Pregnenolone

Pregnenolone is a 21-carbon steroid with a position-3 beta-oriented hydroxyl group, a position-20 ketone and a double bond between carbon positions 5 and 6. Its molecular formula is ${\mathrm{C}}_{21}{\mathrm{H}}_{32}{\mathrm{O}}_2$.^[ref]

A key early step of steroid hormone synthesis converts cholesterol into pregnenolone. The cholesterol side-chain cleavage enzyme first hydroxylates the substrate at two positions and then cleaves the side chain, producing pregnenolone and 4-Methylpentanal. This is a sequence of oxygen-dependent reactions with electron transfer, not simply spontaneous breaking of a carbon chain.^[ref]

The products illustrate a compact material-balance check. The 27 carbons of cholesterol divide into 21 in pregnenolone and six in the aldehyde product: $27=21+6$. This count alone does not balance oxygen or hydrogen, but it makes clear why a pathway can retain the four-ring steroid framework while shortening its side chain. Identifying pregnenolone in a sample establishes a particular chemical intermediate; its presence alone does not measure the rate of every downstream hormone-producing reaction.