The biological conversion of 7-dehydrocholesterol into cholesterol and comments on the reduction of double bonds

Abstract

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-(3)H(2)]NADPH, results in the transfer of radioactivity to the 7alpha-position of cholesterol. When the conversion is carried out in the presence of tritiated water the label is introduced exclusively at the 8beta-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 7alpha- and the 8beta-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.

3β-Hydroxysterol Δ7-reductase and the Smith–Lemli–Opitz syndrome

“…Subsequent work established 7DHC as the sterol intermediate involved in the conversion of lathosterol to cholesterol, and that reduction of 7DHC to yield cholesterol required NADPH [54]. Wilton et al [55,56] established a direct transfer of hydrogen from NADPH to the 7 -position of cholesterol, and that the 8 -hydrogen was derived from water. Recent work by Nishino and Ishibashi [57] suggests that NADPH cytochrome-P450 oxidoreductase activity is essential for DHCR7 catalyzed reduction of 7DHC, and that DHCR7 may be an iron-containing enzyme.…”

Mechanism of Enzymatic Reduction of Steroid Double Bonds

“…7-Dehydrocholesterol, 5,7-choIestadien-3@-01. tion of tritium into the 8p-position of the steroid [7].…”

“…It has not been fully established whether the addition of the hydrogen from NADPH or the addition of the proton is the rate-limiting step in these reactions. Both possibilities have been suggested [7,12]. Wilton et al have shown that when enzymatic reduction of the A 7 double bond in 7-dehydrocholesterol is carried out in a medium containing tritiated water, there is only a small incorpora-…”

“…5a-Reduction of 3-0x0-A4-steroids and reduction of the A24-double bond in lanosterol [4,9] involve a cis addition of hydrogens, whereas the other reactions involve a trans addition [7,9-111. With the exception of the 5p-saturation of A4-3-oxosteroids [3,6] the hydride ion comes from the B-side of NADPH [4,5,7,8]. It has not been fully established whether the addition of the hydrogen from NADPH or the addition of the proton is the rate-limiting step in these reactions.…”

“…The hydrogen derived from NADPH has been assumed to add as a hydride ion and the hydrogen from the enzyme or the medium as a proton. This mechanism has been established for the following reactions : 501-and 5p-reduction of the Ad-double bound in various A4-3-oxosteroids [2-61, reduction of the A7-double bond in 7-dehydrocholesterol [7], of the A14-double bond in 4,4-dimethylcholesta-8,14-dien3p-01 [8] and of the A2*-double bond in lanosterol [S]. 5a-Reduction of 3-0x0-A4-steroids and reduction of the A24-double bond in lanosterol [4,9] involve a cis addition of hydrogens, whereas the other reactions involve a trans addition [7,9-111.…”