Distinct forms of cytochrome P-450 are responsible for 6β-hydroxylation of bile acids and of neutral steroids

Abstract

Cytochrome P-450-dependent 6 beta-hydroxylation of bile acids in rat liver contributes to the synthesis of the quantitatively important pool of 6-hydroxylated bile acids, as well as to the detoxification of hydrophobic bile acids. The lithocholic acid 6 beta-hydroxylation reaction was investigated and compared with androstenedione 6 beta-hydroxylation. Differential responses of these two activities to inducers and inhibitors of microsomal P-450 enzymes, lack of mutual inhibition by the two substrates and differential inhibition by antibodies raised against several purified hepatic cytochromes P-450 were observed. From these results it was concluded that 6 beta-hydroxylation of lithocholic acid is catalysed by P-450 form(s) different from the subfamily IIIA cytochromes P-450 which are responsible for the bulk of microsomal androstenedione 6 beta-hydroxylation. Similar, but more tentative, results revealed that the 7 alpha-hydroxylation of lithocholic acid and of androstenedione may be also catalysed by distinct P-450 enzymes. The results indicate that cytochromes P-450 hydroxylating bile acids are distinct from analogous enzymes that carry out reactions of the same regio- and stereo-specificity on neutral steroids (steroid hormones). A comparison of pairs of cytochromes P-450 that catalyse the same reaction on closely related steroid molecules will help to define those structural elements in the proteins that determine the recognition of their respective substrates.

Peroxisome Proliferator-activated Receptor α Induces Hepatic Expression of the Human Bile Acid Glucuronidating UDP-glucuronosyltransferase 2B4 Enzyme

“…In addition to sulfation, LCA is efficiently 6␣-hydroxylated into HDCA by the hepatic CYP3A4 enzyme, and this modification facilitates its glucuronidation by UGT2B4 at the 6␣-hydroxy position prior to renal excretion (50). Thus, glucuronidation of HDCA has been proposed as an alternative mechanism for reducing the hepatic toxicity of monohydroxylated LCA (44,50). Recent studies indicate that the BA sensors pregnane X-receptor (PXR) and FXR play important roles in LCA detoxification.…”

“…However, conjugation of LCA with sulfate, a conjugation reaction catalyzed by the dehydroepiandrosterone sulfotransferase (SULT2A1) enzyme, allows an increased hydrosolubility of LCA and facilitates its biliary excretion (47)(48)(49). In addition to sulfation, LCA is efficiently 6␣-hydroxylated into HDCA by the hepatic CYP3A4 enzyme, and this modification facilitates its glucuronidation by UGT2B4 at the 6␣-hydroxy position prior to renal excretion (50). Thus, glucuronidation of HDCA has been proposed as an alternative mechanism for reducing the hepatic toxicity of monohydroxylated LCA (44,50).…”

Engineered Three-Dimensional Liver Mimics Recapitulate Critical Rat-Specific Bile Acid Pathways

“…It is synthesized from CDCA in vivo by phenotypically stable hepatocytes. [41][42][43][44] The production of TbMCA was monitored to determine the fate of metabolized CDCA reported in the previous section (Fig. 6).…”

“…In the rat, beta-muricholic acid (bMCA) is considered an additional primary BA since it is directly synthesized by rat hepatocytes from CDCA. [41][42][43][44] The final step before functional BA secretion from the liver is conjugation to either glycine or taurine to increase their solubility in vivo. 8,12,45 In this study, we exploited the 1 ng/mL detection limit of our HPLC/MS methodology to quantify the following primary rat BAs: CA, glycocholic acid (GCA), taurocholic acid (TCA), CDCA, glycochenodeoxycholic acid (GCDCA), taurochenodeoxycholic acid (TCDCA), bMCA, and tauro-beta-muricholic acid (TbMCA).…”

Silymarin as a New Hepatoprotective Agent in Experimental Cholestasis: New Possibilities for an Ancient Medication

“…The recovery from LC-induced cholestasis is highly dependent on biliary disposal of the cholestatic itself, and on its conversion into less toxic metabolites, via Cyp3a-mediated hydroxylation [195]. This phase I metabolism involves preferentially 6 -hydroxylation, with formation of the non-cholestatic BSs, murideoxycholate (MDC) and -MC [196,197].…”