Amino acid oxidase; part of the gene cluster that mediates the biosynthesis of the isoquinoline alkaloids fumisoquin A, fumisoquin B and fumisoquin C; as well as small amounts of fumipyrrole as a shunt metabolite (PubMed:25582336, PubMed:27065235, PubMed:30194285). The products of the cluster lead to a brown coloration and are important for growth and conidiation (PubMed:25582336). The nonribosomal peptide synthetase-like protein fsqF, which lacks a canonical condensation domain, is required for addition of a serine-derived dehydroalanine moiety to activated tyrosine but is not essential for the subsequent steps leading to isoquinoline formation (PubMed:27065235). A different enzyme, most likely the ATP-grasp enzyme fsqD, is responsible for activation of tyrosine (Probable). Three additional enzymes encoded by the fsq cluster, the N-methyltransferase fsqC, the phenol 2-monooxygenase fsqG and the FAD-dependent oxidase fsqB, catalyze the formation of the isoquinoline ring system in the fumisoquins (PubMed:27065235, PubMed:30194285). FsqB converts the fspF thiolation domain-bound (2S,4S,5S)-2-amino-6-(3,4-dihydroxyphenyl)-4-hydroxy-5-(methylamino)hexanoyl into isoquinoline (PubMed:27065235, PubMed:30194285). The cyclization most likely proceeds via a two-step mechanism, beginning with FAD-dependent oxidation of the methyl group to an iminium species followed by electrophilic attack on the deprotonated phenol (Probable). Is able to convert N-methyl-3,4-dihydroxy-DL-phenylalanine (N-methyl-DOPA) directly into cyclic isoquinoline, in vitro (PubMed:27065235, PubMed:30194285). The absence of the meta-hydroxyl group, as in L-N-methyl-tyrosine, leads to a 25-fold lower rate of reduction and the formation of the demethylated product L-tyrosine, instead of a cyclic product (PubMed:30194285). Does not accept the D-stereoisomer of N-methyltyrosine, in contrast to N-methyl-DOPA, for which both stereoisomers are oxidized with similar rates (PubMed:30194285). (2S,4S,5S)-2-amino-6-(3,4-dihydroxyphenyl)-4-hydroxy-5-(methylamino)hexanoyl-[peptidyl-carrier protein] + O2 = (2S,4S)-2-amino-4-[(3S)-7,8-dihydroxy-1,2,3,4-tetrahydroisoquinolin-3-yl]-4-hydroxybutanoyl-[peptidyl-carrier protein] + H2O2 N-methyl-L-dopa + O2 = (3S)-7,8-dihydroxy-1,2,3,4-tetrahydroisoquinoline-3-carboxylate + H2O2 N-methyl-D-dopa + O2 = (3R)-7,8-dihydroxy-1,2,3,4-tetrahydroisoquinoline-3-carboxylate + H2O2 Optimum pH is 7.6. Secondary metabolite biosynthesis. Dimer. Expression is positively regulated by the fumisoquins biosynthesis specific transcription factor fsqA (PubMed:25582336). Leads to complete abolishment of isoquinoline alkaloid production and accumulation of a series of benzyl pyrroles, including fumipyrrole. Belongs to the MSOX/MTOX family.