The molecular mechanisms of two common polymorphisms of drug oxidation--evidence for functional changes in cytochrome P-450 isozymes catalysing bufuralol and mephenytoin oxidation

Drug metabolizing enzymes related to laboratory medicine: cytochromes P-450 and UDP-glucuronosyltransferases

Many studies on drug metabolism have been carried out during the last decades using protein purification, molecular cloning techniques and analysis of polymorphisms at phenotype and genotype levels. These researchers led to a better understanding of the role of drug metabolizing enzymes in the biotransformation of drugs, pollutants or foreign compounds and of their use in laboratory medicine. The metabolic processes commonly involved in the biotransformation of xenobiotics have been classified into functionalization reaction (phase I reactions), which implicate lipophilic compounds. These molecules are modified via monooxygenation, dealkylation, reduction, aromatization, hydrolysis and can be substrates for the phase II reactions, often called conjugation reactions as they conjugate a functional group with a polar, endogenous compound. This review, devoted to cytochromes P-450 (CYP) and UDP-glucuronosyltransferases (UGT), describes essentially the genetic polymorphisms found in humans, their clinical consequences and the methods to assess the phenotypes or genotypes, with a view to studying the interindividual differences in drug monooxygenation and drug glucuronidation. Variations in drug glucuronidation reported here focused essentially on variations due to physiological factors, induction, drug interactions and genetic factors in disorders such as Gilbert's Syndrome and Crigler-Najjar type I and II diseases.

Analysis of imipramine and three metabolites produced by isozyme CYP2D6 expressed in a human cell line

1. A commercially-available human cytochrome P450 isozyme (CYP2D6) preparation was used in imipramine metabolism studies. This isozyme catalysed both aromatic C-oxidation and N-demethylation. 2-Hydroxyimipramine was the major metabolite; desipramine was isolated in a significant amount and 2-hydroxydesipramine was a trace metabolite. 2. To prevent decomposition of metabolites during the analytical procedure, the metabolism mixture was derivatized with acetic anhydride prior to extraction, and the derivatized metabolites were separated and quantified by g.l.c. with N/P detection. The analytical procedure had excellent sensitivity and was capable of routinely quantifying imipramine and its metabolites down to the 0.36 nmol level. 3. In excess of 90% of drug and metabolites was consistently recovered when metabolism was conducted over a 5-60-min duration. 4. The formation of the secondary metabolite, 2-hydroxydesimipramine, from imipramine proceeds by two pathways, via desipramine and via 2-hydroxyimipramine; the former is the preferred pathway. 5. CYP2D6 catalyses C-hydroxylation of imipramine to 2-hydroxyimipramine more efficiently than its N-demethylation to desipramine. Also, the C-hydroxylation of imipramine to 2-hydroxyimipramine proceeds more efficiently than the conversion of desipramine to 2-hydroxydesipramine.

A preliminary 3D model for cytochrome P450 2D6 constructed by homology model building

A homology model building study of cytochrome P450 2D6 has been carried out based on the crystal structure of cytochrome P450 101. The primary sequences of P450 101 and P450 2D6 were aligned by making use of an automated alignment procedure. This alignment was adjusted manually by matching alpha-helices (C, D, G, I, J, K and L) and beta-sheets (beta 3/beta 4) of P450 101 that are proposed to be conserved in membrane-bound P450s (Ouzounis and Melvin [Eur. J. Biochem., 198 (1991) 307]) to the corresponding regions in the primary amino acid sequence of P450 2D6. Furthermore, alpha-helices B, B' and F were found to be conserved in P450 2D6. No significant homology between the remaining regions of P450 101 and P450 2D6 could be found and these regions were therefore deleted. A 3D model of P450 2D6 was constructed by copying the coordinates of the residues from the crystal structure of P450 101 to the corresponding residues in P450 2D6. The regions without a significant homology with P450 101 were not incorporated into the model. After energy-minimization of the resulting 3D model of P450 2D6, possible active site residues were identified by fitting the substrates debrisoquine and dextrometorphan into the proposed active site. Both substrates could be positioned into a planar pocket near the heme region formed by residues Val370, Pro371, Leu372, Trp316, and part of the oxygen binding site of P450 2D6. Furthermore, the carboxylate group of either Asp100 or Asp301 was identified as a possible candidate for the proposed interaction with basic nitrogen atom(s) of the substrates.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of debrisoquine hydroxylation phenotype on the pharmacokinetics of mexiletine

Marked interindividual variation has been observed in the pharmacokinetics of the antiarrhythmic agent mexiletine. The fact that its urinary excretion is dependent on urinary pH may account, in part, for such variation. The influence that genetic differences in hepatic metabolism of the debrisoquine-type may have on mexiletine pharmacokinetics was considered in this study. The pharmacokinetics and urinary excretion of mexiletine (250 mg administered intravenously) were investigated in 5 rapid extensive metabolisers (EM), 5 slow EM and 5 poor metabolisers (PM) of debrisoquine, under conditions of controlled urinary pH. Mexiletine disposition kinetics was found to be altered in PM individuals. These subjects showed higher total area under the curve (AUC), (15.7 versus 8.16 micrograms.h.ml-1) prolonged elimination half-lives (in serum and urine) (serum: 18.5 versus 11.6 h, urine: 19.2 versus 11.7 h) and lower total clearance values compared with EM (216 versus 450 ml.min-1). In this respect, slow EM individuals generally presented intermediate values of those pharmacokinetic parameters. A higher incidence of adverse-effects was also observed among slow EM and PM subjects. It is concluded that genetic differences in mexiletine oxidation of the debrisoquine-type have an influence on its observed pharmacokinetic variability. The clinical consequences are discussed.

