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7-Ketocholesterol is a bioactive sterol, a potent competitive inhibitor of cytochrome P450 7A1, and toxic in liver cells. Multiple origins of this compound have been identified, with cholesterol being the presumed precursor. Although routes for formation of the 7-keto compound from cholesterol have been established, we found that 7-dehydrocholesterol (the immediate precursor of cholesterol) is oxidized by P450 7A1 to 7-ketocholesterol (k(cat)/K(m) = 3 × 10(4) m(-1) s(-1)). P450 7A1 converted lathosterol (Δ(5)-dihydro-7-dehydrocholesterol) to a mixture of the 7-keto and 7α,8α-epoxide products (~1:2 ratio), with the epoxide not rearranging to the ketone. The oxidation of 7-dehydrocholesterol occured with predominant formation of 7-ketocholesterol and with the 7α,8α-epoxide as only a minor product; the synthesized epoxide was stable in the presence of P450 7A1. The mechanism of 7-dehydrocholesterol oxidation to 7-ketocholesterol is proposed to involve a Fe(III)-O-C-C(+) intermediate and a 7,8-hydride shift or an alternative closing to yield the epoxide (Liebler, D. C., and Guengerich, F. P. (1983) Biochemistry 22, 5482-5489). Accordingly, reaction of P450 7A1 with 7-[(2)H(1)]dehydrocholesterol yielded complete migration of deuterium in the product 7-ketocholesterol. The finding that 7-dehydrocholesterol is a precursor of 7-ketocholesterol has relevance to an inborn error of metabolism known as Smith-Lemli-Opitz syndrome (SLOS) caused by defective cholesterol biosynthesis. Mutations within the gene encoding 7-dehydrocholesterol reductase, the last enzyme in the pathway, lead to the accumulation of 7-dehydrocholesterol in tissues and fluids of SLOS patients. Our findings suggest that 7-ketocholesterol levels may also be elevated in SLOS tissue and fluids as a result of P450 7A1 oxidation of 7-dehydrocholesterol.