Identification of the "major" polymorphic carbocysteine metabolite as S-(carboxymethylthio)-L-cysteine

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.

Phenotypic variation in xenobiotic metabolism and adverse environmental response: focus on sulfur-dependent detoxification pathways

Proper bodily response to environmental toxicants presumably requires proper function of the xenobiotic (foreign chemical) detoxification pathways. Links between phenotypic variations in xenobiotic metabolism and adverse environmental response have long been sought. Metabolism of the drug S-carboxymethyl-L-cysteine (SCMC) is polymorphous in the population, having a bimodal distribution of metabolites, 2.5% of the general population are thought to be nonmetabolizers. The researchers developing this data feel this implies a polymorphism in sulfoxidation of the amino acid cysteine to sulfate. While this interpretation is somewhat controversial, these metabolic differences reflected may have significant effects. Additionally, a significant number of individuals with environmental intolerance or chronic disease have impaired sulfation of phenolic xenobiotics. This impairment is demonstrated with the probe drug acetaminophen and is presumably due to starvation of the sulfotransferases for sulfate substrate. Reduced metabolism of SCMC has been found with increased frequency in individuals with several degenerative neurological and immunological conditions and drug intolerances, including Alzheimer's disease, Parkinson's disease, motor neuron disease, rheumatoid arthritis, and delayed food sensitivity. Impaired sulfation has been found in many of these conditions, and preliminary data suggests that it may be important in multiple chemical sensitivities and diet responsive autism. In addition, impaired sulfation may be relevant to intolerance of phenol, tyramine, and phenylic food constituents, and it may be a factor in the success of the Feingold diet. These studies indicate the need for the development of genetic and functional tests of xenobiotic metabolism as tools for further research in epidemiology and risk assessment.

Sulphoxidation and sulphation capacity in patients with primary biliary cirrhosis

We have previously reported an association of impaired S-oxidation with primary biliary cirrhosis. In order to confirm and further define this relationship, we retested S-oxidation capacity via three metabolic pathways and sulphation capacity via a fourth pathway. Metabolism of S-carboxymethyl-L-cysteine is polymorphic -20% of healthy individuals being poor S-oxidisers. We found 26% with primary biliary cirrhosis were poor S-oxidisers, compared with 36% with other liver disease and 25% of healthy controls. Differences were not statistically significant. S-oxidation of ranitidine is dependent upon flavin mono-oxygenases. We showed a non-significant trend toward less S-oxide in primary biliary cirrhosis and other liver disease, compared with healthy controls, with no significant difference between disease groups. Conversion of cysteine to sulphate depends predominantly on cysteine dioxygenase. Impaired activity may be reflected by decreased plasma sulphate and elevated cysteine. We found that the plasma cysteine: sulphate ratio was significantly elevated not only in primary biliary cirrhosis (p < 0.0001), but also in other liver disease (p < 0.0001), compared with healthy individuals. Sulphation capacity was studied by analysing paracetamol metabolism. Paracetamol sulphate and sulphate: glucuronide ratio were reduced in primary biliary cirrhosis compared with normal individuals, (p < 0.05). A trend towards less sulphate in primary biliary cirrhosis compared other liver disease was not significant (p = 0.42). We conclude that although sulphation and some sulphoxidation pathways are impaired in primary biliary cirrhosis, we can currently find no evidence to substantiate the hypothesis that primary biliary cirrhosis is a disease specifically associated with poor S-oxidation, as assessed via these metabolic pathways.

Depletion of total cysteine, glutathione, and homocysteine in plasma by ifosfamide/mesna therapy

