Catalyzes the O-methylation, and thereby the inactivation, of catecholamine neurotransmitters and catechol hormones. a catechol + S-adenosyl-L-methionine = a guaiacol + H(+) + S-adenosyl-L-homocysteine Binds 1 Mg(2+) ion per subunit. Strongly expressed in eye, diencephalon, spinal cord, hindbrain, liver, kidney and telencephalon. Also detected at very low levels in muscle, spleen, anterior gut and heart. In eye, expressed strongly in retina. In brain, expressed in the central part of the telencephalon, the periventricular gray zone of the optic tectum, the periglomerular nucleus, the olfactory bulb, and the region adjacent to the diencephalic ventricle in the hypothalamus. Expressed in gill, with strongest expression in gill filaments nearest the gill arch, and in esophageal epithelium. Belongs to the class I-like SAM-binding methyltransferase superfamily. Cation-dependent O-methyltransferase family.