Studies on 5-hydroxytryptophan decarboxylase. I. In vitro inhibition and substrate interaction - Yuwiler, A.; Geller, E.; Eiduson, S. (Elsevier)
1. An enzyme preparation obtained from quickly frozen hog kidney by the method of Clark et al. (10) was found to decarboxylate l-5-hydroxytryptophan, l-3,4-dihydroxyphenylalanine, l-erythro-3,4-dihydroxyphenylserine, and l-threo-3,4-dihydroxyphenylserine.2. The enzymic activity toward l-5-hydroxytryptophan or l-3,4-dihydroxyphenylalanine was not affected by the addition of 5-hydroxytryptamine, α-methyltryptamine methane sulfonate, 5-(3-indol-acrylolyl)salicylic acid, 1-benzyl-2,5-dimethylserotonin, or (5-oxyindolyl-3) butenone-3 but was inhibited by 2-(3,4-dihydroxyphenyl)ethylamine, m-hydroxypropadrine, norepinephrine, epinephrine, thiosemicarbazide, β-(3-indolyl)acrylic acid, 3,4'-dihydroxy-3'-carboxychalcone and 3,4,4'-trihydroxy-3'-carboxychalcone, and 1-(5-hydroxyindolyl-3)-2-(3-carboxy-4-hydroxybenzoyl)ethylene. The inhibition by 2-(3,4-dihydroxy-phenyl)ethylamine could be reversed by addition of pyridoxal phosphate.3. Mixtures of l-5-hydroxytryptophan and l-3,4-dihydroxyphenylalanine were found to be decarboxylated at a rate intermediate between the decarboxylation rates of the separate substrates.4. The depression in the decarboxylation rate of the mixture was roughly proportional to the molar ratio of l-5-hydroxytryptophan and l-3,4-dihydroxyphenylalanine.5. The intermediate relationship between the decarboxylation rate of the mixture and the individual substrates was approximately the same whether 0 μg., 25 μg., or 20 μmoles of pyridoxal phosphate was included in the system.6. Additive stoichiometry could be obtained with substrate mixtures in the presence of excess enzyme or after relatively long periods of incubation.7. The implications of these findings are discussed with regard to the discrete nature of l-5-hydroxytryptophan and l-3,4-dihydroxyphenylalanine decarboxylases and the possible physiological significance of their interactions.
http://dx.doi.org/10.1016/0003-9861(59)90354-6
Keywords: W; V; U