Macrolide antibiotics inhibit the degradation of the glucocorticoid-responsive cytochrome P-450p in rat hepatocytes in vivo and in primary monolayer culture

Treatment of rats with macrolide antibiotics such as triacetyloleandomycin (TAO) dramatically increases the hepatic concentration of a cytochrome P-450 indistinguishable from P-450p, the major liver cytochrome induced by glucocorticoids such as dexamethasone (Wrighton, S. A., Maurel, P., Schuetz, E. G., Watkins, P. B., Young, B., and Guzelian, P. S. (1985) Biochemistry 24, 2171-2178). To investigate the mechanism of induction of P-450p, we treated rats for 4 days with these agents and found that dexamethasone and TAO induced the synthesis of P-450p at least 70- and 35-fold over control values, respectively, as estimated from measurements of P-450p mRNA translatable in a cell-free system. However, the accumulation of P-450p holocytochrome (measured as TAO-metabolite spectral complex) or P-450p protein (measured by quantitative immunoblotting) increased at least 150-fold by TAO but only 32-fold by dexamethasone. The possibility that TAO decreases the degradation of P-450p was supported by the observation that administration of TAO to dexamethasone-treated rats labeled with NaH[14C]O3 and [3H]-delta-aminolevulinic acid retarded the decay of radioactive immunoprecipitable P-450p protein (t1/2 = 60 versus 14 h) and heme (t1/2 = 73 versus 10 h). To confirm these results, P-450p protein synthesis was measured as radioactivity incorporated into immunoprecipitable P-450p in primary monolayer cultures of adult rat hepatocytes incubated with [3H]leucine. Dexamethasone treatment of the cultures stimulated P-450p synthesis by at least 30-fold whereas macrolides were without effect. However, macrolide antibiotics but not dexamethasone inhibited the disappearance of radiolabeled P-450p from cultured hepatocytes similar to the results obtained in vivo. We conclude that macrolide antibiotics induce P-450p, the most rapidly turning over cytochrome yet reported, by stimulating its synthesis indirectly and by blocking its degradation, significantly.

Purification and characterization of a new form of cytochrome P-450 (LM2b) from untreated male rabbit liver microsomes

A new form of cytochrome P-450 has been purified from untreated male rabbit liver microsomes. This form was designated P-450 LM2b on the basis of its electrophoretic mobility on SDS polyacrylamide gel, where it migrates as a polypeptide of apparent molecular weight of 50,250. This hemoprotein exhibits a maximum at 448.5 nm in the Soret band of the CO-Ferrous state spectrum. On the basis of its molecular, spectral, enzymologic and immunologic data, P-450 LM2b was shown to be distinct from the other P-450 forms, already characterized in rabbit liver microsomes. However P-450 LM2b and P-450 LM3b appear to be immunologically related proteins.

Identification of an inducible form of cytochrome P-450 in human liver

It has not yet been determined whether human liver contains inducible cytochromes P-450 similar to those that catalyze the oxidative metabolism of foreign substances in animals. We carried out immunoblot analyses of liver microsomes isolated from eight patients and found that each contained a cytochrome P-450, termed HLp, that reacted with antibodies directed against P-450p, a rat liver cytochrome that is inducible by the anti-glucocorticoid pregnenolone-16 alpha-carbonitrile, by glucocorticoids, by anti-seizure drugs, and by such macrolide antibiotics as triacetyloleandomycin. In the two patients who received dexamethasone and anti-seizure medications and in the one patient who was given triacetyloleandomycin, the concentrations of immunoreactive HLp and the ability to demethylate erythromycin and/or to convert triacetyloleandomycin to a metabolite that forms a spectral complex with cytochrome P-450 heme (catalytic properties unique to P-450p in rat liver) were significantly higher as compared to the values for patients who received no inducing drugs. We purified HLp to homogeneity and found that it was immunochemically related to P-450p and to its homologue in the rabbit (LM3c), actively demethylated erythromycin in a reconstituted system, exhibited electrophoretic mobility identical to that of P-450p, and shared 57% homology in its NH2-terminal amino acid sequence with that of a pregnenolone-16 alpha-carbonitrile-inducible rat cytochrome P-450. We conclude that HLp is a human representative of the multigene family of the glucocorticoid-inducible cytochromes P-450.