In vitro forecasting of drugs that may interfere with codeine bioactivation

The O-demethylation of codeine (methylmorphine) into morphine is mediated by the polymorphic cytochrome P450 DB1 (P450 IID6). By means of in vitro screening in human liver microsomes we have studied the effect on codeine bioactivation of several drugs used as analgesics or as adjuvants for pain control. In microsomes from an extensive metabolizer subject, paracetamol (acetaminophen) and NSAIDs (acetylsalicylic acid, diclofenac, indomethacin, piroxicam, and pirprofen), benzodiazepines (chlordiazepoxide, clonazepam, diazepam, flunitrazepam, and midazolam), and anticonvulsants (carbamazepine and phenytoin) did not alter the reaction. There was marked inhibition of in vitro morphine production by neuroleptics (chlorpromazine, haloperidol, levomepromazine, and thioridazine), metoclopramide, and tricyclic antidepressants (amitriptyline, clomipramine, desipramine, imipramine, and nortriptyline). Enzyme kinetics showed competitive inhibition by neuroleptics (chlorpromazine Ki = 0.5 microM) and antidepressants (clomipramine Ki = 6.8 microM), which are substrates of the polymorphic monooxygenase. Due to the low affinity of codeine for P450 DB1 (Km = 100-200 microM), its bioactivation in extensive metabolizers, and thus its analgesic efficacy, is liable to vary greatly when it is combined with any drug that has a high affinity for the polymorphic isozyme.

Differential foetal development of the O- and N-demethylation of codeine and dextromethorphan in man

1. Codeine and dextromethorphan were N-demethylated in human foetal liver microsomes at high rates which were close to the activities in adult livers. In contrast, foetal liver microsomes did not catalyze the O-demethylation of these drugs at mid-gestation. 2. The metabolic data were in accordance with the absence of P450IID6 and the presence of P450 IIIA as determined by Western blotting with anti-human P450 IID6 (MAb 114/2) and anti-rat P450 IIIA (PCN 2-13-1/C2) monoclonal antibodies, respectively. 3. The inhibitory effects of midazolam and dehydroepiandrosterone support the contention that the N-demethylase is a human foetal form of the cytochrome P450 IIIA family. Consistent with this we found that blotting with the MAb PCN 2-13-1/C2, which recognizes an epitope specific for the P450 III family, correlated well with the N-demethylase activities.

Differences in phenytoin biotransformation and susceptibility to congenital malformations: a review

The clinical variability of teratogenic response to fetal drug exposure has been well documented. Metabolic differences in biotransformation have been shown to extend to multiple drugs and may involve many steps in drug metabolism with alterations of key intermediates. Although metabolic differences have been reported to be associated with complications of medication use, it has only recently been appreciated that such differences also may be associated in the unborn with the potential for the disruption of normal embryologic development and the production of congenital malformations. It has long been suspected that the teratogenicity of phenytoin may be mediated not only by the parent compound, but also by toxic intermediary metabolites that are produced during the biotransformation of the parent compound. Recent work elucidating differences in isoenzyme forms of cytochrome P-450 enzyme systems, glutathione, and microsomal epoxide hydrolase has provided increased interest in the multiple individual pharmacogenetic differences that may be significant factors affecting increased susceptibility to birth defects in individuals and families with fetal exposure to phenytoin.

Regioselectivity and stereoselectivity of the metabolism of the chiral quinolizidine alkaloids sparteine and pachycarpine in the rat

1. The metabolism of (-)-sparteine and (+)-sparteine (pachycarpine) was investigated in male Sprague-Dawley rats by g.l.c.-mass spectrometry, and 13C- and 2H-n.m.r. spectroscopy. The structure of the major metabolite of (-)-sparteine was confirmed to be 2,3-didehydrosparteine by g.l.c.-mass spectrometry after alkaline sample work-up. 2H-n.m.r. spectroscopy showed that this metabolite exhibits the structure of the carbinolamine (2S)-hydroxysparteine in aqueous solution of neutral pH. No other metabolites with an enamine structure were observed by g.l.c.-mass spectrometry and 13C-n.m.r. spectroscopy. 2. Pachycarpine is metabolized in vivo and in vitro stereoselectively to the aliphatic alcohol (4S)-hydroxypachycarpine as the main metabolite. 3. The formation of the 2,3-didehydrosparteine proceeds via stereospecific abstraction of the axial 2 beta hydrogen atom. Inhibition in vitro studied with purified rat liver microsomes demonstrated that both sparteine enantiomers are metabolized by the same cytochrome P450 isozyme. Therefore this enzyme exhibits marked substrate and product stereoselectivity for the metabolism of the two enantiomeric quinolizidine alkaloids.

