Acetylation of serotonin in the rabbit pineal gland: an N-acetyltransferase with properties distinct from NAT1 and NAT2 is responsible.

Two rabbit arylamine N-acetyltransferases (NAT1 and NAT2, EC 2.3.1.5) have been cloned and characterized recently in this laboratory. They catalyze the acetylation of primary arylamine and hydrazine drugs and other substrates in the liver, including sulfamethazine, p-aminosalicylic acid, and p-aminobenzoic acid. In the pineal gland, serotonin is metabolized to N-acetylserotonin by an unknown N-acetyl-transferase. Similarity of the liver enzymes and the pineal gland arylalkylamine N-acetyltransferase (AA-NAT) has been suggested, because pineal gland homogenates were shown to metabolize arylamine substrates as p-phenetidine, aniline, or phenylethylamine, and liver homogenates or partially purified liver enzyme preparations catalyzed the N-acetylation of serotonin. The present study was undertaken to elucidate the possible role of NAT1 or NAT2 in serotonin acetylation in the pineal gland. We transiently expressed rNAT1 and rNAT2 genes in COS cells, studied the kinetics of the enzymes produced with various substrates, and compared these data with activities of rabbit pineal glands and livers. These enzymatic studies were complemented with western blot analysis with antibodies against NAT1 and NAT2. Cross-hybridization of rNAT1 or rNAT2 to the gene for the pineal gland AA-NAT was tested by Southern blot studies of genomic rabbit DNA. Our results indicate that although NAT1 is expressed in the pineal gland, it is not involved in the physiologically important step of N-acetylation of serotonin.