Demonstration in multiple species of inducible hepatic cytochromes P-450 and their mRNAs related to the glucocorticoid-inducible cytochrome P-450 of the rat

We have recently demonstrated that P-450p, a form of rat liver cytochrome P-450 inducible by steroids such as dexamethasone and pregnenolone-16 alpha-carbonitrile, by the macrolide antibiotic triacetyloleandomycin, and by phenobarbital, is immunochemically related to and shares 73% NH2-terminal amino acid sequence homology with rabbit cytochrome LM3c. Extending this interspecies comparison we now report that liver microsomes prepared from the rabbit, hamster, gerbil, and mouse contain inducible cytochromes P-450 that resemble P-450p in: (a) converting triacetyloleandomycin to a metabolite that forms a distinct spectral complex with cytochrome P-450 heme, (b) catalyzing the demethylation of erythromycin, and (c) reacting on immunoblots with antibodies directed against P-450p or LM3c. These three characteristics changed in parallel within treatment groups of a given species receiving different inducers of cytochrome P-450. However, there were striking qualitative and quantitative interspecies differences in the responses to inducers. For example, rifampicin was the most efficacious inducer of LM3c in the rabbit and yet was not at all an inducer of P-450p in the rat whereas pregnenolone-16 alpha-carbonitrile, an inducer in the rat, failed to induce LM3c in the rabbit. Immunoblot analysis of these microsomes revealed in each species except the rabbit a single immunochemically related protein. A second immunoreactive protein was present in microsomes from male and female and rifampicin- and dexamethasone-treated female rabbits. Two cloned cDNAs, which hybridized to a species of liver mRNA directing the synthesis of P-450p in a cell-free translation system, were used to probe Northern blots of liver RNAs. These revealed a single band of hybridizable mRNA in each species (except RNA from the rabbit which gave no signal even under conditions of reduced stringency) that was induced in qualitative proportions to that of the accumulated immunoreactive protein. We conclude that P-450p appears to be conserved in evolution and is represented in each of the species tested by one or more immunochemically related proteins which exhibit similar catalytic activities to those of P-450p.

Triacetyloleandomycin as inducer of cytochrome P-450 LM3c from rabbit liver microsomes

A cytochrome P-450 LM3 isozyme has been isolated and purified to electrophoretic homogeneity from liver microsomes of New Zealand white rabbits treated with TAO. On the basis of N-terminal sequence analysis and Ouchterlony double diffusion experiments, this isozyme appeared to be closely related to P-450 LM3c isolated from control animals and was designated LM3c (TAO). Anti LM3c (TAO) IgG totally inhibited both erythromycin demethylase and P-450-TAO metabolite complex formation, two monooxygenase activities specifically stimulated by TAO in liver microsomes from male and female rabbits. Moreover, immunoquantitation experiments showed that the level of LM3c (TAO) was increased 10-15 times above control values in liver microsomes from TAO treated male and female rabbits. We conclude that an isozyme identical or closely related to LM3c is the major form of P-450 induced by TAO in rabbit liver microsomes.

Identification of the cytochrome P-450 induced by macrolide antibiotics in rat liver as the glucocorticoid responsive cytochrome P-450p

We administered triacetyloleandomycin (TAO) to rats and found that this macrolide antibiotic is the most efficacious inducer of liver microsomal cytochrome P-450 (P-450) examined to date. Liver microsomes prepared from TAO-treated rats contained greater than 5.0 nmol of P-450/mg of protein and a single induced protein as judged by analysis on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. This protein comigrated with P-450p, the major form of P-450 induced in liver microsomes of rats treated with pregnenolone-16 alpha-carbonitrile (PCN) or dexamethasone (DEX). On immunoblots of such gels developed with antibodies to P-450p, the TAO-induced protein reacted strongly as a single band. There was strict parallelism between the amount of immunoreactive P-450p in liver microsomes prepared from untreated rats or from rats treated with phenobarbital, TAO, DEX, or PCN, the ability of these microsomes to catalyze conversion of TAO to a metabolite which forms a spectral complex, and the ethylmorphine and erythromycin demethylase activities. Antibodies to P-450p specifically blocked microsomal TAO metabolite complex formation and ethylmorphine and erythromycin demethylase activities. Moreover, anti-P-450p antibodies completely immunoprecipitated solubilized TAO metabolite complexes prepared by detergent treatment of liver microsomes obtained from TAO-treated rats. Finally, we found that the major form of P-450 isolated from liver microsomes of TAO-treated rats and purified to homogeneity was indistinguishable from purified P-450p as judged by molecular weights, spectral characteristics, enzymatic activities, ability to bind TAO, peptide maps, and amino-terminal amino acid sequences. We concluded that, in addition to glucocorticoids, macrolide antibiotics are specific inducers of P-450p.

