Secreted L-amino-acid oxidase that acts as a key immunoregulator (PubMed:32818467). Has preference for L-aromatic amino acids: converts phenylalanine (Phe), tyrosine (Tyr) and tryptophan (Trp) to phenylpyruvic acid (PP), hydroxyphenylpyruvic acid (HPP), and indole-3-pyruvic acid (I3P), respectively (PubMed:15383589). Also has weak L-arginine oxidase activity (By similarity). Acts as a negative regulator of anti-tumor immunity by mediating Trp degradation via an indole pyruvate pathway that activates the transcription factor AHR (PubMed:21469114, PubMed:28405502, PubMed:32818467). IL4I1-mediated Trp catabolism generates I3P, giving rise to indole metabolites (indole-3-acetic acid (IAA) and indole-3-aldehyde (I3A)) and kynurenic acid, which act as ligands for AHR, a ligand-activated transcription factor that plays important roles in immunity and cancer (By similarity). AHR activation by indoles following IL4I1-mediated Trp degradation enhances tumor progression by promoting cancer cell motility and suppressing adaptive immunity (PubMed:32818467). Also has an immunoregulatory function in some immune cell, probably by mediating Trp degradation and promoting downstream AHR activation: inhibits T-cell activation and proliferation, promotes the differentiation of naive CD4(+) T-cells into FOXP3(+) regulatory T-cells (Treg) and regulates the development and function of B-cells (PubMed:25778793, PubMed:29288206). Also regulates M2 macrophage polarization by inhibiting T-cell activation (PubMed:26599209). Also has antibacterial properties by inhibiting growth of Gram negative and Gram positive bacteria through the production of NH4(+) and H2O2 (By similarity). an L-alpha-amino acid + H2O + O2 = a 2-oxocarboxylate + H2O2 + NH4(+) H2O + L-tryptophan + O2 = H2O2 + indole-3-pyruvate + NH4(+) H2O + L-phenylalanine + O2 = 3-phenylpyruvate + H2O2 + NH4(+) H2O + L-tyrosine + O2 = 3-(4-hydroxyphenyl)pyruvate + H2O2 + NH4(+) H2O + L-arginine + O2 = 5-guanidino-2-oxopentanoate + H2O2 + NH4(+) Optimum pH is 4.0. Amino-acid degradation; L-tryptophan degradation via pyruvate pathway. Secreted at the immunological synapse. Primarily found in immune tissues. Primarily found in immune tissues, mostly in B-lymphocytes. Restricted to the testis, predominantly in Sertoli cells at the periphery of the ducts, and the brain, including Purkinje cells, hippocampus and mitral cells in the olfactory bulb. No isoform 2 expression in fetal tissues. Expression increases during bone marrow-derived macrophage (BMDM) differentiation: expression is much higher in primary macrophages than monocytes. By interleukin-4. Mice display an accelerated B-cell egress from the bone marrow, resulting in the accumulation of peripheral follicular B-cells. Uses the promoter of the upstream NUP62 gene and shares the first 2 non-coding exons with NUP62. Belongs to the flavin monoamine oxidase family. FIG1 subfamily.