title: Histamine N-methyl transferase: inhibition by drugs.
abstract: 1. Histamine N-methyl transferase activity was measured in samples of human liver, brain, kidney, lung and intestinal mucosa. The mean (+/- s.d.) rate (nmol min-1 mg-1 protein) of histamine N-methylation was 1.78 +/- 0.59 (liver, n = 60), 1.15 +/- 0.38 (renal cortex, n = 8), 0.79 +/- 0.14 (renal medulla, n = 8), 0.35 +/- 0.08 (lung, n = 20), 0.47 +/- 0.18 (human intestine, n = 30) and 0.29 +/- 0.14 (brain, n = 13). 2. Inhibition of histamine N-methyl transferase by 15 drugs was investigated in human liver. The IC50 for the various drugs ranged over three orders of magnitude; chloroquine was the most potent inhibitor. 3. The average IC50 values for chloroquine were 12.6, 22.0, 19.0, 21.6 microM in liver, renal cortex, brain and colon, respectively. These values are lower than the Michaelis-Menten constant for histamine N-methyltransferase in liver (43.8 microM) and kidney (45.5 microM). Chloroquine carried a mixed non-competitive inhibition of hepatic histamine N-methyl transferase. Some side-effects of chloroquine may be explained by inhibition of histamine N-methyl transferase.
authors.affiliation: Department of Biomedicine, Medical School, University of Pisa, Italy.
authors.forename: G M
authors.initials: GM
authors.lastname: Pacifici
authors.forename: P
authors.initials: P
authors.lastname: Donatelli
authors.forename: L
authors.initials: L
authors.lastname: Giuliani
mesh_terms: D000328:Adult; D000368:Aged; D005260:Female; D006637:Histamine N-Methyltransferase / Q000032:analysis* / Q000037:antagonists & inhibitors*; D006801:Humans; D008297:Male; D008875:Middle Aged; D014018:Tissue Distribution
chemical_list: D006637:Histamine N-Methyltransferase