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

The toxicity of hydrophilic (cholate) and lipophilic (deoxycholate, ch enodeoxycholate, and lithocholate) bile acids on the function of the e lectron transport chain was investigated in intact and disrupted rat l iver mitochondria. In intact mitochondria, lipophilic bile acids used at a concentration of 100 mu mol/L (0.1 mu mol/mg protein) inhibited s tate 3 and state 3u (dinitrophenol-uncoupled) oxidation rates for L-gl utamate, succinate, duroquinol or ascorbate/N,N,N',N'-tetramethyl-p-ph enylenediamine as substrates. In contrast, state 4 oxidation rates and ADP/oxygen ratios were not significantly affected. At a bile acid con centration of 10 mu mol/L (0.01 mu mol/mg protein), the state 3 oxidat ion rate for L-glutamate was decreased in the presence of deoxycholate , chenodeoxycholate or lithecholate, whereas only lithocholate inhibit ed state 3 oxidation for succinate or duroquinol. In broken mitochondr ia, inhibition of oxidative metabolism was found for NADH or duroquino l as substrate in the presence of 100 mu mol/L lithocholate (0.2 mu mo l/mg protein) and for duroquinol in the presence of 100 mu mol/L cheno deoxycholate. Direct assessment of the activities of the enzyme comple xes of the electron transport chain revealed decreased activities of c omplex I and complex III in the presence of 100 mu mol/L deoxycholate or chenodeoxycholate or 10 mu mol/L lithecholate. Inhibition of comple x IV required higher bile acid concentrations (300 mu mol/L for chenod eoxycholate or 30 mu mol/L for lithocholate), and complex II was not a ffected. Both chenodeoxycholate and lithecholate were incorporated int o mitochondrial membranes. The phospholipid content of mitochondrial m embranes 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 l iver mitochondria. The inhibitory effect of lipophilic bile acids on t he electron transport chain can be explained by an unspecific effect o n the inner mitochondrial membrane of intact mitochondria at high conc entrations (100 mu mol/L) and by a specific impairment of complex I an d complex III in broken mitochondria or in intact mitochondria incubat ed with low bile acid concentrations (10 mu mol/L). The impairment of mitochondrial function by bile acids may be clinically relevant in pat ients or animals with chronic cholestasis.