A methodological and metabolite identification study of the metabolism of S-carboxymethyl-L-cysteine in man

Comparison of the sulfur-oxygenation of cysteine and S-carboxymethyl-l-cysteine in human hepatic cytosol and the rôle of cysteine dioxygenase

OBJECTIVES

To determine the K_m , V_max , cofactor, activator and inhibitor requirements of human cysteine dioxygenase and S-carboxymethyl-l-cysteine S-oxygenase with respect to both l-Cysteine and S-carboxymethyl-l-cysteine as substrates.

METHODS

In vitro human hepatic cytosolic fraction enzyme assays were optimised for cysteine dioxygenase activity using l-Cysteine as substrate and the effect of various cofactors, activators and inhibitors on the S-oxidations of both l-Cysteine and S-carboxymethyl-l-cysteine were investigated.

KEY FINDINGS

The results of the in vitro reaction phenotyping investigation found that although both cysteine dioxygenase and S-carboxymethyl-l-cysteine S-oxygenase required Fe²⁺ for catalytic activity both enzymes showed considerable divergence in cofactor, activator and inhibitor specificities. Cysteine dioxygenase has no cofactor but uses NAD⁺ and NADH(H⁺ ) as pharmacological chaperones and is not inhibited by S-carboxymethyl-l-cysteine. S-carboxymethyl-l-cysteine S-oxygenase requires tetrahydrobiopterin as a cofactor, is not activated by NAD⁺ and NADH(H⁺ ) but is activated by l-Cysteine. Additionally, the sulfydryl alkylating agent, N-ethylmaleimide, activated carboxymethyl-l-cysteine S-oxygenase but inhibited cysteine dioxygenase.

CONCLUSIONS

Human hepatic cytosolic fraction cysteine dioxygenase activity is not responsible for the S-oxidation of the substituted cysteine, S-carboxymethyl-l-cysteine.

An investigation into possible xenobiotic-endobiotic inter-relationships involving the amino acid analogue drug, S-carboxymethyl-L-cysteine and plasma amino acids in humans

The amino acid derivative, S-carboxymethyl-L-cysteine, is an anti-oxidant agent extensively employed as adjunctive therapy in the treatment of human pulmonary conditions. A major biotransformation route of this drug, which displays considerable variation in capacity in man, involves the oxidation of the sulfide moiety to the inactive S-oxide metabolite. Previous observations have indicated that fasted plasma L-cysteine concentrations and fasted plasma L-cysteine/free inorganic sulfate ratios were correlated with the degree of sulfoxidation of this drug and that these particular parameters may be used as endobiotic biomarkers for this xenobiotic metabolism. It has been proposed also that the enzyme, cysteine dioxygenase, was responsible for the drug sulfoxidation. Further in this theme, the degree of S-oxidation of S-carboxymethyl-L-cysteine in 100 human volunteers was investigated with respect to it potential correlation with fasted plasma amino acid concentrations. Extensive statistical analyses showed no significant associations or relationships between the degree of drug S-oxidation and fasted plasma amino acid concentrations, especially with respect to the sulfur-containing compounds, methionine, L-cysteine, L-cysteine sulfinic acid, taurine and free inorganic sulfate, also the derived ratios of L-cysteine/L-cysteine sulfinic acid and L-cysteine/free inorganic sulfate. It was concluded that plasma amino acid levels or derived ratios cannot be employed to predict the degree of S-oxidation of S-carboxymethyl-L-cysteine (or vice versa) and that it is doubtful if the enzyme, cysteine dioxygenase, has any involvement in the metabolism of this drug.

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.

Spectrophotometric determination of penicillamine and carbocisteine based on formation of metal complexes

A simple spectrophotometric method was developed for the determination of penicillamine and carbocisteine. The method depends on complexation of penicillamine with Ni, Co and Pb ions in acetate buffer pH of 6.3, 6.5 and 5.3, respectively, and carbocisteine with Cu and Ni ions in borate buffer pH of 6.7; 1-70 microg/ml of these drugs could be determined by measuring the absorbance of each complex at its specific lambdamax. The results obtained are in good agreement with those obtained using the official methods. The proposed method was successfully applied for the determination of these compounds in their dosage forms. Also, the molar ratio and stability constant of the metal complexes were calculated and a proposal of the reaction pathway was postulated.

Phenylalanine 4-Monooxygenase and the S-Oxidation of S-Carboxymethyl-L-cysteine

The identity of the enzyme responsible for the S-oxidation of the mucolytic S-substituted L-cysteine drug, S-carboxymethyl-L-cysteine (SCMC), has been actively investigated for the last 10 years. A genetic polymorphism exists in the oxidation of the thioether moiety that has been identified as a disease susceptibility factor in a number of degenerative diseases. This polymorphism has also been implicated in the wide variation in clinical response to SCMC therapy in man. To date little is known about the molecular enzymology of this reaction but a previous investigation revealed that rat activated phenylalanine 4-monooxygenase (PAH) could S-oxidise both Met- and S-methyl-L-cysteine (SMC) to their S-oxide metabolites. We have investigated the hypothesis that SCMC was also a substrate for activated PAH in the cytosolic faction of the Wistar rat. 1. Substrate and inhibitor investigation revealed that SCMC was a substrate for activated PAH activity in vitro. 2. The large aromatic amino acid hydroxylase monoclonal antibody and the Fe3+ chelator, deferoxamine, completely inhibited both Phe and SCMC oxidation to their respective metabolites. 3. Analysis of the Dixon plots revealed that both Phe and SCMC competitively inhibited each other's oxidation. 4. Correlation studies showed that the rate of production of Tyr was positively correlated to the production of both SCMC and SMC S-oxides in 20 female Wistar rat hepatic cytosolic fractions. These results strongly support the hypothesis that PAH is the enzyme responsible for SCMC S-oxidation in the rat.