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PubMed1986

Direct measurements of intramolecular electron transfer rates between cytochrome c and cytochrome c peroxidase: effects of exothermicity and primary sequence on rate.

Rapid mixing of ferrocytochrome c peroxidase [cyt c peroxidase(II)] and ferricytochrome c [cyt c(III)] results in the reduction of cyt c(III) by cyt c peroxidase(II). In 10 mM phosphate, pH 7.0, the rate of decay of cyt c peroxidase(II) and the rate of accumulation of cyt c(II) give equal first-order rate constants: k = 0.23 +/- 0.02 s-1. Equivalent results are obtained by pulse radiolysis using isopropanol radical as the reducing agent. This rate is independent of the initial cyt c(III):cyt c peroxidase(II) ratios. These results are consistent with unimolecular electron transfer occurring within a cyt c(III)-cyt c peroxidase(II) complex. When cyt c is replaced by porphyrin cyt c (iron-free cyt c), a complex still forms with cyt c peroxidase. On radiolysis, using e-aq as the reducing agent, intracomplex electron transfer occurs from the porphyrin cyt c anion radical to cyt c peroxidase(III) with k = 150 s-1. This large rate increase with increasing delta G degrees suggests that the barrier for intracomplex electron transfer is large. Finally, we have briefly investigated how the cyt c peroxidase(II)----cyt c(III) rate depends on the primary structure of cyt c(III). We find the reactivity order to be as follows: yeast (k = 3.4 s-1) greater than horse (k = 0.3 s-1) greater than tuna (k = 0.2 s-1). These results mirror a report [Ho, P. S., Sutoris, C., Liang, N., Margoliash, E. & Hoffman, B. M. (1985) J. Am. Chem. Soc. 107, 1070-1071] on excited state reactions of the cyt c/cyt c peroxidase couple.

Amino Acid Sequence↗
PubMed2001

Cytochrome P450 2D6.1 and cytochrome P450 2D6.10 differ in catalytic activity for multiple substrates.

CYP2D6 is involved in the metabolism of several classes of drugs, including tricyclic antidepressants, selective serotonin reuptake inhibitors and various amphetamines. CYP2D6*10 is an allelic variant, producing an enzyme with Pro34Ser and Ser486Thr amino acid substitutions. Approximately 75% of Asians possess the *10 allele. We sought to further characterize CYP2D6.10 catalytically in vitro in a baculovirus expression system using various substrates and inhibitors, in comparison to CYP2D6.1 (wild-type). Using dextromethorphan (DEX), P-methoxyamphetamine, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and (+/-)3,4-methylenedioxymethamphetamine (MDMA), the ratios of intrinsic clearance (Vmax/Km) of *1 to *10 were 50, 34, 22 and 123, respectively. The CYP2D6 substrates amitriptyline, and (+) and (-) methamphetamine (MAMP) are both p-hydroxylated and N-demethylated (NDM). The intrinsic clearance *1/*10 ratios were 42, 30 and 67 for the p-hydroxylation; and 60, 120 and 157 for the NDM, respectively, illustrating chemical pathway and enantiomeric selectivity for MAMP. It was apparent that (+) and (-) MAMP NDM and MDMA demethylenation were most significantly different in CYP2D6.10. Using DEX as the substrate, the ratios of Ki(*10)/Ki(*1) for inhibitors were: budipine (1.3), sparteine (1.6), debrisoquine (8.1), fluoxetine (16), norfluoxetine (30), paroxetine (14), MDMA (21) and MMDA-2 (7.1), indicating that CYP2D6.10 shows drug-specific altered susceptibility to inhibition. Taken together, these data suggest that CYP2D6*10/*10 individuals may be expected to require different drug doses; and show altered susceptibility to toxicity, interaction risk and, in the case of the amphetamines, drug dependence and toxicity compared to CYP2D6*1/*1 individuals.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine↗