Enables L-amino-acid oxidase activity. Involved in negative regulation of T cell mediated immune response to tumor cell; positive regulation of regulatory T cell differentiation; and regulation of B cell differentiation. Acts upstream of or within aromatic amino acid family metabolic process. Located in lysosome. Orthologous to human IL4I1 (interleukin 4 induced 1). [provided by Alliance of Genome Resources, Apr 2022]