Isolation and partial characterization of a rifampicin induced rabbit liver microsomal cytochrome P-450

Rifampicin administration to New Zealand male rabbits increased the concentration of an LM3 form of cytochrome P-450 to up to 30% of the microsomal P-450 concentration. This enzyme was purified to electrophoretic homogeneity with a yield of 8% of the original total microsomal P-450 concentration. Isolated as a low spin hemoprotein in its substrate free oxidized form, it displays in its reduced CO-complexed form an absorption maximum at 449 nm. Immunological assays, as well as activity measurements, in particular its stereospecific progesterone hydroxylation in the 6 beta-position, show a relationship between LM3,Rif and LM3c (from untreated rabbits).

Inductive and repressive effects of rifampicin on rabbit liver microsomal cytochrome P-450

New Zealand White rabbits were treated with rifampicin at a dose of 50 mg/kg for 4 days. The total amount of microsomal hepatic cytochrome P-450 was not modified in treated, with respect to control, animals. However, further studies involving SDS-PAGE analysis, monooxygenase activity measurements and radial immunodiffusion assays indicated that rifampicin strongly affects the level of two P-450 isoenzymes. An LM3 form was induced; this form, apparently associated with erythromycine demethylase activity and hydroxylation of progesterone preferentially in position 6 beta, was shown to be immunologically and functionally different from LM3a and LM3b. On the other hand, an LM4 form, typically induced by beta-naphthoflavone, was repressed. The concomitant inductive/repressive effect of rifampicine on two cytochrome P-450 isoenzymes makes this drug a very atypical inducer, at least in the rabbit.

Cytochrome P-450 isozyme LM3b from rabbit liver microsomes. Induction by triacetyloleandomycin purification and characterization

Oral administration of triacetyloleandomycin (TAO), 1 mmol/kg/day for 7 days to mature male New Zealand White rabbit results in a significant increase in the content of liver microsomal cytochrome P-450. This increase is accompanied by the occurrence on sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the microsomes of a strong band in the zone of electrophoretic mobility associated with the LM3 isozymes and the stimulation of a number of monooxygenase activities of these microsomes including aminopyrine, chlorcyclizine, TAO, and erythromycin demethylation as well as 2-OH-estradiol and 6 beta OH-testosterone hydroxylation. Cytochrome P-450 LM3 (TAO) from these liver microsomes, purified to electrophoretic homogeneity, had Mr = 52,000 as determined by calibrated sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Comparison with isozymes LM3a, LM3b, and LM3c isolated from control animals, by a number of criteria including spectral data, amino acid content, NH2-terminal sequence analysis, peptide mapping, immunological properties, and monooxygenase activities of reconstituted system, indicated that isozymes LM3 (TAO) and LM3b are very similar, if not identical, proteins. We conclude that TAO must be considered as a new type of inducer of microsomal cytochrome P-450 from rabbit liver.

Polyamines as modulators of drug oxidation reactions catalyzed by cytochrome P-450 from liver microsomes

The effect of polyamines on the activity of the mixed-function oxidase (MFO) system from human, rat and rabbit liver microsomes was investigated in detail. It was shown that polyamine (spermine) stimulates NADPH-dependent activity of the MFO system several-fold whatever the substrate (foreign drug or natural), not only with microsomes but also with the reconstituted system consisting of highly purified cytochrome P-450 (LM2 isozyme), cytochrome P-450 NADPH reductase and dilauroylphosphorylcholine. Stimulation (extent and concentration dependence) appeared to be dependent on a number of parameters such as ionic strength, pH, animal species and treatment, nature of the substrate, and was stereospecific (different effect on 6 beta-and 16 alpha-testosterone hydroxylation). Further, the spermine effect was evaluated on some elementary steps of the cytochrome P-450 reaction cycle, like substrate binding, P-450 reduction and second electron transfer. Finally, it was shown that the organic peroxide dependent activity was not stimulated by spermine with microsomes nor with the purified P-450 LM2 isozyme. On the basis of this study, it was concluded that the locus of polyamine action is cytochrome P-450 and that stimulation could result either from increased stability of the oxyferrous intermediate of P-450 or from an increased rate of second electron transfer from reductase to P-450.

