A cDNA clone for human catechol-Omethyltransferase (hCOMT; S-adenosyl-L-methionine:catechol O-methyltransferase; EC 2.1.1.6) was isolated from a human hepatoma cell line (Hep G2) cDNA library by hybridization screening with a porcine cDNA probe. The cDNA clone was sequenced and found to have an insert of 1226 nucleotides. The deduced primary structure of hCOMT is composed of 271 amino acid residues with the predicted molecular mass of 30 kDa. At its N terminus it has a hydrophobic segment of 21 amino acid residues that may be responsible for insertion of hCOMT into the endoplasmic reticulum membrane. The primary structure of hCOMT exhibits high homology to the porcine partial cDNA sequence (93%). The deduced amino acid sequence contains two tryptic peptide sequences ( Catechol-O-methyltransferase (COMT; S-adenosyl-L-methionine; catechol O-methyltransferase; EC 2.1.1.6) is an enzyme that catalyzes the transfer of a methyl group from S-adenosyl-L-methionine to the m-hydroxy group of catecholamine neurotransmitters (dopamine, noradrenaline, adrenaline), their metabolites, and L-dopa, thereby inactivating them. The enzyme has a broad substrate specificity accepting as substrate also catechol steroids, a-methyldopa, and apomorphine (1). It is widely distributed in various cerebral and extracerebral tissues of all mammalian species (2), including erythrocytes (3). The occurrence of at least two distinct isoforms of COMT has been demonstrated, of which one is soluble (S-COMT) and the other membrane-bound

The requirements for killer cell production in the course of a mixed leukocyte reaction and the specificity of target cell (PHA‐blasts) lysis in the mouse were investigated using inbred strains carrying intra‐H‐2 recombinant chromosomes. Strong lytic activity was generated in all, and only those, responder‐stimulator combinations which differed at either the H‐2D or the H‐2K, or both regions, even if the MLR incompatibility between responder and stimulator was very weak. Killing activity was specific and directed against determinants controlled by genes in the H‐2K and H‐2D regions. The slope of the killer dose‐response curves is the same for either type of specificity. Quantitative comparison of the lytic activity of a given killer cell population on different targets demonstrated a dose effect of the number of specificities recognized. No significant killing against the Ir or the Ss‐Slp regions of the H‐2 complex could be detected. AntiH2 sera, if directed against the killer cells, do not inhibit their activity, while they can block killing, if directed against the target. This inhibition is specific in that a serum that blocks killing against the H‐2K specificity of a given target does not inhibit the lytic activity directed against the H‐2D determinants on the same target.

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