Cholesterol biosynthesis from lanosterol: Development of a novel assay method and characterization of rat liver microsomal lanosterol Δ²⁴-reductase

The membrane-bound sterol Δ²⁴-reductase (24-reductase) catalyses anaerobic reduction of the 24(25)-enes of lanosterol and other obligatory intermediates of cholesterol biosynthesis from lanosterol. A novel assay method and properties of the 24-reductase are described. More than a 120-fold induction of the 24-reductase activity was achieved by feeding rats a diet containing 5% cholestyramine plus 0.1% lovastatin in chow and by modulating diurnal variation. With this enzyme induction condition, lanosterol was converted efficiently into dihydrolanosterol in both intact hepatic microsomes and freshly isolated hepatocytes only when either miconazole or CO was added to inhibit 14α-demethylation of lanosterol. AR45 cells, which are deficient in 14α-methyl demethylase (14α-DM), exhibit lanosterol 24-reductase activity without addition of either CO or miconazole. Conversely, inhibition of the 24-reductase was not required for the expression of 14α-DM activity. Studies on the substrate specificities for the 24-reductase using different 24(25)-enes showed that the most reactive substrate was 5α-cholesta-7,24-dien-3β-ol, which exhibited a maximal 18-fold higher k_cat than that of lanosterol without the aid of the 14α-DM inhibitor. In addition, both the kinetic behaviour of lanosterol substrate in relation to the 24-reductase and a non-competitive inhibition mode of U18666A (K_i 0.157 μM) as well as Triparanol (K_i 0.523 μM), two well-known 24-reductase inhibitors, were determined. On the basis of our new findings on the preferred substrate and on the negative effect of 14α-DM on the 24-reductase, we suggest that C-24 reduction of sterols takes place straight after sterol Δ^8→7 isomerization of zymosterol, which occurs several steps after C-32 demethylation of lanosterol in the 19-step pathway of cholesterol biosynthesis from lanosterol.