It is shown that the 7-dehydrocholesterol reductase-catalysed conversion of 7-dehydrocholesterol into cholesterol (II), with a 105000g microsomal pellet of rat liver in the presence of [4−3H2]NADPH, results in the transfer of radioactivity to the 7α-position of cholesterol. When the conversion is carried out in the presence of tritiated water the label is introduced exclusively at the 8β-position. However, when the conversion of 7-dehydrocholesterol into cholesterol is performed with a 500g supernatant of rat liver homogenate the radioactivity is incorporated at both the 7α- and the 8β-position. Evidence is provided for the presence of an enzyme system in the 500g supernatant that catalyses an equilibration of hydrogen atoms between those at the 4-position of NADPH and those of water. The work with stereospecifically labelled cofactors shows that both the equilibrating system and the 7-dehydrocholesterol reductase utilize the 4B-hydrogen atom of NADPH. In the light of these results a mechanism for the reduction of carbon–carbon double bonds is discussed.
