Retina-specific copper amine oxidase (AOC2) is present on the cell surface of most cells but especially retinal cells. It is classed as a semicarbazide-sensitive amine oxidase (SSAO) and catalyses the oxidative deamination of aromatic amines such as tyramine (TYR, shown here), 2-phenylethylamine and tryptamine (Kaitaniemi et al. 2009, Bour et al. 2007). Overexpression of AOC2 could result in tissue destruction seen in ocular pathologies.
Monoamine oxidases (MAOA and B), present in the outer mitochondrial membrane, catalyse the oxidation of biogenic amines, releasing hydrogen peroxide (H2O2). H2O2 produced during the oxidative deamination of these amines appears to be involved in the progress of neurodegenerative disorders such as Parkinson disease, presumably via oxidative damage to the mitochondrial membrane. MAOB (also MAOA but not show here), with FAD as cofactor, can deaminate tyramine (TYR), a naturally-occuring monoamine that can act as a catecholamine releasing agent (Pearce & Roth 1985).
Cytosolic tyrosine is transaminated to form 3-(4-hydroxyphenyl)pyruvate which in four further reactions is converted to fumarate and acetoacetate. Tyrosine is thus both a glucogenic (fumarate) and a ketogenic (acetoacetate) amino acid. Defects in any of the steps lead to metabolic diseases with accumulation of tyrosine (tyrosinemia) (Mitchell et al. 2001; Held, 2006).
The hydroxylation of phenylalanine, an essential amino acid, to form tyrosine is a major source of the latter amino acid in the body under normal conditions and is also the first step in phenylalanine catabolism. To continue the catabolic process, tyrosine is transaminated to 3-(4-hydroxyphenyl)pyruvate which is broken down to fumarate and acetoacetate (Blau et al. 2001; Mitchell et al. 2001).