Genetically determined impaired drug sulphoxidation

Phenylalanine 4-monooxygenase: the "sulfoxidation polymorphism"

1. Consistent differences in the proportion of an orally administered dose of S-carboxymethyl-l-cysteine subsequently excreted in the urine as S-oxide metabolites were reported 40 years ago. This observation suggested the existence of inter-individual variation in the ability to undertake the enzymatic S-oxygenation of this compound. Pedigree studies and investigations employing twin pairs indicated a genetically controlled phenomenon overlaid with environmental influences. It was reproducible and not related to gender or age.2. Studies undertaken in several healthy volunteer cohorts always provided similar results that were not significantly different when statistically analysed. However, when compared to these healthy populations, a preponderance of subjects exhibiting the characteristic of poor sulfoxidation of S-carboxymethyl-l-cysteine was found within groups of patients suffering from various disease conditions. The most striking of these associations were witnessed amongst subjects diagnosed with neurodegenerative disorders; although, underlying mechanisms were unknown.3. Exhaustive investigation has identified the enzyme responsible for this S-oxygenation reaction as the tetrahydrobiopterin-dependent aromatic amino acid hydroxylase, phenylalanine 4-monooxygenase classically assigned the sole function of converting phenylalanine to tyrosine. The underlying principle is discussed that enzymes traditionally associated solely with intermediary metabolism may have as yet unrecognised alternative roles in protecting the organism from potential toxic assault.

Lack of congruence between cysteine dioxygenase activity and S-carboxymethyl-l-cysteine S-oxidation activity in rat cytosol

The identity of the enzyme(s) responsible for the S-oxidation of the mucoactive drug S-carboxy-methyl-l-cysteine (SCMC) is unknown but the protein(s) are a susceptibility factor for a number of chronic degenerative diseases. The structural similarities between the amino acid l-cysteine and SCMC have raised the possibility that cysteine dioxygenase (CDO) may be responsible for this biotransformation reaction. Both CDO and SCMC S-oxygenase were found to require Fe2+ for enzymatic activity, and both enzyme activities were inhibited by Fe2+ and Fe3+ chelators. However, sulphydryl group modification of the enzymes resulted in the activation of the S-oxidation of SCMC but inhibition of the S-oxidation of l-cysteine. When the two enzyme activities were quantified in 20 female hepatic cytosolic fractions no linear correlation in the production of their respective metabolites was seen. The results of this investigation indicate that CDO is not responsible for the S-oxidation of SCMC in the rat.

PHENYLALANINE 4-MONOOXYGENASE AND THE S-OXIDATION OF S-CARBOXYMETHYL-L-CYSTEINE IN HepG2 CELLS

The role of phenylalanine 4-monooxygenase (PAH) in the S-oxidation of S-carboxymethyl-L-cysteine (SCMC) in the rat has now been well established in rat cytosolic fractions in vitro. However, the role of PAH in the S-oxidation of SCMC in human cytosolic fractions or hepatocytes has yet to be investigated. The aim of this investigation was to analyse the kinetic parameters of PAH oxidation of both L-phenylalanine (Phe) and SCMC in the human HepG2 cell line in order to investigate the use of these cells as a model for the cellular regulation of SCMC S-oxidation. The experimentally determined Km and V(max) were 7.14 +/- 0.32 mM and 0.85 +/- 0.32 nmole Tyr formed min(-1) x mg protein(-1) using Phe as substrate. For SCMC the values were 25.24 +/- 5.91 mM and 0.79 +/- 0.09 nmole SCMC (RIS) S-oxides formed min(-1) x mg protein(-1). The experimentally determined Km and V(max) for the cofactor BH4 were 6.81 +/- 0.21 microM and 0.41 +/- 0.004 nmole Tyr formed min(-1) x mg protein(-1) for Phe and 7.24 +/- 0.19 microM and 0.42 +/- 0.002 nmole SCMC (R/S) S-oxides formed min(-1) x mg protein(-1) for SCMC. The use of various PAH inhibitors confirmed that HepG2 cells contained PAH and that the enzyme was capable of converting SCMC to its (R) and (S) S-oxide metabolites in an in vitro PAH assay. Thus HepG2 cells have become a useful additional tool for the investigation of the cellular regulation of PAH in the S-oxidation of SCMC.

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.

