Dietary modifications of the biliary bile acid glycine: taurine ratio and activity of hepatic bile acid-CoA:amino acidN-acyltransferase (EC2.3.1) in the rat

Abstract

Effects of dietary manipulations on the biliary bile acid glycine: taurine (G: T) ratio and the activity of hepatic bile acid-Co A: amino acidN-acyltransferase (EC2.3.1) in the post-mitochondrial fraction of liver homogenates were examined in the rat. The G:T ratio in rats fed on the diet containing 100 g pectin/kg (2.18) was markedly higher than that in the animals fed on the diet containing 100 g cellulose/kg (0.09). The diets containing either 10 g cholesterol/kg or 5 g sodium cholate/kg, especially the latter, also increased the G:T ratio (0.77 and 2.33 respectively) compared with a control diet free of these steroids (0.34). When the saturating concentrations of taurine (20 mM) and glycine (100 mM) were the substrates, dietary pectin relative to cellulose significantly increased the activity of both taurine- and glycine-dependent bile acid-CoA: amino acidN-acyltransferase, but neither dietary bile acid nor cholesterol influenced it. In spite of the marked difference in the G:T ratio among the rats given various types of experimental diet, the bile acid-CoA: amino acidN-acyltransferase reaction produced taurine- but little glycine-conjugated bile acid when both taurine and glycine coexisted at physiological concentration ranges in the assay media. Dietary manipulations modified the hepatic taurine concentrations and the changes were inversely correlated with those in the G: T ratio. However, hepatic concentration of taurine (1.67–4.82 μmol/g) in rats given various types of experimental diet was comparable with or even higher than the reported Michaelis constant (Km) value ofN-acyltransferase for this compound (0.8–2.5 mM). In contrast, glycine concentrations (1.81–2.58 μmol/g) were much lower than theKmvalue for this amino acid (35–40 mM) under various dietary conditions. Thus, neither the substrate specificity of the bile acid conjugation enzyme nor the alteration in the hepatic concentration of taurine or glycine accounted for the change in the G: T ratio in the present study.