Propranolol oxidation by human liver microsomes--the use of cumene hydroperoxide to probe isoenzyme specificity and regio- and stereoselectivity

1. Three oxidations of the enantiomers of propranolol were studied in human liver microsomes under two reaction conditions. Previous in vitro studies had established that two of the livers were from poor metaboliser (PM) phenotypes for the debrisoquine 4-hydroxylase (cytochrome P-450IID6) and the remaining seven were from extensive metaboliser (EM) phenotypes. 2. In the presence of NADPH and oxygen 4- and 5-hydroxylation of propranolol occurred in microsomes from all nine livers, as did propranolol N-desisopropylation. R(+)-propranolol was oxidized preferentially along the three pathways, although enantioselectivity observed for N-desisopropylation may have arisen not only from stereoselectivity in formation rates, but also from stereoselectivity in subsequent microsomal metabolism, possibly by monoamine oxidase. The involvement of monoamine oxidase in the further microsomal metabolism of N-desisopropylpropranolol was indicated by inhibition of the metabolism of this compound when incubated with phenelzine. 3. Cumene hydroperoxide has been proposed to support only the activity of cytochrome P450IID6. This is consistent with the observations that a) propranolol 4- and 5-hydroxylation occurred in microsomes from the EM livers only and b) side-chain oxidation was not observed under these conditions in either PM or EM livers. 4. Using cumene hydroperoxide to support the reactions, the 4-hydroxylation of propranolol showed little enantioselectivity, whereas S(-)-propranolol was 5-hydroxylated about twice as fast as the R(+)-enantiomer. There were highly significant correlations between the rates of 4- and 5-hydroxylation of R(+)-propranolol (r = 0.96, P less than 0.001, n = 7 livers) and of S(-)-propranolol (r = 0.98, P less than 0.001). Both oxidations were described by single-site Michaelis-Menten kinetics. 5. The findings suggest that P450IID6 is involved in both the 4- and 5-hydroxylations of propranolol, but that these metabolites can also be formed by other P450 isoenzymes. It is confirmed that P450IID6 does not contribute to the N-desisopropylation of propranolol. Furthermore, the finding that mephenytoin did not inhibit the appearance of this metabolite is not consistent with the results of in vivo studies suggesting the involvement of the same enzyme in the side-chain oxidation of propranolol and the 4-hydroxylation of mephenytoin.

Identification of the primary gene defect at the cytochrome P450 CYP2D locus

The mammalian cytochrome P450-dependent monooxygenase system is involved in the metabolism of drugs and chemical carcinogens. The role of these enzymes in toxicological response is exemplified by an autosomal recessive polymorphism at the cytochrome P450 CYP2D6 debrisoquine hydroxylase locus which results in the severely compromised metabolism of at least 25 drugs, and which in some cases can lead to life-threatening side-effects. In addition, this polymorphism, which affects 8-10% of the caucasian population, has been associated with altered susceptibility to lung and bladder cancer. Here we report the identification of the primary mutation responsible for this metabolic defect and the development of a simple DNA-based genetic assay to allow both the identification of most individuals at risk of drug side-effects and clarification of the conflicting reports on the association of this polymorphism with cancer susceptibility.

Genetic polymorphism of sparteine/debrisoquine oxidation: a reappraisal

Polymorphic oxidation of the sparteine/debrisoquine-type has been shown to account for much of the interindividual variation in the metabolism, pharmacokinetics and pharmacodynamics of an increasing number of drugs, including some antiarrhythmic, antidepressant and beta-adrenoceptor antagonist agents. Impaired hydroxylation of these drugs results from the absence of the enzyme cytochrome P450IID6 in the livers of poor metabolisers, who constitute 6% to 10% of Caucasian populations. The clinical importance of the phenomenon has to be explored further and for most sparteine/debrisoquine-related substrates there is a need for controlled prospective studies to define the consequences to the patient of impaired or enhanced drug oxidation.

Rapid screening for polymorphisms in dextromethorphan and mephenytoin metabolism

1. The phenotyping parameters for dextromethorphan and mephenytoin were assessed in 48 normal male volunteers following administration of each metabolic probe drug on separate occasions and together according to a randomized 3-way crossover design. 2. Neither the urinary S-/R-mephenytoin ratio nor the dextromethorphan metabolic ratio were altered significantly by coadministration of the probe drugs. 3. Five-hundred and nineteen subjects were screened for expression of mephenytoin 4-hydroxylase and dextromethorphan O-demethylase activity following the coadministration of mephenytoin and dextromethorphan. The activity was determined in each case by methods not requiring any quantitative measurements. 4. Nineteen (3.7%) of the subjects were identified as poor metabolizers (PMs) of mephenytoin and 35 subjects (6.7%) as PMs of dextromethorphan. 5. All PMs of dextromethorphan were confirmed by more rigorous evaluation of the metabolic ratio.

The genetic polymorphism of debrisoquine/sparteine metabolism--clinical aspects

It has been established that the metabolism of more than twenty drugs, including antiarrhythmics, beta-adrenoceptor antagonists, antidepressants, opiates and neuroleptics is catalyzed by cytochrome P-450dbl. The activity of this P-450 isozyme is under genetic rather than environmental control. This article discusses the therapeutic implications for each of the classes of drugs affected by this genetic polymorphism in drug metabolism. Not only are the problems associated with poor metabolizers who are unable to metabolize the compounds discussed, but it is also emphasized that it is difficult to attain therapeutic plasma concentrations for some drugs in high activity extensive metabolizers.

