TOXICITY OF BILE-ACIDS ON THE ELECTRON-TRANSPORT CHAIN OF ISOLATED RAT-LIVER MITOCHONDRIA

The toxicity of hydrophilic (cholate) and lipophilic (deoxycholate, chenodeoxycholate, and lithocholate) bile acids on the function of the electron transport chain was investigated in intact and disrupted rat liver mitochondria. In intact mitochondria, lipophilic bile acids used at a concentration of 100 mu mol/L (0.1 mu mol/mg protein) inhibited state 3 and state 3u (dinitrophenol-uncoupled) oxidation rates for L-glutamate, succinate, duroquinol or ascorbate/N,N,N',N'-tetramethyl-p-phenylenediamine as substrates. In contrast, state 4 oxidation rates and ADP/oxygen ratios were not significantly affected. At a bile acid concentration of 10 mu mol/L (0.01 mu mol/mg protein), the state 3 oxidation rate for L-glutamate was decreased in the presence of deoxycholate, chenodeoxycholate or lithecholate, whereas only lithocholate inhibited state 3 oxidation for succinate or duroquinol. In broken mitochondria, inhibition of oxidative metabolism was found for NADH or duroquinol as substrate in the presence of 100 mu mol/L lithocholate (0.2 mu mol/mg protein) and for duroquinol in the presence of 100 mu mol/L chenodeoxycholate. Direct assessment of the activities of the enzyme complexes of the electron transport chain revealed decreased activities of complex I and complex III in the presence of 100 mu mol/L deoxycholate or chenodeoxycholate or 10 mu mol/L lithecholate. Inhibition of complex IV required higher bile acid concentrations (300 mu mol/L for chenodeoxycholate or 30 mu mol/L for lithocholate), and complex II was not affected. Both chenodeoxycholate and lithecholate were incorporated into mitochondrial membranes. The phospholipid content of mitochondrial membranes decreased in incubations containing 100 mu mol/L (0.1 mu mol/mg protein) chenodeoxycholate but was not affected in the presence of 100 mu mol/L lithecholate. The studies show that lipophilic bile acids impair the function of the electron transport chain in isolated rat liver mitochondria. The inhibitory effect of lipophilic bile acids on the electron transport chain can be explained by an unspecific effect on the inner mitochondrial membrane of intact mitochondria at high concentrations (100 mu mol/L) and by a specific impairment of complex I and complex III in broken mitochondria or in intact mitochondria incubated with low bile acid concentrations (10 mu mol/L). The impairment of mitochondrial function by bile acids may be clinically relevant in patients or animals with chronic cholestasis.