title: In vitro characteristics of guinea pig lung aromatic azaheterocycle N-methyltransferase.
abstract: 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.
authors.forename: K C
authors.initials: KC
authors.lastname: Cundy
authors.forename: P A
authors.initials: PA
authors.lastname: Crooks
mesh_terms: D000818:Animals; D003367:Cotinine / Q000378:metabolism; D006168:Guinea Pigs; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007700:Kinetics; D008168:Lung / Q000201:enzymology*; D008297:Male; D008745:Methylation; D008780:Methyltransferases / Q000378:metabolism*; D009536:Niacinamide / Q000378:metabolism; D009538:Nicotine / Q000378:metabolism / Q000494:pharmacology; D013237:Stereoisomerism; D013347:Subcellular Fractions / Q000201:enzymology; D013696:Temperature
chemical_list: D009536:Niacinamide; D009538:Nicotine; D008780:Methyltransferases; C045058:azaheterocycle N-methyltransferase; D003367:Cotinine