The genetic polymorphism of debrisoquine/sparteine metabolism-molecular mechanisms

The genetic polymorphism of debrisoquine/sparteine metabolism is one of the best studied examples of a genetic variability in drug response. 5-10% of individuals in Caucasian populations are 'poor metabolizers' of debrisoquine, sparteine and over 20 other drugs. The discovery and the inheritance of deficient debrisoquine/sparteine metabolism are briefly described, followed by a detailed account of the studies leading to the characterization of the deficient reaction and the purification of cytochrome P-450IID1, the target enzyme of this polymorphism. It is demonstrated by immunological methods that deficient debrisoquine hydroxylation is due to the absence of P-450IID1 protein in the livers of poor metabolizers. The cloning and sequencing of the P-450IID1 cDNA and of IID1 related genes are summarized. The P-450IID1 cDNA has subsequently led to the discovery of aberrant splicing of P-450IID1 pre-mRNA as the cause of absent P-450IID1 protein. Finally, the identification of mutant alleles of the P-450IID1 gene (CYP 2D) by restriction fragment length polymorphisms in lymphocyte DNA of poor metabolizers is presented.

Alternative splicing in the human cytochrome P450IIB6 gene generates a high level of aberrant messages

Polymorphisms within the human cytochrome P450 system can have severe clinical consequences and have been associated with adverse drug side effects and susceptibility to environmentally linked diseases such as cancer. Aberrant splicing of cytochrome P450 mRNA has been proposed as a potential mechanism for these polymorphisms. We have isolated aberrantly, as well as normally, spliced mRNAs (cDNAs) from the human P450IIB6 gene which either contain part of intron 5 and lack exon 8 or which contain a 58-bp fragment (exon 8A) instead of exon 8. Sequence analysis of the P450IIB6 gene demonstrates the presence of cryptic splice sites in intron 8 which will account for the generation of exon 8A. The mRNAs were therefore generated by alternative splicing. These data gain significance as the mRNAs will not encode a functional P450 enzyme and appear to represent a high proportion of the P450IIB6 mRNA population. Analysis of mRNA from fifteen individual human livers and cDNA libraries constructed from a variety of human tissues using the polymerase chain reaction shows that the aberrant splicing occurs in all cells and all individuals tested. This suggests a high level of infidelity in the processing of P450IIB6 mRNAs and demonstrates that the presence of abnormal transcripts does not imply the presence of a functionally inactive gene.

Effect of quinidine on the dextromethorphan O-demethylase activity of microsomal fractions from human liver

1. The kinetics of dextromethorphan O-demethylation were measured in microsomes prepared from five human livers, both in the absence and in the presence of quinidine. 2. For each liver and over the concentration range of dextromethorphan examined (4.2-3400 microM), this reaction involved an enzymatic component of high affinity, with an apparent Michaelis-Menten constant (Km) of 4.6 +/- 1.8 microM (mean +/- s.d.) and a maximum velocity (Vmax) of 4.2 +/- 3.5 nmol mg-1 h-1 (mean +/- s.d.). 3. Quinidine was a potent and competitive inhibitor of the activity of this component (mean Ki +/- s.d. of 0.025 +/- 0.008 microM) as it is for other oxidation reactions which have already been found to co-segregate with the debrisoquine-type polymorphism. 4. With microsomes from four of the five livers studied, there was evidence of a second enzymatic component of activity characterized by a similar Vmax and about 20-fold higher Km compared with the high affinity component. The activity of this low affinity component was unaffected by quinidine in the concentrations studied.

Selective in vivo inhibition by quinidine of methoxyphenamine oxidation in rat models of human debrisoquine polymorphism

1. Lewis and Dark Agouti (DA) rat strains (n = 4), models of human extensive and poor metabolizer phenotypes of debrisoquine/sparteine, respectively, were dosed with methoxyphenamine with and without prior administration of quinidine. Methoxyphenamine and its three metabolites, namely N-desmethylmethoxyphenamine, O-desmethylmethoxyphenamine and 5-hydroxymethoxyphenamine were quantified in 0-24 h urine. 2. The oxidative metabolic routes of methoxyphenamine which had been previously shown to involve the debrisoquine/sparteine isozyme, namely O-demethylation and aromatic 5-hydroxylation, were both significantly inhibited by quinidine in the two rat strains. 3. The oxidative metabolic route of methoxyphenamine which had been previously shown to not involve the debrisoquine/sparteine isozyme, namely N-demethylation, was not significantly inhibited by quinidine in either rat strain. 4. The Lewis strain pretreated with quinidine resembled the DA strain without such pretreatment in terms of O-desmethylmethoxyphenamine and 5-hydroxymethoxyphenamine in that the mean percentages of the dose excreted as these two metabolites and the mean O-desmethylmethoxyphenamine/methoxyphenamine and 5-hydroxymethoxyphenamine/methoxyphenamine ratios were similar to one another. 5. Ten days after quinidine administration to the Lewis strain of rat, all parameters of methoxyphenamine and its metabolites returned to normal. 6. A protocol involving substrate administration to Lewis strain rats with and without prior administration of quinidine could be developed as an attractive approach to screen substrates for metabolism in vivo by the debrisoquine/sparteine isozyme. Such an approach obviates interstrain differences.

Enantioselective gas chromatographic assays with electron-capture detection for methoxyphenamine and its three primary metabolites in human urine

Sensitive and enantioselective gas chromatographic assays have been developed and applied to the quantitation in human urine of the enantiomers of methoxyphenamine and its three primary oxidative metabolites, namely, N-desmethylmethoxyphenamine, O-desmethylmethoxyphenamine and 5-hydroxymethoxyphenamine. The separation of the various analytes was achieved through the combined use of high-resolution gas chromatography coupled with electron-capture detection and employing a capillary OV-225 column. The formation of diastereometric derivatives involved the chiral acylating reagent N-heptafluorobutyryl-L-prolyl chloride. The assays for methoxyphenamine and O-desmethylmethoxyphenamine were linear over the range 0.25-2.0 micrograms/ml for each analytes' enantiomers, while in the case of the enantiomers for N-desmethylmethoxyphenamine and 5-hydroxymethoxyphenamine linearity was shown over the ranges 0.094-0.75 and 0.188-1.5 micrograms/ml, respectively. The mean coefficients of variation in all cases were less than 4%.

