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M J Coon

Publications and source records attributed to M J Coon.

15 recordsLinked to original sources
PubMed1979

Mechanistic studies with purified components of the liver microsomal hydroxylation system: spectral intermediates in reaction of cytochrome P-450 with peroxy compounds.

Recent investigations in this laboratory on the mechanism of action of liver microsomal cytochrome P-450 (P-450 LM) and its interaction with other components of the hydroxylation system are presented. Two electrophoretically homogeneous forms of the cytochrome, phenobarbital-inducible P-450 LM2 and 5,6-benzoflavone-inducible P-450 LM4, so designated according to their relative electrophoretic mobilities, were used in these studies. Phosphatidylcholine is required in the reconstituted enzyme system for rapid electron transfer from NADPH to P-450 LM, catalyzed by NADPH-cytochrome P-450 reductase, as well as for maximal hydroxylation activity with either molecular oxygen or a peroxy compound serving as oxygen donor to the substrate. The phospholipid facilitates the binding of both substrate and reductase to P-450 LM and apparently causes a structural change in the cytochrome as shown by an increase in alpha-helical content, determined by circular dichroic spectrometry. P-450LM3 and LM4 are one-electron acceptors under anaerobic conditions, in accord with previous potentiometric titrations and product yield data, but in disagreement with previous titrations with reducing agents. The cause for the discrepancy between the present and earlier results is not yet fully understood. Stopped flow spectrophotometry was employed to detect intermediates in the reaction of peroxy compounds with P-450LM2. With m-chloroperbenzoic acid the intermediate formed has absorption maxima at 375, 425, and 540 nm in the absolute spectrum and at 370, 436, and 540 nm in the difference spectrum (intermediate minus oxidized form). A study of the magnitude of the spectral change at various peracid concentrations indicated that with this oxidant the reaction shows a dependence resembling a binding curve. These and other experiments with various oxidants, including cumente hydroperoxide, suggest a reversible two-step mechanism according to the reaction: P-450 LM + oxidant equilibrium C equilibrium D, where C may be an enzyme-oxidant complex and D is a spectral intermediate of unknown structure. A scheme is proposed for the mechanism of action of P-450 LM based on these and earlier studies, including evidence from deuterium isotope experiments for the formation of a substrate carbon radical prior to oxygen transfer.

Anaerobiosis↗
PubMed1979

Kinetics of reduction of purified liver microsomal cytochrome P-450 in the reconstituted enzyme system studied by stopped flow spectrophotometry.

Stopped flow studies were undertaken to examine the kinetics of reduction of 5,6-benzoflavone-inducible P-450 LM4 by NADPH in the presence of NADPH-cytochrome P-450 reductase and phospholipid under anaerobic CO at 25 degrees C. The reaction exhibited biphasic kinetics irrespective of NADPH concentration or of the molar ratio of reductase to P-450 LM4. The apparent first order rate constants for the fast and slow phases were determined to be 0.9 to 1.0 and 0.25 s-1, respectively. With the reductase and P-450 LM4 present in equimolar amounts, the total amount of P-450 LM4 reduced increased linearly with NADPH concentration; the titration gave a stoichiometry of 2 mol of NADPH per mol of reductase-cytochrome complex. The NADPH concentration had no appreciable effect on the magnitude of the first order rate constants for the fast and slow phases. The kinetics obtained in the presence of benzphetamine were essentially indistinguishable from those seen in the absence of this substrate, while the amount of P-450 LM4 reduced in the fast phase, but not the rate constant for this phase, decreased when phospholipid was omitted from the reaction mixture. Nearly maximal rates of NADPH oxidation by P-450 LM2 OR LM4 were obtained with a molar ratio of reductase to P-450 LM of 1.0. Benzphetamine enhanced the oxidation of NADPH by P-450 LM2 but had no effect on the activity of P-450 LM4. Rates of NADPH oxidation in the presence of P-450 LM2 and LM4 decreased by 80 and 40%, respectively, when phospholipid was omitted from the reconstituted enzyme system. These studies provide evidence for the formation of a catalytically functional 1:1 complex between the reductase and P-450 LM4, and indicate that P-450 LM2 and LM4 differ in their dependence on phospholipid.

Animals↗
PubMed1978

Regio- and stereoselectivity of various forms of purified cytochrome P-450 in the metabolism of benzo[a]pyrene and (-) trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene as shown by product formation and binding to DNA.

