STEROL METHYLTRANSFERASE FROM UROMYCES PHASEOLI: AN INVESTIGATION OF THE FIRST AND THE SECOND TRANSMETHYLATION REACTIONS

Article abstract of DOI:10.1016/0031-9422(81)80020-9

Key Word Index-Uromyces phaseoli; Pucciniacetate; Basidiomycete; phytosterol biosynthesis; sterol methyltransferase.Abstract-The two-carbon unit at C-24 of many plant, algal and fungal sterols is known to be synthesized by two successive transmethylations with S-adenosylmethionine as the carbon donor.Enzyme(s) for the two transmethylations were isolated from Uromyces phaseoli in an attempt to determine if the two-step reaction is catalysed by separable enzymic activities.The sensitivity of the methyltransferase assay was improved and this modification removed the apparent inhibition of the enzyme(s) by detergents.Several substrates for the second transmethylation step were synthesized by a route developed to convert Δ24-sterols to 24-methylene sterols.The enzymes for the two transmethylation reactions were solubilized and purified.Both activities were purified together approximately the same degree, 155-fold.Degradation products from zymosterol were found to inhibit both reaction steps equally.The data suggest that a single enzyme or complex is responsible for the catalysis of both steps.