Absence of isotope effects in the microsomal hydroxylation of acetanilide

Kinetic Deuterium Isotope Effects in Cytochrome P450 Reactions

Cytochrome P450 (P450, CYP) research provides many opportunities for the application of kinetic isotope effect (KIE) strategies. P450s collectively catalyze oxidations of more substrates than any other group of enzymes, and CH bond cleavage is a major feature in a large fraction of these reactions. The presence of a significant primary deuterium KIE is evidence that hydrogen abstraction is at least partially rate-limiting in the reactions, and this appears to be the case in many P450 reactions. The first report of a KIE in (P450-linked) drug metabolism appeared in 1961 (for morphine N-demethylation), and in a number of cases, it has been possible to modulate the in vivo metabolism or toxicity of chemicals by deuterium substitution. A number of efforts are in progress to utilize deuterium substitution to alter the metabolism of drugs in an advantageous manner.

The effect of temperature on monoxygenase reactions in the microsomal membrane

The effect of temperature on the rates of monoxygenase reactions was studied with microsomes prepared from phenobarbital pretreated rats. The rates of the N-demethylation of ethylmorphine, benzphethamine, aminopyrine, and p-nitroanisole were studied. Breaks at temperatures around 24 degrees C were observed in the Arrhenius plots of all these reactions. The energy of activation of these reactions has values of 10-12 and 19-21 kcal per mol at temperature ranges above and below the break temperature, respectively. The break, however, was not observed if 30% glycerol was added to the microsomes. The Arrhenius plot of the microsomal NADPH-cytochrome c reductase activity also did not show any break. The implications of these observations in relationship to the fluidity of the membrane, the translational mobility of membrane enzymes, and the rate of monoxygenase reactions are discussed.

Studies on the metabolism of tripamide, a new antihypertensive agent. I. Characterization of metabolites in rats

1. Tripamide (N-(4-aza-endo-tricyclo[5.2.1.0(2,6)]decan-4-yl)-4-chloro-3-sulphamoyl [carbonyl-14C]benzamide) was metabolized in rat to yield five metabolites which were separated by chromatography, and characterized by mass spectrometry and reverse isotope dilution analysis. 2. The five metabolites were identified as follows; N-(3(or 5)-hydroxy-4-aza-endo-tricyclo[5.2.1.0(2,6)]decan-4-yl)-4-chloro-3-sulphamoylbenzamide (3-hydroxy-tripamide); N-8(or 9)-hydroxy-4-aza-endo-tricyclo[5.2.1.0(2,6)]decan-4-yl)-4-chloro-3-sulphamoylbenzamide (8-hydroxy-tripamide); 4-chloro-3-sulphamoylbenzamide; 4-chloro-3-sulphamoylbenzoic acid; and 4-chloro-3-sulphamoylbenzoic acid-(N'-acetyl)hydrazide. 3. Tripamide was metabolized rapidly and the major metabolite was 4-chloro-3-sulphamoylbenzoic acid in rat blood. 4. After intravenous injection, the excretion rate of the hydroxylated tripamide into bile was about 2-3 times faster than that into urine.