O-methyltransferase that mediates the methylation of 3-hydroxy-5-methyl-L-tyrosine (3-OH-5-Me-Tyr) into 3-hydroxy-5-methyl-O-methyltyrosine (3-OH-5-Me-OMe-Tyr), a core structure of saframycin A, a potent antitumor antibiotic that belongs to the tetrahydroisoquinoline family. 5-hydroxy-3-methyl-L-tyrosine + S-adenosyl-L-methionine = 5-hydroxy-3-methyl-O-methyl-L-tyrosine + H(+) + S-adenosyl-L-homocysteine Antibiotic biosynthesis. Belongs to the class I-like SAM-binding methyltransferase superfamily. Cation-independent O-methyltransferase family. COMT subfamily.