Clinical significance of the sparteine/debrisoquine oxidation polymorphism

The sparteine/debrisoquine oxidation polymorphism results from differences in the activity of one isozyme of cytochrome P450, the P450db1 (P450 IID1). The oxidation of more than 20 clinically useful drugs has now been shown to be under similar genetic control to that of sparteine/debrisoquine. The clinical significance of this polymorphism may be defined by the value of phenotyping patients before treatment. The clinical significance of such polymorphic elimination of a particular drug can be analyzed in three steps: first, does the kinetics of active principle of a drug depend significantly on P450db1?; second, is the resulting pharmacokinetic variability of any clinical importance?; and third, can the variation in response be assessed by direct clinical or paraclinical measurements? It is concluded from such an analysis that, in general, the sparteine/debrisoquine oxidation polymorphism is of significance in patient management only for those drugs for which plasma concentration measurements are considered useful and for which the elimination of the drug and/or its active metabolite is mainly determined by P450db1. At present, this applies to tricyclic antidepressants and to certain neuroleptics (e.g. perphenazine and thioridazine) and antiarrhythmics (e.g. propafenone and flecainide). Phenotyping should be introduced in to clinical routine under strictly controlled conditions to afford a better understanding of its potentials and limitations. The increasing knowledge of specific substrates and inhibitors of P450db1 allows precise predictions of drug-drug interactions. At present, the strong inhibitory effect of neuroleptics on the metabolism of tricyclic antidepressants represents the best clinically documented and most relevant example of such an interaction.

Xenobiotic and endobiotic inhibitors of cytochrome P-450dbl function, the target of the debrisoquine/sparteine type polymorphism

Five to 10% of Caucasians are poor metabolizers (PM) of debrisoquine, sparteine, bufuralol and numerous other drugs. A deficiency in cytochrome P-450dbl (P-450dbl) function is the cause of this polymorphism of drug oxidation with autosomal recessive inheritance. In the present study, inhibition of bufuralol-1'-hydroxylase in human liver microsomes by drugs and chemicals was performed in a search for potential new substrates for this polymorphic enzyme. Among the 80 alkaloids and drugs tested, 25 were competitive inhibitors. In vitro competitive inhibition of bufuralol oxidation by a substance indicates that this compound is able to bind to the same enzymatic site as bufuralol. This may mean that the competing drug also is metabolized by P-450dbl and that its metabolism is subject to the same genetic variation as the oxidation of bufuralol. However, some of these competitive inhibitors are not oxidized by P-450dbl. In this case, however, they may interfere with the in vivo phenotyping procedure by inhibiting the formation of metabolites of test drugs such as debrisoquine, sparteine, metoprolol or dextrometorphan.

Ifosfamide plasma clearance in relation to polymorphic debrisoquine oxidation

Ifosfamide (IF) pharmacokinetics and the plasma (NBP)-alkylating activity were determined in 33 patients with different tumours after the administration of IF as single-agent chemotherapy. All subjects had been phenotyped for debrisoquine oxidation. There is a lack of correlation between the debrisoquine metabolic ratio (DMR) and either the total plasma clearance of IF (CLIF) or the AUC of the plasma NBP-alkylating activity.

Absence of hepatic cytochrome P450bufI causes genetically deficient debrisoquine oxidation in man

The common genetic deficiency of drug oxidation known as debrisoquine/sparteine-type polymorphism was investigated with bufuralol as prototype substrate. In human liver microsomes the 1'-hydroxylation of bufuralol is catalyzed by two functionally distinct P-450 isozymes, the high-affinity/highly stereoselective P450bufI and the low-affinity/nonstereoselective P450bufII. We demonstrate that P450bufI is unique in hydroxylating bufuralol in a cumene hydroperoxide (CuOOH) mediated reaction whereas P450bufII is active only in the classical NADPH- and O2-supported monooxygenation. In microsomes of liver biopsies of in vivo phenotyped poor metabolizers of debrisoquine or sparteine, the CuOOH-mediated activity was drastically reduced. Rabbit antibodies against a rat P-450 isozyme with high bufuralol 1'-hydroxylase activity (P450db1) precipitated exclusively P450bufI-type activity from solubilized microsomes. Western blotting of microsomes with these antibodies revealed a close correlation between the immunoreactive protein and CuOOH-mediated (+)-bufuralol 1'-hydroxylation. No immunoreactive protein was detected in liver microsomes of in vivo phenotyped poor metabolizers. These data provide evidence for a specific deficiency of P450bufI and are consistent with the complete or almost complete absence of this protein in the liver of poor metabolizers.

