Stereospecificity in the Enzymatic Conversion of Δ7-Cholesten-3β-ol to 7-Dehydrocholesterol

[6β-3H]-Δ7-Cholesten-3β-ol and [6β-2H]-Δ7-cholesten-3β-ol have been prepared by chemical synthesis. Reduction of 7-dehydrocholesterol by diimide has been found to yield Δ7-cholesten-3β-ol. With the use of this reaction we have prepared [4-14C]-Δ7-cholesten-3β-ol, [5α,6α-3H]-Δ7-cholesten-3β-ol, and [5α,6α-2H]-Δ7-cholesten-3β-ol. Virtually complete retention of labeled hydrogen of [4-14C], [6β-3H]-Δ7-cholesten-3β-ol was observed upon enzymatic conversion to 7-dehydrocholesterol and cholesterol. The location of the tritium at carbon atom 6 of cholesterol was established by the virtually complete loss of labeled hydrogen upon conversion of the 3H-cholesterol to 6-nitrocholesteryl nitrate. The enzymatic conversion of [4-14C], [5α,6α-3H]-Δ7-cholesten-3β-ol to 7-dehydrocholesterol and cholesterol occurred with complete loss of labeled hydrogen. A simple method is described for the rapid removal of 7-dehydrocholesterol from samples of Δ7-cholesten-3β-ol. A significant chromatographic fractionation of 7-dehydrocholesterol and cholesterol labeled with tritium at carbon atom 6 from the same compounds labeled with 14C at carbon atom 4 or containing no isotopic label was observed on columns of alumina impregnated with silver nitrate.