Biosynthesis of the 4-O-Methyl-d-glucuronic Acid Unit of Hemicellulose B by Transmethylation from S-Adenosyl-l-methionine

A particulate enzyme preparation from immature corn cobs is capable of transferring the 14C-labeled methyl group from S-adenosyl-l-methionine to a macromolecular acceptor present in the particles. The radioactive product formed is shown to be hemicellulose B, labeled in the 4-O-methyl-d-glucuronic acid unit. The transferase has optimal activity at pH 8.1, and is specifically stimulated by CoCl2 or MnCl2. Evidence is presented that the methylation of the plant heteropolysaccharide occurs at the macromolecular level.