Comparison between the classic and the alternative pathways of BA synthesis
| Classic (Neutral) Pathway | Alternative (Acidic) Pathway |
|---|---|
| Operates in the liver. | Initial steps involve many different cell types. |
| Initiated by cholesterol 7α-hydroxylase (CYP7A1). | Initiated by sterol 27-hydroxylase (CYP27A1). |
| CYP7A1 is located in the hepatic endoplasmic reticulum. | CYP27A1 is located in mitochondria of many different cell types. |
| CYP27A1–catalyzed oxidation is involved much later in the pathway. | CYP27A1–catalyzed oxidation is involved at the very beginning of the pathway. |
| Modification of the sterol nucleus precedes the side chain oxidation. | Side-chain oxidation precedes the modification of the sterol nucleus. |
| The classic pathway dominates in healthy humans, producing over 90% of the total BAs. | The alternative pathway dominates in neonates. In adults, this pathway produces 5–10% of the total BAs. |