Fluorometric Assay of Rat Tissue N-Methyltransferases with Nicotinamide and Four Isomeric Methylnicotinamides

Abstract

Nicotinamide (NA) and its four isomeric methyl analogs [2-, 4-, 5- and 6-methylnicotinamides (MNs)] were tested as substrates for nicotinamide /V-methyltransferase (NNMT) and amine /V-methyltransferase (ANMT) using rat liver, kidney, spleen and brain 9000 x g supernatant fluids as model enzyme preparations. The A-methylated products were determined fluorometrically by their reaction with acetophenone or 4-methoxybenzaldehyde to form fluorescent 2,7-naphthyridine derivatives, and the lower limits of the determination were 8—30pmol/100/il. A-Methyltransferase activities were detected in the liver with NA, 4-MN and 5-MN, and in the brain and spleen with 4-MN. On this basis, 5-MN is considered to be a selective substrate for NNMT in addition to NA, which is a known methyl acceptor for this enzyme. Although 4-MN appears to serve as a methyl acceptor for both ANMT and NNMT, it seems to be essentially a selective substrate for brain ANMT because of the absence of NNMT in brain. The fluorometric methods used here are also very useful because of their simplicity, sensitivity and selectivity. © 1992, The Pharmaceutical Society of Japan. All rights reserved.