The sulfhydryl status of cells, particularly the intracellular concentration of glutathione, is a critical determinant of the response of tumor and normal cells to cytostatic drugs. Recent data indicate that the administration of mercaptoethane sulfonate (mesna), which is often combined with ifosfamide, markedly decreases the circulating concentration of total cysteine and could thereby influence the response of the organism to the cytotoxic effects of chemotherapy. The aim of the present study was to assess the effects of the combination of ifosfamide/mesna on sulfhydryl and disulfide homeostasis in tumor patients. Ifosfamide was infused into 14 patients with advanced sarcoma for 5 days at a dose of 2.4-3.2 g/m2 per day together with mesna. The plasma concentrations of total mesna, cysteine, glutathione, and homocysteine were measured before and on days 1 and 6 of the first course of ifosfamide/mesna therapy and prior to the next course of chemotherapy, and the urinary excretion of cysteine and mesna was monitored daily using a high-performance liquid chromatography (HPLC) method. Ifosfamide/mesna resulted in a marked depletion of circulating total cysteine, i.e., cysteine, cystine, and cysteine mixed disulfides [from 245 +/- 36 to 50 +/- 14 nmol/ml (mean +/- 95% CI) on day 6], total glutathione (from 6.9 +/- 1.1 to 2.5 +/- 1.1 nmol/ml), and total homocysteine (from 12.3 +/- 2.1 to 1.4 +/- 1.1 nmol/ml). The values returned to baseline levels prior to the next course of chemotherapy. The urinary excretion of cysteine increased significantly from 0.28 to 1.82 mmol/day on the 1st day, whereupon it returned toward baseline. An average of 62% +/- 6% of the delivered dose of mesna was recovered in urine. The combination of ifosfamide/mesna results in depletion of circulating total cysteine, glutathione, and homocysteine. This marked derangement of sulfhydryl and disulfide homeostasis could modulate the efficacy and toxicity of ifosfamide/mesna therapy.

Metabolic polymorphisms

Polymorphisms have been detected in a variety of xenobiotic-metabolizing enzymes at both the phenotypic and genotypic level. In the case of four enzymes, the cytochrome P450 CYP2D6, glutathione S-transferase mu, N-acetyltransferase 2 and serum cholinesterase, the majority of mutations which give rise to a defective phenotype have now been identified. Another group of enzymes show definite polymorphism at the phenotypic level but the exact genetic mechanisms responsible are not yet clear. These enzymes include the cytochromes P450 CYP1A1, CYP1A2 and a CYP2C form which metabolizes mephenytoin, a flavin-linked monooxygenase (fish-odour syndrome), paraoxonase, UDP-glucuronosyltransferase (Gilbert's syndrome) and thiopurine S-methyltransferase. In the case of a further group of enzymes, there is some evidence for polymorphism at either the phenotypic or genotypic level but this has not been unambiguously demonstrated. Examples of this class include the cytochrome P450 enzymes CYP2A6, CYP2E1, CYP2C9 and CYP3A4, xanthine oxidase, an S-oxidase which metabolizes carbocysteine, epoxide hydrolase, two forms of sulphotransferase and several methyltransferases. The nature of all these polymorphisms and possible polymorphisms is discussed in detail, with particular reference to the effects of this variation on drug metabolism and susceptibility to chemically-induced diseases.

Cancer risk related to genetic polymorphisms in carcinogen metabolism and DNA repair

Chemical carcinogenesis involves metabolism in the body of the carcinogen to the ultimate carcinogen and its interaction with DNA. There is considerable interindividual variation in the metabolic ability to activate as well as detoxify the carcinogens and in the ability to repair the carcinogen-DNA adducts. In many cases such differences occur as genetic polymorphisms and form the basis for variation in susceptibility to carcinogens and thereby to cancer risk. The activation mechanism is particularly related to the cytochromes P-450 (CYPs), and four of these are known to activate carcinogens: CYP1A1, CYP1A2, CYP2E1, and CYP3A4. Increased cancer risk has been related to polymorphisms in the CYPs and other activating enzymes. The DNA repair mechanisms show considerable complexity, and deficient repair mechanisms in certain human disorders are clearly related to increased cancer risk. Yet, there is no unambiguous epidemiological evidence available for cancer risk among individuals in general. In vivo methods have to be refined and developed for use in epidemiological studies.

Magnetic resonance spectroscopy: a powerful tool for drug metabolism studies

Studies on the metabolism and disposition of drugs using nuclear magnetic resonance spectroscopy (MRS) as the analytical technique are reviewed. An overview of the main studies classed in terms of the observed magnetic nucleus (1H, 2H, 7Li, 13C, 19F, 31P, 77Se) is followed by some typical examples of the way in which 19F and 31P MRS can be profitably employed to gain more understanding about the metabolism and disposition of the anticancer fluoropyrimidines (5-fluorouracil (FU) and its prodrugs) and ifosfamide (IF). The results of three recent studies carried out in our laboratory are developed. They concern the direct quantitative monitoring of the hepatic metabolism of FU in the isolated perfused mouse liver, the elucidation of the origin of the cardiotoxicity of FU and the metabolism of IF from an analysis of biofluids of patients. Finally, the advantages and limitations of MRS for investigations on drug metabolism are discussed.