The 14 alpha-demethylation of 24,25-dihydrolanosterol (DHL) derivatives having trimmed side chains, 27-nor-DHL, 26,27-dinor-DHL, 25,26,27-trinor-DHL, 24,25,26,27-tetranor-DHL, 23,24,25,26,27-pentanor-DHL and 22,23,24,25,26,27-hexanor-DHL, was studied with the reconstituted lanosterol 14 alpha-demethylase system consisting of cytochrome P-450(14DM) and NADPH-cytochrome P-450 reductase both purified from yeast microsomes. The demethylase catalyzed the 14 alpha-demethylation of the derivatives having the side chains longer than tetranor but the activities for the trinor- and tetranor-derivatives were lower. Kinetic analysis indicated that affinity of the trinor-derivative for the demethylase was considerably higher than that of DHL. The affinities of the 27-nor- and dinor-derivatives were increased by this order and were the intermediates of DHL and the trinor derivative. On the other hand, Vmax values of the demethylase for the DHL derivatives were decreased depending on their side-chain lengths, and the substrate-dependent reduction rate of cytochrome P-450(14DM) was also decreased in the same manner. Based on these observations, it was concluded that interaction of the side chain of lanosterol especially C-25, 26 and 27 with the substrate site of lanosterol 14 alpha-demethylase was necessary for enhancing the catalytic activity of the enzyme. However, this interaction was considered not to be essential for substrate binding.
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