Involved in the biosynthesis of the quinone methoxy group present in the mitomycin A and B, which are used as anticancer agents (PubMed:10099135, PubMed:17461583). In vitro, catalyzes the 6-O-methylation of both C9-beta- and C9-alpha-configured 6-hydroxymitomycins via the transfer of the S-methyl group of S-adenosyl-L-methionine (AdoMet) to the 6-demethylmitomycin A and B. It can also use hydroxyquinone as substrate (PubMed:17461583). 6-demethylmitomycin A + S-adenosyl-L-methionine = mitomycin A + S-adenosyl-L-homocysteine 6-demethylmitomycin B + S-adenosyl-L-methionine = mitomycin B + S-adenosyl-L-homocysteine Completely inhibited by Zn(2+) and Cu(2+). kcat is 0.110 min(-1) for methyltransferase activity with S-adenosyl-L-methionine as substrate. kcat is 0.103 min(-1) for methyltransferase activity with 6-demethylmitomycin A as substrate. Homodimer. Cells lacking this gene are unable to produce mitomycins A and B, and accumulate 6-demethylmitomycin A and 6-demethylmitomycin B. Belongs to the class I-like SAM-binding methyltransferase superfamily. Cation-independent O-methyltransferase family. COMT subfamily.