A. ThithapandhaP. SrinootMahidol University2018-03-122018-03-121972-01-01Comparative and General Pharmacology. Vol.3, No.10 (1972), 139-144001040352-s2.0-0015351514https://repository.li.mahidol.ac.th/handle/20.500.14594/100251. 1. The effects of naturally occurring vitamins on two important methylating enzymes, 2. histamine N-methyltransferase and catechol O-methyltransferase, were investigated. It was found that, of all the vitamins tested in vitro, vitamin B 6 (pyridoxal-5′-phosphate) was the most potent inhibitor of both enzymes. 3. 2. Vitamin B 6 inhibited both enzymes competitively with respect to their substrates (histamine or norepinephrine) and non-competitively with respect to S-adenosylmethionine. 4. 3. The inhibition of catechol O-methyltransferase by vitamin B 6 was complex and dependent on magnesium ion concentration. Maximum inhibition was observed when the metal ion concentration was equal to 10 -3 M. © 1972.Mahidol UniversityMedicineArticleSCOPUS10.1016/0010-4035(72)90019-5