Direct evidence for electron transfer from ferrous cytochrome b5 to the oxyferrous intermediate of liver microsomal cytochrome P-450 LM2

Interaction and electron transfer between highly purified microsomal cytochrome P-450 from phenobarbital-induced rabbits and cytochrome b5 from uninduced rabbits was investigated by difference and stopped-flow spectrophotometry. Formation of a 1:1 complex between ferric P-450 and b5, demonstrated by difference spectrophotometry, was observed only when both cytochromes were incorporated into micelles of phosphatidylcholine. The dissociation constant (Kd) of the complex was decreased from 2.3 microM to 0.4 microM in the presence of 1 mM benzphetamine. The apparent Kd of benzphetamine was reduced from 220 microM to 50 microM upon addition of b5. The influence of ferrous b5 on the autooxidation of the oxyferrous intermediate of P-450 in the presence and in the absence of substrate was investigated by stopped-flow spectrophotometry. Both cytochromes were reduced photochemically, so that experiments could be carried out in the absence of the corresponding reductases and reduced pyridine nucleotides. Kinetic analysis of the data showed that formation of a 1:1 complex between ferrous P-450 and b5 was a prerequisite for electron transfer between the cytochromes. Here again, incorporation of both cytochromes into micelles was absolutely required for this process. Kd was decreased from 7.5 microM to 2.2 microM in the presence of 1 mM benzphetamine. The rate of electron transfer from b5 to oxyferrous P-450, derived from the kinetics of reoxidation of either b5 or P-450, was increased from 2.5 s-1 to about 4 to 7 s-1 in the presence of 1 mM benzphetamine. These results provide the first quantitative data on the electron transfer between the b5 and P-450, whose rate constant is compatible with the observations made on the effect of b5 on hydroxylation reactions catalyzed by the P-450 enzyme system.

Fast photochemical reactions of cytochrome P450 at subzero temperatures

Several reactions of the cytochrome P450 multi-step cycle have been studied by fast light activation combined with subzero temperatures. A flash device was adapted to an Aminco-Chance DW 2 spectrophotometer equipped for subzero temperature thermostatisation. The first electron can be introduced into the cycle by non specific reducing agents such as reduced flavin mononucleotide (FMNH2) or methylviologen radical (MV.). This first reduction remains a fast process even at subzero temperatures. The oxy-compound Fe2+-O2 can thus be formed either directly from Fe2+ or via the photodissociation of the carboxy-ferro adduct. Fe2+-O2 is stable at subzero temperatures towards spontaneous autoxidation as well as further reduction by FMNH2 or MW.. In addition, the recombination of CO after flash photodissociation of Fe2+-CO was used to study in more details the specific behaviors of the purified microsomal cytochrome.

Enzyme behaviour and molecular environment. The effects of ionic strength, detergents, linear polyanions and phospholipids on the pH profile of soluble cytochrome oxidase

The activity vs. pH profile for the oxidation of ferrocytochrome c by purified cytochrome oxidase (ferrocytochrome c:oxygen oxidoreductase, EC 1.9.3.1) was investigated as a function of ionic strength (from 10 to 200 mM) in the absence and in the presence of various perturbants: Tween 20, linear polyanions (RNA, heparin, polyglutamic acid) and phospholipids (asolectin, phosphatidylcholine, phosphatidic acid and cardiolipin). The activation induced by Tween 20 and "zero net charge" phospholipid liposomes was not pH dependent. On the other hand, linear polyanions and polyanionic liposomes strongly perturbed the pH profile, mostly at low ionic strength, by shifting the pH optimum about 1.7 pH units towards alkaline pH values. This effect was reversed by increasing ionic strength. These observations are interpreted in the light of polyelectrolyte theory. Since these results show striking with membrane-bound enzyme, it is concluded that in vivo cytochrome oxidase is located within polyanionic sites of the micochondrial membrane. The activation broght about by phospholipids may result from two posible processes: creation of a hydrophobic environment by the non-polar tails, preventing autoaggregation; and creation of a suitable polyelectrolytic environment by the polar heads (of non zero net charge), increasing the intrinsic reaction rate.