Quantitative investigation of copper(II) and zinc(II) complexes with S-carboxymethyl-L-cysteine and computer-simulated appraisal of their potential significance in vivo

S-carboxymethyl-L-cysteine (SCC) is a mucolytic agent extensively used in the treatment of respiratory tract disorders. Some of the undesirable side effects observed during SCC therapy being reminiscent of symptoms characteristic of copper and zinc imbalances, the objective of this paper was to test the possible interference of SCC with the metabolism of these two metals. Copper(II)- and zinc(II)-SCC complex equilibria have thus been investigated under physiological conditions by means of classical potentiometry combined with computer-assisted calculation techniques. Formation constants derived from these studies have then been used to simulate 1) the potential influence of SCC on the distribution of the above metals in blood plasma and 2) the extent to which gastrointestinal interactions between the drug and each metal ion in turn are likely to affect the bioavailability of each other. The results of these simulations show that 1) plasma therapeutic levels of SCC are not likely to induce dramatic changes in the distributions of copper(II) and zinc(II) low molecular weight fractions, 2) the gastrointestinal distribution of the drug is not affected by standard dietary doses of these metals, and 3) in contrast, therapeutic concentrations of SCC are capable of mobilizing significant fractions of both metals into tissue-diffusible electrically neutral complexes. In conclusion significant depletions of neither copper nor zinc are to be expected from oral administration of SCC. While the drug may to some extent facilitate the excretion of Cu2+ and Zn2+ ions from blood plasma, its gastrointestinal influence is, on the contrary, favorable to a better absorption of these two metals.

An investigation of the metabolism of S-carboxymethyl-L-cysteine in man using a novel HPLC-ECD method

The metabolism of an oral dose of S-carboxymethyl-L-cysteine (SCMC) in man has been studied; the quantitative determination of SCMC, S-methyl-L-cysteine (SMC) and their sulphoxide metabolites (SCMCO and SMCO), in urine, was carried out using high performance liquid chromatography (HPLC) with electrochemical detection (ECD); the possibility of stereospecific sulphoxidation was investigated.

Relative bioavailability of carbocysteine from three dosage forms, investigated in healthy volunteers

The aim of the present study was to evaluate the bioavailability of a new tablet formulation of carbocysteine relative against two other oral carbocysteine containing dosage forms, viz. a syrup and capsules. Plasma levels and urine concentrations of carbocysteine were monitored, following oral administration of all three dosage forms to healthy human volunteers, by direct derivatization of carbocysteine using dabsylchloride and subsequent high performance liquid chromatography. There was no difference in bioavailability of carbocysteine from these dosage forms as expressed by the respective areas under the plasma concentration-time curves and total amounts of unchanged carbocysteine excreted in urine.

Phenacetin O-deethylase activity of the rat: strain differences and the effects of enzyme-inducing compounds

Phenacetin O-deethylase activity in microsomal fractions from liver of DA and Fischer rats has been determined. No major sex or strain differences were found. Kinetic analysis revealed two major components of O-deethylase activity in the liver of both strains of rats. Michaelis-Menten analysis revealed no major difference between the strains. Phenacetin O-deethylase activity is inducible by both 3-methylcholanthrene and phenobarbitone in DA and Fischer rats. 3-Methylcholanthrene selectively increases the high-affinity component of activity, by 20- to 25-fold, whereas phenobarbitone selectively increases the low-affinity component, by two- to three-fold. It is concluded that there is no major difference between the DA and Fischer strains in their ability to O-deethylate phenacetin. Thus, unlike poor metabolizers of debrisoquine in the human population, who appear also to have impaired phenacetin O-deethylase activity, the DA rat is deficient in only the former activity.

Strain differences in the metabolic activation of aflatoxin B1 in the rat

It has been reported that female Fischer rats are much more susceptible to the hepatocarcinogenic effects of aflatoxin B1 than female DA rats. Female Fischer rats are approximately twice as active as female DA rats in producing adducts of aflatoxin B1 with DNA in vivo, in freshly isolated hepatocytes and with hepatic microsomal fractions. There was no difference between the hepatic microsomal fractions from Fischer and DA rats in the production of adducts between aflatoxin B1 and microsomal protein. The difference between the strains in the formation of adducts with DNA was not due to either the activity of glutathione S-transferases or to the selective destruction of cytochrome P-450 in the DA strain. None of the differences reported here was of sufficient magnitude to explain the difference in susceptibility of the rat strains to the hepatocarcinogenic effects of aflatoxin B1.

