Identification and characterization of a catechol-o-methyltransferase cDNA in the catfish Heteropneustes fossilis: tissue, sex and seasonal variations, and effects of gonadotropin and 2-hydroxyestradiol-17β on mRNA expression

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English. Catechol-O-methyltransferase (COMT) is involved in the methylation and inactivation of endogenous and xenobiotic catechol compounds, and serves as a common biochemical link in the catecholamine and catecholestrogen metabolism. Studies on cloning, sequencing and function characterization <em>comt</em> gene in lower vertebrates like fish are fewer. In the present study, a full-length comt cDNA of 1,442 bp with an open-reading frame (ORF) of 792 bp, and start codon (ATG) at nucleotide 162 and stop codon (TAG) at nucleotide 953 was isolated and characterized in the stinging catfish <em>Heteropneustes fossilis</em> (accession no. KT597925). The ORF codes for a protein of 263 amino acid residues, which is also validated by the catfish transcriptome data analysis. The catfish <em>Comt</em> shared conserved putative structural regions important for S-adenosyl methionine (AdoMet)- and catechol- binding, transmembrane regions, two glycosylation sites (N-65 and N-91) at the N-terminus and two phosphorylation sites (Ser-235 and Thr-240) at the C-terminus. The gene was expressed in all tissues examined and the expression showed significant sex dimorphic distribution with high levels in females. The transcript was abundant in the liver, brain and gonads and low in muscles. The transcripts showed significant seasonal variations in the brain and ovary, increased progressively to the peak levels in spawning phase and then declined. The brain and ovarian <em>comt</em> mRNA levels showed periovulatory changes after <em>in vivo</em> and <em>in vitro</em> human chorionic gonadotropin (hCG ) treatments with high fold increases at 16 and 24 h in the brain and at 16 h in the ovary. The catecholestrogen 2-hydroxyE2 up 35 regulated ovarian <em>comt</em> expression <em>in vitro</em> with the highest fold increase at 16 h. The mRNA and protein was localized in the follicular layer of the vitellogenic follicles and in the cytoplasm of primary follicles. The data were discussed in relation to catecholamine and catecholestrogen mediated functions in the brain and ovary of the stinging catfish.