Thiol-disulfide interchange between cystine and N-2-mercaptoethyl-1, 3-diaminopropane as a potential treatment for cystinuria

Determination and metabolism of dithiol chelating agents. VI. Isolation and identification of the mixed disulfides of meso-2,3-dimercaptosuccinic acid with L-cysteine in human urine

Virtually nothing is known about the biotransformation of the heavy metal chelating agent, meso-2,3-dimercaptosuccinic acid (DMSA). Two fasted, normal, young men were given 10.0 mg DMSA/kg po, and their urines were collected over a 14-hr period. Urine samples were analyzed, before and after electrolytic reductive treatment, for DMSA and its biotransformants using bromobimane derivatization, HPLC separation, and fluorescence detection. Metabolites were isolated by HPLC, ion-pairing extraction, ion-exchange extraction, and TLC. By 14 hr after DMSA administration, 87% of the total DMSA and 95% of the total L-cysteine found in urine consisted of altered forms of these compounds. The urinary excretion of altered DMSA, at 1, 2, 4, 6, 9, and 14 hr after administration of DMSA, when compared to the urinary excretion of altered L-cysteine had a correlation coefficient of 0.952 and p less than 0.003. Approximately 90% of the altered DMSA excreted in the 2- to 4-hr urine was found in disulfide linkage with L-cysteine. The remaining 10% was found as cyclic disulfides of DMSA. Of the mixed disulfides found in 4- to 6-hr urine, 97% consisted of two L-cysteine residues per one DMSA and the remaining 3% consisted of one L-cysteine per one DMSA. The 2:1 mixed disulfides (97%) were isolated as three distinct species by TLC, consisting of 77, 12, and 8% of the total mixed disulfides found. In addition to the novelty of these biotransformants of DMSA, the DMSA-cysteine mixed disulfides indicate a thiol-disulfide interchange between DMSA and L-cystine. The discovery of the formation of these water soluble DMSA-cysteine mixed disulfides should encourage the evaluation of DMSA in the treatment of cystinuria.

Mechanisms of synergistic toxicity of the radioprotective agent, WR2721, and 6-hydroxydopamine

WR2721 is a "prodrug" for a radioprotective thiol which has been proposed for adjunctive use as a free radical scavenger in cancer chemotherapy. When used adjunctively with oxygen radical generating chemotherapeutic agents in mice, however, WR2721 produces synergistic toxicity rather than attenuation of the toxic effects of such agents. The present paper discusses potential mechanisms for such synergistic toxicity. The pathway for glutathione synthesis appeared to be inactivated in mice treated with WR2721. The disulfide metabolite of WR2721 was a potent inactivator of gamma-glutamylcysteine synthetase, the rate-limiting enzyme in glutathione synthesis. The inactivation of the enzyme by this compound was similar to that reported for cystamine, a compound known to form a mixed disulfide with a cysteine residue near the glutamic acid binding site of the enzyme. Oxygen radicals not only inactivated the synthetase, as well, but hastened the oxidation of the free thiol metabolite of WR2721 to its corresponding disulfide.

Cystinuria

Cystinuria is an inherited metabolic disease resulting in renal stone formation. An incidence of 1 in 7000 makes it a relatively common genetic disease. The biochemical defect is a carrier protein in the epithelial cells of certain organs. This carrier protein is responsible for the transport of cystine and the dibasic amino acids. Cystine is a poorly soluble compound which precipitates in acid urine and results in renal calculi. Cystine stones account for 1 to 2% of all renal calculi. Homozygotes are detected by the high concentration of cystine in their urine. Treatment consists of sulfhydryl compounds that form more soluble compounds with cystine through sulfhydryl exchange as well as alkalinization of urine and hydration to make cystine more soluble.

Reversal of the biological activity of Escherichia coli heat-stable enterotoxin by disulfide-reducing agents

Various disulfide-reducing agents, mostly thiols and thiol precursors, were examined for their ability to reduce the disulfide bonds in the Escherichia coli heat-stable enterotoxin STa; reduction of the bonds results in loss of biological activity. The biological activity measured was the stimulation of guanylate cyclase in pig intestinal brush border membranes by STa. Nearly all of the compounds inactivated STa, although at different rates; a smaller number appreciably decreased guanylate cyclase activity when they were introduced into the reaction mixture after STa bound to its receptor. With dithiothreitol, the decrease in reaction rate was both time and concentration dependent and resulted in a reversal to basal activity. The anionic thiols were relatively ineffective in reversing activation, the neutral monothiols were moderately effective, and the aminothiols and neutral dithiols were the most effective. The order of effectiveness of the compounds was S-2-(3-aminopropylamino)ethanethiol greater than 2,3-dimercaptopropanol = 2-aminoethylisothiuronium bromide greater than dithiothreitol greater than 2-mercaptoethylamine greater than alpha-thioglycerol. These compounds were used in weanling pig ligated-intestinal-loop bioassays to determine if STa-induced secretion was reduced when they were injected 20 min after the STa. Instead of S-2-(3-aminopropylamino)ethanethiol we used the phosphorylated derivative S-2-(3-aminopropylamino)ethylphosphorothioic acid; this compound and 2,3-dimercaptopropanol were the only compounds that reduced STa-induced secretion and had no direct secretory or pathological effects.

Cystine depletion by WR-1065 in cystinotic cells. Mechanism of action

Cystinotic leucocytes and skin fibroblasts incubated with the aminothiol N-(2'-mercaptoethyl)-1,3-propanediamine (WR-1065) exhibited substantial intralysosomal cystine depletion within 2 hr. Wr-2721, the thiol phosphorylated derivative of WR-1065, did not lower cystinotic leucocyte cystine in 1 hr but depleted cystinotic fibroblasts of cystine after 21 hr. Concentrations of cysteamine (beta-mercaptoethylamine) equimolar with those of WR-1065 depleted cystine more rapidly than did WR-1065, but the extent of cystine depletion by WR-1065 approached that for cysteamine when longer periods of incubation or higher concentrations were used. Cystine depletion by WR-1065 was slower for leucocyte lysosomal granular fractions than for whole leucocytes. L-[35S]Cystine-labeled fibroblasts exposed to WR-1065 exhibited new compounds not seen when cells were incubated without WR-1065: WR-1065-cysteine, cysteamine-cysteine and cysteamine-glutathione mixed disulfides. L-[35S]Cystine-loaded lysosome-rich granular fractions from cystinotic leucocytes incubated with WR-1065 formed WR-1065-cysteine mixed disulfide but no cysteamine-cysteine mixed disulfide. We suggest that WR-2721 is dephosphorylated intracellularly to the free thiol, WR-1065, which subsequently is converted to cysteamine by an unknown route. Intracellular cysteamine then enters the lysosome and reacts with free cystine to form cysteamine-cysteine mixed disulfide and cysteine which move into the cytosol and the incubation medium where they participate in further interchange reactions with free thiols present there, namely WR-1065 and glutathione.

Trypanothione: a novel bis(glutathionyl)spermidine cofactor for glutathione reductase in trypanosomatids

Glutathione reductase from trypanosomes and leishmanias, unlike glutathione reductase from other organisms, requires an unusual low molecular weight cofactor for activity. The cofactor was purified from the insect trypanosomatid Crithidia fasciculata and identified as a novel glutathione-spermidine conjugate, N1,N8-bis(L-gamma-glutamyl-L-hemicystinyl-glycyl)spermidine, for which the trivial name trypanothione is proposed. This discovery may open a new chemotherapeutic approach to trypanosomiasis and leishmaniasis.