Highly purified cytochromes P-450(LM2) and P-450(LM4) and partially purified P-450(LM1), P-450(LM3b), and P-450(LM7) from rabbit liver microsomes exhibit different catalytic activities in the metabolism of benzo[a]pyrene (BzP) and (-)-trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene [(-)trans-7,8-diol] in a reconstituted enzyme system. The two highly purified cytochromes also exhibit differences in the activation of BzP and (-)trans-7,8-diol to intermediates that bind to DNA, as well as in the stereoselective conversion of (-)trans-7,8-diol to the highly mutagenic and carcinogenic diol-epoxides r-7,t-8-dihydroxy-t-9,10-oxy-7,8,9,10- tetrahydrobenzo[a]pyrene (diol-epoxide I) and r - 7,t - 8 - dihydroxy - c - 9,10 - oxy - 7,8,9,10 - tetrahydrobenzo[a]pyrene (diol-epoxide II). P-450(LM2) is more active than P-450(LM4) in the metabolism of BzP and in its conversion to products that bind to DNA. In contrast, P-450(LM4) is more active than P-450(LM2) in the metabolism of (-)trans-7,8-diol and in its conversion to products that bind to DNA. The ratio of activity (percent substrate metabolized) with BzP relative to that with (-)trans-7,8-diol is 21 for P-450(LM2) and 0.3 for P-450(LM4); P-450(LM1), P-450(LM3b), and P-450(LM7) gave intermediate ratios. Marked stereoselectivity in the oxygenation of the (-)trans-7,8-diol to the highly mutagenic and putatively carcinogenic diol-epoxides I and II was observed with P-450(LM4), whereas the other preparations showed less selectivity. The ratio of diolepoxide I to diol-epoxide II ranges from 0.3 for P-450(LM7) to 11 for P-450(LM4). The substrate specificity and regio- and stereo-selectivity of the different forms of cytochrome P-450 may regulate the balance between activation and detoxification pathways of BzP and therefore determine the susceptibility of individual tissues, strains, and species to the carcinogenic action of BzP.

Animals↗
PubMed1976

Properties of electrophoretically homogeneous phenobarbital-inducible and beta-naphthoflavone-inducible forms of liver microsomal cytochrome P-450.

Procedures are described for the isolation of two forms of rabbit liver microsomal liver microsomal cytochrome P-450 (P-450LM) in homogeneous state. They are designated by their relative electrophoretic mobilities on polyacrylamide gel in the presence of sodium dodecyl sulfate as P-450LM2 and P-450LM4. P-450LM2, which was isolated from phenobarbital-induced animals, has a subunit molecular weight of 48,700. The best preparations contain 20 nmol of the cytochrome per mg of protein and 1 molecule of heme per polypeptide chain. P-450LM4, which is induced by beta-naphthoflavone but is also present in phenobarbital-induced and untreated animals, was isolated from all three sources and found to have a subunit molecular weight of 55,300. The best preparations contain 17nmol of the cytochrome per mg of protein and 1 molecule of heme per polypeptide chain. Some of the purified preparations of the cytochromes, although electrophoretically homogeneous, contain apoenzyme due to heme loss during purification. The purified proteins contain no detectable NADPH-cytochrome P-450 reductase, cytochrome b5, or NADH-cytochrome b5 reductase, and only low levels of phospholipid (about 1 molecule per subunit). Amino acid analysis indicated that P-450LM2 and P-450LM4 are similar in composition, but the latter protein has about 60 additional residues. The COOH-terminal amino acid of P-450LM2 is arginine, as shown by carboxypeptidase treatment, whereas that of P-450LM4 is lysine. NH2-terminal amino acid residues could not be detected. Carbohydrate analysis indicated that both cytochromes contain 1 residue of glucosamine and 2 of mannose per polypeptide subunit. The optical spectra of the oxidized and reduced cytochromes and carbon monoxide complexes were determined. Oxidized P-450LM2 has maxima at 568, 535, and 418 nm characteristic of a low spin hemeprotein, and P450LM4 from beta-naphthoflavone-induced, phenobarbital-induced, or control microsomes has maxima at 645 and 394 nm, characteristic of the high spin state. The spectrum of -450lm4 becomes similar to that of P-450LM2 at high protein concentrations or upon the addition of detergent (Renex), whereas the spectrum of P-450LM2 is unaffected by the protein concentration or the presence of detergent. Electron paramagnetic resonance spectrometry of the purified cytochromes indicated that oxidized -450lm2 is in the low spin state, whereas P-450LM4 is largely, but not entirely, in the high spin state.

Amino Acid Sequence↗
PubMed1975

Biochemical characterization of highly purified cytochrome P-450 and other components of the mixed function oxidase system of liver microsomal membranes.

Cytochrome P-450 has been purified from liver microsomes of phenobarbital-induced rabbits in the presence of ionic and nonionic detergents to concentrations over 17 nmoles per mg of protein. The purified cytochrome P-450 LM gives a single major band on SDS-polyacrylamide gel electrophoresis representing about 90 per cent of the total protein. The polypeptide chain has a molecular weight of about 49,000 daltons. NADPH-cytochrome P-450 reductase has been purified from liver microsomes of phenobarbital-induced rats in the presence of ionic and nonionic detergents to a stage where it catalyzes the reduction of 33,000 nmoles of cytochrome c per min per mg of protein. The ratio of activities toward cytochrome P-450 and cytochrome c is constant throughout purification. The purified reductase contains equimolar amounts of FMN and FAD and gives a single major band on SDA-polyacrylamide gel electrophoresis accounting for about 70 per cent of the total protein; the molecular weight is about 80,000 daltons. The purified cytochrome P-450 is free of cytochrome b5 but contains another electron acceptor, provisionally called Factor C, which is equivalent in amount to the heme present. Two electrons are taken up per molecule of cytochrome P-450 from dithionite or from NADPH in the presence of catalytic amounts of the reductase, and both electrons are readily transferred from the reduced cytochrome P-450 to molecular oxygen or artificial electron acceptors. The reconstituted enzyme system containing purified cytochrome P-450, purified NADPH-cytochrome P-450 reductase, and phosphatidylcholine retains the ability to catalyze the hydroxylation of drugs, fatty acids, hydrocarbons, and aniline in the presence of NADPH and molecular oxygen.

Anaerobiosis↗