A novel human cytochrome P450 gene (P450IIB): chromosomal localization and evidence for alternative splicing

We have isolated from a single human liver cDNA library two clones which are highly homologous (78% over the coding region) to the major phenobarbital-inducible P450 from rat (P450IIB1). This is the first direct demonstration of the presence of the P450IIB gene subfamily in humans. This subfamily is much less extensive than the rodent homologues, but does appear to contain at least two genes. Of the cDNA clones isolated one is apparently normally spliced, whereas the other lacks exon 8 and retains all or part of intron 5. Both clones contain transcribed Alu sequences. The human P450IIB gene has been located to chromosome 19q12----19q13.2 using a probe derived from intron 5, and is close to the CYP 2A locus encoding cytochrome P450IIA2. Restriction fragment length polymorphisms have been found with the enzymes BamHI and MspI which will enable linkage to be determined between these two loci.

Two mutant alleles of the human cytochrome P-450db1 gene (P450C2D1) associated with genetically deficient metabolism of debrisoquine and other drugs

The "debrisoquine polymorphism" is a clinically important genetic defect of drug metabolism affecting 5-10% of individuals in Caucasian populations. It is inherited as an autosomal recessive trait. A full-length cDNA for human cytochrome P-450db1, the deficient enzyme (also designated P450IID1 for P450 family II subfamily D isozyme 1), has recently been cloned. Leukocyte DNA from "extensive metabolizers" (EMs) or "poor metabolizers" (PMs) of debrisoquine was examined by Southern analysis. Two polymorphic restriction fragments were associated with the PM phenotype when DNAs from 24 unrelated PM and 29 unrelated EM individuals were probed with P-450db1 cDNA after digestion with Xba I restriction endonuclease and Southern blotting: a polymorphic 44-kilobase (kb) fragment was found in 58% of PMs but only in 3.4% of EMs, and a polymorphic 11.5-kb fragment was present in 33% of PMs but in none of the EMs. Seventy-five percent of PMs had either the 44-kb or the 11.5-kb fragment or both. Segregation of these restriction fragment length polymorphisms in the families of six PM probands demonstrated that each of the two fragments is allelic with the 29-kb fragment present in all EM individuals and suggests that they identify two independent mutated allels of the P-450db1 gene (designated P450C2D1). At least a third mutated allele not detected by these restriction fragment length polymorphisms must be present in the population. The Xba I 44-kb fragment and 11.5-kb fragment were in linkage disequilibrium with restriction fragment length polymorphisms generated by four and five additional restriction endonucleases, respectively, which can be used to identify the same mutant alleles for the P-450db1 gene.

Enantioselectivity of 4-hydroxylation in extensive and poor metabolizers of debrisoquine

Debrisoquine (DQ) has no chiral centre, but hydroxylation in position 4 leads to formation of an asymmetric carbon centre with two possible enantiomers, their absolute configuration being R(-) and S(+)-4-hydroxydebrisoquine (4-OHDQ). Since the absolute stereochemistry of the 4-hydroxylation of DQ in man is unknown, the enantioselectivity of this process was studied in panels of extensive (EM) and poor metabolizers (PM) of DQ. In EM subjects 4-hydroxylation of DQ leads almost exclusively to the formation of S(+)-4-OHDQ. In contrast, PM subjects were not only characterized by a decreased total 4-OHDQ formation but also a marked loss of enantioselectivity in product formation. Between 5 to 36% of total 4-OHDQ was excreted as R(-)-4-OHDQ.

Characterization of the common genetic defect in humans deficient in debrisoquine metabolism

In population studies of individuals given the antihypertensive drug debrisoquine, two distinct phenotypes have been described: extensive metabolizers excrete 10-200 times more of the urinary metabolite 4-hydroxydebrisoquine than poor metabolizers. In family studies the poor-metabolizer phenotype behaves as an autosomal recessive trait with an incidence between 5% and 10% in the white population of Europe and North America, and extends to the deficient metabolism of more than 20 commonly prescribed drugs. Clinical studies have shown that such individuals are at high risk for the development of adverse side effects from these and probably many other drugs. Here we show that poor metabolizers have negligible amounts of the cytochrome P450 enzyme P450db1. We have cloned the human P450db1 complementary DNA and expressed it in mammalian cell culture. Furthermore, by directly cloning and sequencing cDNAs from several poor-metabolizer livers, we have identified three variant messenger RNAs that are products of mutant genes producing incorrectly spliced db1 pre-mRNA, providing a molecular explanation for one of man's most commonly defective genes (frequency of mutant alleles 35-43%).

Human debrisoquine 4-hydroxylase (P450IID1): cDNA and deduced amino acid sequence and assignment of the CYP2D locus to chromosome 22

The enzyme P450db1 (db1) is responsible for the common human defect in drug oxidation known as the "debrisoquine/sparteine polymorphism." Polyclonal antibody against the rat db1 protein was used to screen a human liver lambda gt11 library for the db1 cDNA clone. A cDNA containing the full protein coding sequence was isolated; the deduced NH2-terminal sequence of this cDNA was identical to that derived from direct sequencing of the purified human db1 protein. Comparison of the human db1 with rat db1 revealed 71 and 73% similarities of nucleotides and amino acids, respectively. By use of human-rodent somatic cell hybrids the db1 gene was localized to human chromosome 22 (CYP2D locus).

Pharmacokinetic and pharmacodynamic consequences of stereoselective drug metabolism in man

The examples discussed demonstrate the importance of stereoselective drug metabolism and raise the question of whether the therapeutic use of racemic drugs is still justified. There is no straightforward answer to this question. If only quantitative differences in therapeutic activity exist and the less active enantiomer is not predominantly responsible for side effects, the therapeutic benefit gained by using the more active enantiomer is only marginal and does not justify the substantial increase in costs involved in manufacturing such a drug preparation. However, if stereoselectivity in therapeutic activity is pronounced and adverse drug reactions are caused mainly by the less active isomer then an isomeric pure drug preparation should be used.

