Cytochrome P-450, specificity

Figure 6. Elution profile of protein, radioactivity, and thiocyanate from a Sepha-dex G-25 column of reconstituted monooxygenase system from rat liver that had been incubated with [ 5] parathion. The 5-mL incubation mixture contained 20 nmol Cytochrome P-450 (specific activity 16.4 nmol/mg protein), 5 units NADPH-Cytochrome c reductase, 600 fig dilauroyl L-3-phosphatidylchoUne, 600 fig sodium deoxycholate, and 1 X IO M p 5] parathion. The remainder of the incubation mixture is described in Figure 4. The incubation time was 5 min. One-milliliter fractions were collected. The radioactivity (x) represents cpm/0.1 mL. The OOggo (o) was measured on each 1-mL fraction (20).

Recently Hansson and Wikvall studied 12 -hydroxylation in a reconstituted system consisting of highly purified cytochrome P-450 LM4 from rabbits [101]. The cytochrome P-450 fractions used were electrophoretically homogenous, but the cytochrome P-450 from starved rabbits had up to 4 times higher capacity to catalyse 12a-hydroxylation than had cytochrome P-450 from untreated, phenobarbital-treated, or -naphthoflavone-treated rabbits. It should be mentioned that treatment with /8-naphthoflavone increases the amount of cytochrome P-450 LM4 in the liver. Amino acid analyses, peptide-mapping experiments as well as absorption spectral and circular dichroism spectral analyses revealed physical differences between cytochrome P-450 LM4 preparations from starved and phenobarbital-treated animals. It was concluded that the cytochrome P-450 LM4 fraction was heterogenous and contained a species of cytochrome P-450 specific for 12a-hydroxylation. 

Wang, H.R and T. Kimura (1976). Purification and characterization of adrenal cortex mitochondrial cytochrome P-450 specific for cholesterol side chain cleavage activity. J. Biol. Chem. 251, 6068-6074. 

Hydroxy vitamin D pools ia the blood and is transported on DBF to the kidney, where further hydroxylation takes place at C-1 or C-24 ia response to calcium levels. l-Hydroxylation occurs primarily ia the kidney mitochondria and is cataly2ed by a mixed-function monooxygenase with a specific cytochrome P-450 (52,179,180). 1 a- and 24-Hydroxylation of 25-hydroxycholecalciferol has also been shown to take place ia the placenta of pregnant mammals and ia bone cells, as well as ia the epidermis. Low phosphate levels also stimulate 1,25-dihydtoxycholecalciferol production, which ia turn stimulates intestinal calcium as well as phosphoms absorption. It also mobilizes these minerals from bone and decreases their kidney excretion. Together with PTH, calcitriol also stimulates renal reabsorption of the calcium and phosphoms by the proximal tubules (51,141,181—183). 

Komori M, Nishio K, Kitada M, et al. 1990. Fetus-specific expression of a form of cytochrome P-450 in human liver. Biochemistry 29 4430-4433. 

Another area which needs further investigation is the role of specific forms of cytochrome P-450 in the metabolism of cyclic... 

The metabolism of alkyl lead compounds appears to begin with dealkylation mediated by cytochrome P-450 in the rat, mouse, and rabbit. This step creates triethyl and trimethyl metabolites from tetraethyl and tetramethyl lead. Further biotransformation of these metabolites is highly species specific (Bolanowska 1968 EPA 1986a Kehoe and Thamann 1931). 

It has earlier been suggested to make cytochrome c a more specific reagent for superoxide detection by its acetylation or succinoylation [9-11], It was proposed that acetylation and succinoylation must cause a greater decrease in the reaction of cytochrome c with NADPH cytochrome P-450 reductase than with superoxide due to a decrease in the electrostatic charge of native cytochrome c [12]. However, the rate constant for the most selective succinoylated cytochrome c became about 10% of native cytochrome [13], making this assay even less sensitive. 

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