Formation of Δ²²-bile acids in rats is not gender specific and occurs in the peroxisome

We recently demonstrated that the formation of Δ²²-bile acids is a quantitatively major pathway for normal bile acid synthesis in the adult male Sprague-Dawley rat. This pathway is specific for 7β-hydroxy bile acids and, when ursodeoxycholic acid is administered, Δ²²-ursodeoxycholic acid appears as a major metabolite in the liver tissue, bile, intestinal contents, and plasma. The aims of this study were, therefore, to determine whether this metabolic pathway was gender specific, and to establish that the peroxisome is a site of formation of Δ²²-bile acids. Bile acids were determined by gas chromatography-mass spectrometry in liver tissue, jejunum, and plasma of adult female rats and in animals fed a diet containing 0.4% and 1% ursodeoxycholic acid. Bile acid metabolism in female rats was found to be similar to that of male rats, and Δ²²-β-muricholic acid, rather than β-muricholate, was likewise confirmed as the major muricholic acid synthesized. Ursodeoxycholic acid administration resulted in the appearance of Δ²²-ursodeoxycholic acid as a major metabolite. When adult male Sprague-Dawley rats were treated with clofibrate, a drug that induces peroxisomal proliferation, liver weight increased 40-60% and total bile acid synthesis decreased markedly, but the relative composition of individual bile acids was unchanged. When ursodeoxycholic acid was added to the diet, the proportion of Δ²²-bile acids relative to the corresponding saturated analogues increased significantly compared with untreated rats, indicating that clofibrate had "amplified" the pathway for formation of Δ²²-bile acids. When UDCA was incubated in vitro with a peroxisomal-enriched fraction from normal adult male rat liver, Δ²²-ursodeoxycholic acid was formed in proportions comparable to that observed in vivo when this bile acid was given orally. These studies establish that the pathway for the formation of Δ²²-bile acids is not gender specific and mainly occurs in hepatic peroxisomes.