Substrate and product stereoselectivity in monooxygenase-mediated drug activation and inactivation

In this overview, stereoselective aspects of drug metabolism have been examined in a biochemical and pharmacodynamic perspective. From the facts and concepts presented, the conclusion to emerge is that the pharmacokinetic behaviour of mixtures of stereoisomers (e.g. racemates) is not always the simple addition of the behaviour of individual stereoisomers; as a consequence, stereoisomeric mixtures might display pharmacodynamic effects differing somewhat from those caused by the separate eutomers and distomers. In some circles, the notion of "isomeric ballast" is being mentioned with increasing regularity, leading almost fatally to the conclusion that eutomers should be purified from their distomeric ballast for therapeutic use. A number of examples discussed here show that in vitro and also in vivo, a racemate often displays a pharmacokinetic and pharmacodynamic behaviour which is not the mere addition of the behaviour of its separate enantiomers. This may seem as an additional argument for the therapeutic use of pure eutomers since a number of interactions are thus avoided. But does this imply that distomers must always be considered as detrimental ballast? Enforcing compulsory resolution of stereoisomeric mixtures, in particular racemates, would increase severalfold the cost of many drugs. This is a small price to pay if the benefit is an improved therapeutic index. But, to reword the question, would such a legislation automatically result in therapeutic benefits?

Stereoselective delivery and actions of beta receptor antagonists

These studies have revealed that the delivery and actions of beta receptor antagonist drugs are controlled by a cascade of stereoselective processes involving multiple enzymes, transport proteins and receptors. In essence, the free concentration of the pharmacologically active (-)-enantiomer species of these drugs presented to cell surface beta receptors appears to be a function of the stereoselective clearance by hepatic cytochrome P-450 isoenzymes, enantiomer selective binding to alpha 1-acid glycoprotein and albumin and perhaps predominantly by stereoselective sequestration (and release) by the vesicular amine transport protein within adrenergic neurons. Stereoselectivity in the clearance of beta blocking drugs, which can favor either the (+)- or (-)-enantiomer, only appears to be important for the lipophilic drugs which are cleared by hepatic metabolism. Such stereoselectivity is due to differential stereochemical substrate requirements of individual hepatic cytochrome P-450 isoenzymes. Interindividual variations in the stereoselectivity can occur as a result of differences in the amount and expression of cytochrome P-450 isoenzymes due to genetic predisposition or other factors. In the same context, we have observed a significant correlation between the extent and stereoselectivity of binding of beta blocking drugs to plasma proteins. This is another finding which suggests that variability in the expression of individual proteins involved in the beta blocking drug-protein cascade determines the free concentration of the pharmacologically active enantiomer. However, since most observations have been made in young normal subjects, the extent of stereoselectivity in metabolism, binding and other processes is unknown in the general population where steady-state plasma concentrations can vary widely due to multiple biological factors. The observations from neural studies support the concept that adrenergic nerve endings provide a depot for the stereoselective storage and release of the active enantiomer of beta receptor antagonists. The mechanism of this release appears to involve exocytotic secretion of drug that has been stereoselectively accumulated by the neurotransmitter storage vesicles. In terms of this idea, beta receptor antagonists released during nerve stimulation may achieve concentrations of the (-)-enantiomer within the adrenergic synapse greatly in excess of those found in plasma. Such a mechanism could significantly influence both the intensity and duration of beta receptor blockade in the heart, blood vessels, brain and other target tissues.(ABSTRACT TRUNCATED AT 400 WORDS)

Conversion of leukotriene A4 to leukotriene B4: catalysis by human liver microsomes under anaerobic conditions

During a 2-min incubation of leukotriene A4 (LTA4) with human liver microsomes, 1.7 mol% was converted into leukotriene B4 (LTB4). The reaction was dependent on protein concentration, time, and substrate concentration, was not supported by heat-inactivated microsomes, and did not require NADPH. Kinetic analysis of the reaction revealed apparent Michaelis-Menten type behavior (app Km approximately 20 microM). Production rates varied widely among three patients examined. Piperonyl butoxide, propanethiol, and cyclohexene oxide (1 mM) inhibited LTB4 formation by microsomal LTA4-hydrolase by 52, 40, and 60%, respectively. The latter two compounds were shown not to inhibit cytosolic LTA4-hydrolase activity. The activity of microsomal and cytosolic LTA4-hydrolase was decreased in the presence of 100% O2 by 45 and 64%, respectively. Direct chemical ionization mass spectrometry was used to obtain a mass spectrum of 50 ng of underivatized synthetic LTB4 free acid and show that this spectrum is identical with that of 10 ng of the product isolated from LTA4 hydrolysis by human liver microsomes. The authenticity of the biologically generated LTB4 was confirmed by functional characterization in a receptor displacement assay. Displacement of [3H]LTB4 from the high affinity receptors of LTB4 on human neutrophils revealed KD50 values of 8.2 and 5.1 nM for human liver microsome derived and synthetic LTB4, respectively. The nearly two-fold higher KD50 of the microsomally generated LTB4 is suggested to result from an epimeric mixture of the active 5(S),12(R)- and the less active 5(S),12(S)-dihydroxy-6,14-cis-8,10-trans-eicosatetraenoic acid.

