Specific photoactivated covalent binding of S-adenosylmethionine to phenylethanolamine-N-methyl transferase.

S-Adenosyl-L-methionine, the common methyl donor in enzymatic methylation systems, has been directly cross-linked to phenylethanolamine-N-methyltransferase from bovine adrenal medulla. Cross-linking was achieved by direct irradiation of a mixture of S-adenosyl-L-methionine and phenylethanolamine-N-methyl-transferase in Tris-HCl buffer (pH 7.5) by ultraviolet light at 254 nm. The cross-linking reaction is highly specific, as shown by a number of criteria. Phenylethanolamine-N-methyltransferase inhibitors S-adenosyl-homocysteine and SK & F 64139 can block this photoactivated cross-linkage. The S-adenosyl-L-methionine-phenylethanolamine-N-methyltransferase adduct is shown to be a homogeneous protein by Sephadex gel filtration, SDS-polyacrylamide gel electrophoresis and electrofocussing. Proteolytic digestion of this adduct releases one major S-adenosyl-L-methionine-labelled peptide, separable by paper chromatography. The results suggest that the cross-linking occurs at the specific active S-adenosyl-L-methionine-binding site of phenylethanolamine-N-methyltransferase.