Equatorial hydroxylase

Esters of cholesterol cannot be directly attacked by the enzyme [60,61]. Rat liver microsomes catalyse 7a-hydroxylation of cholestanol at a rate comparable to that of cholesterol [62]. Since 7a-hydroxylation of cholestanol is stimulated after treatment with cholestyramine, it appears likely that the same enzyme is involved as in hydroxylation of cholesterol. Shght changes in the side chain lead to marked loss of activity [63,64]. Loss of a terminal methyl group reduces the rate to about 50% and addition of an ethyl group at C-24 (sitosterol) leads to almost complete loss of the activity. From a study with a great number of structurally closely related steroids, Aringer concluded that cholesterol 7a-hydroxylase requires a rather flat steroid (A", A or 5a) and an equatorial or quasi-equatorial hydroxyl group at C-3 [65]. 

A particular enzyme of the XO family is the Clostridium barkerii Nicotinic Acid Hydroxylase, in which the thio ligand is replaced by a seleno ligand. The Mo(v) signal is characterized by g = 2.067, 1.982 and 1.974 and does not correlate with any other enzymes of the XO family. The g-tensor is significantly affected by covalency effects due to the presence of the seleno equatorial ligand directly bound to the metal. 

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