MPTP, the neurotoxin inducing Parkinson's disease, is a potent competitive inhibitor of human and rat cytochrome P450 isozymes (P450bufI, P450db1) catalyzing debrisoquine 4-hydroxylation

In human liver microsomal preparations the neurotoxic chemical N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and several of its analogs competitively inhibited bufuralol 1'-hydroxylase activity of cytochrome P450bufI. This enzyme is the target of the common genetic polymorphism of drug oxidation known as debrisoquine polymorphism. Bufuralol 1'-hydroxylase activity was detectable in rat brain tissue. The activity was inhibited by antisera raised against a rat liver cytochrome P450 called P450db1. Immunoblotting experiments revealed the presence of a protein in rat and human brain microsomes with the same electrophoretic properties as the liver enzyme. These data suggest that P450bufI may be involved in the metabolism and neurotoxicity of MPTP.

Variability in drug metabolism: importance of genetic constitution

In man wide variability exists in the rate of metabolism of drugs and among factors which contribute to this phenomenon genetic constitution is of major importance. The metabolism of a number of drugs is subject to polymorphism and the frequency distribution of particular pharmacokinetic parameters shows bimodality, with poor (PM) and extensive metabolizers (EM). Acetylation of a number of drugs is known to be polymorphic and the incidence of poor metabolizers varies markedly among different populations. Debrisoquine and sparteine are frequently applied model substrates for the characterization of a polymorphism in oxidative metabolism. Polymorphic drug oxidation may have important clinical implications, because when standard dosage regimens are applied plasma concentrations will reach far above the maximum acceptable in poor metabolizers and consequently side effects may arise. Regarding the multiplicity of the drug oxidizing enzyme system (cytochrome P-450) it could be of interest to combine model substrates in a cocktail to be able to characterize human subjects simultaneously for a number of independent polymorphisms.

Chromosomal assignment of human cytochrome P-450 (debrisoquine/sparteine type) to chromosome 22

In order to determine on which chromosome the gene controlling human cytochrome P-450 (debrisoquine/sparteine type) is located a linkage study of polymorphic sparteine oxidation (PSO) to various polymorphic markers was carried out. Positive information for linkage between PSO and the P1 blood group was obtained with a maximal LOD-score of LOD = 3.35 for both male and female recombination fraction estimates of theta m = theta f = 0.0. The P1 blood group has been recently mapped to the long arm of chromosome 22. Thus it can be concluded that the gene controlling human cytochrome P-450 (debrisoquine/sparteine) is situated on the long arm of chromosome 22 in close vicinity to P1.

High-performance liquid chromatographic assays for bufuralol 1'-hydroxylase, debrisoquine 4-hydroxylase, and dextromethorphan O-demethylase in microsomes and purified cytochrome P-450 isozymes of human liver

Bufuralol, debrisoquine, and dextromethorphan are three prototype substrates of the common genetic deficiency of oxidative drug metabolism in man known as debrisoquine/sparteine-type polymorphism. We describe assays for the in vitro metabolism of (+)- and (-)-bufuralol, debrisoquine, and dextromethorphan in human liver microsomes and reconstituted purified cytochrome P-450 isozymes. These assays combine nonextractive sample preparation by precipitation of protein with perchloric acid with reversed-phase inorganic ion-pair HPLC and fluorescence detection. The minimal detectable levels of the major metabolites formed are 1'-hydroxybufuralol, 0.1 ng/ml; 4-hydroxydebrisoquine, 0.8 ng/ml; and dextrorphan, 0.1 ng/ml. Formation of these metabolites is linear for at least 45 min and between 1 and 100 micrograms of microsomal protein. Comparative kinetic analysis of the three monooxygenase reactions in human liver microsomes revealed an apparent biphasicity of (+)- and (-)-bufuralol 1'-hydroxylation and dextromethorphan O-demethylation but monophasic formation of 4-hydroxydebrisoquine in the substrate concentration range (less than 1 mM) studied. These data, in combination with those obtained by purified human cytochrome P-450 isozymes indicate the involvement of the same enzyme in the metabolism of all three substrates investigated. However, additional and distinct activities contribute to the metabolism of (+)- and (-)-bufuralol and dextromethorphan.

Genetic variation in the human hepatic cytochrome P-450 system

Studies in rodents indicate that the cytochrome P-450 system consists of a superfamily of heme proteins, produced by clusters of structural genes on different chromosomes. Equivalent P-450s of different species show more homologies than members of different P-450 families within a species. The Ah receptor serves the induction of members of one of the cytochrome families. The human structural gene for the methylcholanthrene-inducible P1-450 is located on Chromosome 15. This gene has been completely sequenced. The human Ah receptor is also measurable. New methods to measure inducibility in man involve new lymphocyte bioassays and mRNA determinations, while in vivo biotransformation studies of caffeine allow estimates of the state of induction. Structural genes for phenobarbital-inducible cytochromes have been localized to Chromosome 19. The deficiency of biotransformation of debrisoquine and sparteine continues to be explored intensely. Linkage studies indicate the gene for the variable cytochrome P-450 to be located on Chromosome 22. The deficiency is more likely due to structural variation than absence of the cytochrome. Inhibiting drugs can mimic the genetic defect. Many pharmacological and toxicological consequences of the deficiency have been defined. The main characteristics of the genetic deficiencies affecting the metabolisms of mephenytoin, phenytoin, tolbutamide, nifedipine and of methyl cysteine were outlined briefly.