= 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 $\mathrm{C_{21}H_{32}O_2}$.<Pregnenolone in ChEBI>[CHEBI:16581, Definition and Formula]{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.<Cholesterol side-chain cleavage enzyme in IUBMB Enzyme Nomenclature>[EC 1.14.15.6, reactions 1a–1c and Comments]{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.
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