You are measuring bile acid metabolites produced by a cell line isolated from a hepatoma. Mass spec and gas chromatographic studies indicate that the cell line produces chenodeoxycholic acid and related metabolites but does not produce cholic acid nor any metabolites of this compound. Which of the following is the most likely enzyme missing or defective in these cells?

Question

Quizplus · Accepted Answer

The Answer is C
EXPLANATION: The major pathway for the synthesis of the bile acids is initiated via hydroxylation of cholesterol at the 7 position via the action of cholesterol 7a-hydroxylase (CYP7A1). The pathway initiated by CYP7A1 is referred to as the "classic" or "neutral" pathway of bile acid synthesis. The hydroxyl group on cholesterol at the 3 position is in the β-orientation and must be epimerized to the α-orientation during the synthesis of the bile acids. This epimerization is initiated by conversion of the 3β-hydroxyl to a 3-oxo group catalyzed by 3β-hydroxy-Δ⁵ C₂₇-steroid oxidoreductase (HSD3B7). Following the action of HSD3B7 the bile acid intermediates can proceed via 2 pathways whose end products are chenodeoxycholic acid (CDCA) and cholic acid (CA). The alternative bile acid synthesis pathway initiated by sterol 27-hydroxylase (CYP27A1) yields chenodeoxycholic acid, the same as for the classic pathway initiated by CYP7A1. However, the acidic pathway does not lead to the synthesis of cholic acid

You Are Measuring Bile Acid Metabolites Produced by a Cell