STEROL METHYLTRANSFERASE FROM UROMYCES-PHASEOLI - AN INVESTIGATION OF THE 1ST AND THE 2ND TRANSMETHYLATION REACTIONS

The 2 C unit at C-24 of many plant, algal and fungal sterols is synthesized by 2 successive transmethylations with S-adenosylmethionine as the C donor. Enzyme(s) for the 2 transmethylations were isolated from U. phaseoli to determine if the 2-step reaction is catalyzed 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 2nd transmethylation step were synthesized by a route developed to convert .DELTA.24-sterols to 24-methylene sterols. The enzymes for the 2 transmethylation reactions were solubilized and purified. Both activities were purified together to approximately the same degree, 155-fold. Degradation products from zymosterol inhibited both reaction steps equally. Evidently, a single enzyme or complex is responsible for the catalysis of both steps.