Catechol-O-methyltransferase in the brain of the male African catfish,Clarias gariepinus; distribution and significance for the metabolism of catecholestrogens and dopamine.

Catechol-O-methyltransferase, involved in the methylation of catechol substrates, was localized in the brain of the male African catfish,Clarias gariepinus, by means of a radiometric assay using [Methyl-(3)H]S-adenosylmethionine as methyldonor and catecholestrone as substrate. Fore- and midbrain were divided into eighteen, 500 μm thick, transverse sections. With a hollow needle (diameter 1 mm), specific areas of the brain were punched out and assayed. The catechol-O-methyltransferase activity was calculated from the amount of radioactive methoxyestrone formed from catecholestrone and expressed in pmol.mg(-1) tissue.h(-1).The enzyme could be demonstrated throughout the brain. Although the enzyme activity did not differ very much between the various brain regions (max. 15.4; min. 7.5 pmol), there were some areas in the brain with a more than average activity,i.e., the lateral telencephalon (10.3 pmol), the nucleus preopticus (13.1 pmol), nucleus lateralis tuberis (11.0 pmol) and nucleus recessus posterioris (12.0 pmol) of the diencephalon, the tectum opticum (15.4 pmol) and torus semicircularis (13.6 pmol) of the mesencephalon, and the caudal cerebellum of the metencephalon (10.8 pmol). The lowest activity was detected in the caudal metencephalon (7.5 pmol).The presence of the enzyme catechol-O-methyltransferase in the brain of the African catfish and the observation that both catecholestrogens and dopamine can be methylated by this enzyme suggest that catecholestrogens can influence the methylation (inactivation) of dopamine. Incubations of forebrain homogenates with dopamine and catecholestrone or catecholestradiol confirmed that both catecholestrogens can inhibit the methylation of dopamine. Lineweaver-burk plots with various concentrations of the catecholestrogens indicated that the inhibition is competitive. Dixon plots from the inhibition studies gave inhibition constants of 1.4 and 0.6 μM for catecholestrone and catecholestradiol, respectively, indicating that catecholestradiol is a two times stronger inhibitor than catecholestrone.The significance of the inhibition of the methylation of dopamine by the catecholestrogens in the brain is discussed in the light of the negative feedback of gonadal steroids on the central regulation of reproductive processes.