In vitro characteristics of guinea pig lung aromatic azaheterocycle N-methyltransferase.

The in vitro characteristics of guinea pig lung aromatic azaheterocycle N-methyltransferase have been determined, using R-(+)-nicotine as substrate. The enzyme is a cytosolic, S-adenosyl-L-methionine-dependent N-methyltransferase exhibiting apparent KM values of 14.2 and 13.2 microM for R-(+)-nicotine and S-adenosyl-L-methionine, respectively. The pH and temperature optima for enzyme activity were 7.4 and 37 degrees C, respectively. An interesting stereospecificity was demonstrated for nicotine enantiomers toward the N-methyltransferase. S-(-)-Nicotine acts as a competitive inhibitor (Ki = 6.25 X 10(-5) M) of the N-methylation of its optical antipode by the above enzyme. The enzyme also exhibits potent feedback inhibition by S-adenosyl-L-homocysteine (Ki = 3.2 X 10(-5) M). Neither S-(-)-cotinine nor nicotinamide is a substrate for the enzyme; however, the latter compound did act as a weak competitive inhibitor (Ki = 41.8 X 10(-5) M) of the N-methylation of R-(+)-nicotine.