False positives with current carbocisteine protocol for sulphoxidation phenotyping

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.

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.

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.

Increased prevalence of poor sulphoxidation in patients with rheumatoid arthritis: effect of changes in the acute phase response and second line drug treatment

A minority of normal subjects have an impaired ability to oxidise sulphur, which is associated with an increased risk of side effects when they receive sulphur containing drugs. In 114 patients with rheumatoid arthritis a greatly increased prevalence of poor sulphoxidation was found in 82 (72%) patients compared with 70/200 (35%) healthy controls, 45/121 (37%) controls matched for age, and 4/35 (11%) of the normal aged general population. In a longitudinal study of 37 patients there was no significant alteration in sulphoxidation status after the introduction of a second line drug or with marked changes in the acute phase response. It seems, therefore, that the poor sulphoxidation status in patients with RA is not an epiphenomenon and may be an important factor in determining the clinical features of rheumatoid disease.

Evaluation of proposed sulphoxidation pathways of carbocysteine in man by HPLC quantification

A quantitative study has been made of the metabolism of S-carboxymethyl-L-cysteine (CMC) and its sulphoxides in volunteers by HPLC. Precolumn derivatization was applied prior to gradient reversed phase HPLC separation and fluorescence detection. For CMC and its metabolites containing a primary amino group the reagent 9-fluorenylmethylchloroformate was used. The other metabolites of CMC were derivatized at their carboxylic group with 1-pyrenyldiazomethane to give stable fluorescent products. Urine samples were collected for 8 h after oral administration of 1.125 g CMC to 33 healthy volunteers. Elimination of CMC in urine as sulphoxides did not account for more than 1% of the dose in any of the volunteers. Thus, CMC-sulphoxide metabolites are not quantitatively important. Recovery of the original substance in 8-hour urines ranged from 10 to 30% and a further 2 to 20% was recovered as the metabolite thiodiglycolic acid. Oral doses of 0.19, 1.125, and 2.25 g CMC in a second group of 12 healthy volunteers did not reveal dose dependence of the urinary excretion of the sulphoxides or of thiodiglycolic acid. Serum concentration-time-curves of CMC, (S)- and (R)-CMC sulphoxide were measured in a group of 9 healthy volunteers. The CMC sulphoxides in serum reached 1.5% of the parent substance after 4 hours. The ratio of CMC to its sulphoxide metabolites was similar in serum and urine. Pharmacogenetic polymorphism of sulphoxidation was not confirmed by the specific HPLC methods used.