Quinidine and the identification of drugs whose elimination is impaired in subjects classified as poor metabolizers of debrisoquine

Quinidine and its diastereoisomer quinine were tested in vitro for their effect on the 4-hydroxylation of debrisoquine, the O-deethylation of phenacetin and the 1'-hydroxylation of bufuralol, by human liver microsomal samples; quinidine was studied for its effect on debrisoquine 4-hydroxylation in vivo. Quinidine was a potent inhibitor of the 4-hydroxylation of debrisoquine and the 1'-hydroxylation of bufuralol, with IC50 values of 0.7 and 0.2 microM, being around 100 times more potent in this respect than quinine. Very much higher (1000-fold) levels of quinidine were required to inhibit the O-deethylation of phenacetin, being rather less potent in this than quinine. Eight subjects were phenotyped for their debrisoquine oxidation status and found to be extensive metabolisers (EM). They were tested again after the co-administration of 50 mg of quinidine with the debrisoquine. The concomitant administration of quinidine increased the metabolic ratios (MRs) by a mean of 26-fold. The effects of quinidine at a dose of only 50 mg, on the metabolism of a new drug in EM subjects may prove a useful method of assessing the contribution of the debrisoquine 4-hydroxylase isozyme to the elimination of the drug tested.

Lack of congruence of S-carboxymethyl-L-cysteine sulphoxidation and debrisoquine 4-hydroxylation in a Caucasian population

One-hundred-and-twenty volunteers and three families were investigated for possible association between the sulphoxidation of S-carboxymethyl-L-cysteine and the debrisoquine hydroxylation polymorphism. The observed individual variations in these two metabolic reactions were shown not to be concordant (rs = 0.068) and any heritable factors controlling the major aspects of these phenomena do not co-segregate.

Genetic aspects of the polymodally distributed sulphoxidation of S-carboxymethyl-L-cysteine in man

Interindividual variation in the sulphoxidation of S-carboxymethyl-L-cysteine (750 mg p.o.) was investigated in 200 healthy volunteers. Nearly a 100-fold difference was observed between individuals with respect to the amount of sulphoxide metabolites detected in their 0-8 h urine (0.6 to 59.1% recovery). Such a difference was shown to be reproducible over several months in 40 subjects who spanned the entire range of capacities. Cumulative plots and maximum likelihood analysis of the distribution indicated that a bimodal model was most probable. Analysis of pedigree data obtained from 12 families suggested a genetic effect with overlying environmental influences.

Contribution of the genetic status of oxidative metabolism to variability in the plasma concentrations of beta-adrenoceptor blocking agents

The oxidative metabolism of bufuralol is under the same genetic control as that of debrisoquine and sparteine. 154 fasting volunteers received a 30 mg tablet of bufuralol and a blood sample was taken 3 h later. In poor metabolizers (8% of the sample) the plasma bufuralol concentrations were very high and the metabolite concentrations were low. The genetic oxidative status is a major source of interindividual variation in the plasma concentration of drugs that undergo oxidative metabolism.

Assay and characterisation of debrisoquine 4-hydroxylase activity of microsomal fractions of human liver

1 A method for the assay of debrisoquine 4-hydroxylase activity in vitro by microsomal fractions of human liver is described. The assay utilises gas chromatography-mass spectrometry with d9-4-hydroxydebrisoquine as internal standard. 2 The limit of detection of 4-hydroxydebrisoquine was 2 ng ml -1 and the coefficient of variation was 4.4%. 3 Debrisoquine 4-hydroxylase activity was linear with protein to concentrations above 2.1 mg ml -1 and with incubation times of at least 15 min. 4 Debrisoquine 4-hydroxylase is a microsomal enzyme with a requirement for NADPH. Activity was inhibited by carbon monoxide. It is concluded that the activity is catalysed by cytochrome P-450. 5 In three samples of human liver the mean value for Vmax of debrisoquine 4-hydroxylase activity was 69.9 +/- 14.3 pmol mg -1 min -1 and for Km it was 130 +/- 24 microM. 6 The only variable from smoking status, alcohol ingestion, sex of the patients, source of liver sample and presence of liver disease that had a significant effect on 4-hydroxylation of debrisoquine was the presence of liver disease. This was associated with a decrease in enzyme activity.