly involved in ductal bile modification (85).…”