Catalyzes the O-methylation, and thereby the inactivation, of catecholamine neurotransmitters and catechol hormones. Shows highest activity towards catecholestrogens and dobutamine. Also has lower activity towards L-DOPA, dopamine and epinephrine. Active towards the xenobiotic compounds methyl-DOPA, carbidopa, isoproterenol, and apomorphine. a catechol + S-adenosyl-L-methionine = a guaiacol + H(+) + S-adenosyl-L-homocysteine Binds 1 Mg(2+) ion per subunit. Optimum pH is 7.5-10.5. Active from pH 5.5-11.5. Widely expressed (PubMed:28844929). Has higher expression in females compared to males (PubMed:21371608). Strongly expressed in liver and diencephalon (PubMed:28844929). Expressed at lower levels in hindbrain, spinal cord, eye, telencephalon, spleen, gut, gill and muscle (PubMed:28844929). Detected in ovary and testis (PubMed:28844929). In eye, detected in all layers of the retina with highest expression in the inner nuclear layer (PubMed:28844929). In gut, expressed in the lamina propria but has little or no expression in gut epithelium (PubMed:28844929). In brain, has strongest expression near the midline of the telencephalon, in the periventricular gray zone of the optic tectum, in the preglomerular nucleus, and near the walls of the diencephalic ventricle (PubMed:28844929). Highly expressed in unfertilized eggs. Has low expression during embryogenesis. Expression then increases after hatching (48 hours post-fertilization), reaching maximum levels at 3 weeks of age. Belongs to the class I-like SAM-binding methyltransferase superfamily. Cation-dependent O-methyltransferase family.