Metabolism of Thioamides by Ralstonia pickettii TA

Abstract

Information on bacterial thioamide metabolism has focused on transformation of the antituberculosis drug ethionamide and related compounds by Mycobacterium tuberculosis. To study this metabolism more generally, a bacterium that grew using thioacetamide as the sole nitrogen source was isolated via enrichment culture. The bacterium was identified as Ralstonia pickettii and designated strain TA. Cells grown on thioacetamide also transformed other thioamide compounds. Transformation of the thioamides tested was dependent on oxygen. During thioamide degradation, sulfur was detected in the medium at the oxidation level of sulfite, further suggesting an oxygenase mechanism. R. pickettii TA did not grow on thiobenzamide as a nitrogen source, but resting cells converted thiobenzamide to benzamide, with thiobenzamide S-oxide and benzonitrile detected as intermediates. Thioacetamide S-oxide was detected as an intermediate during thioacetamide degradation, but the only accumulating metabolite of thioacetamide was identified as 3,5-dimethyl-1,2,4-thiadiazole, a compound shown to derive from spontaneous reaction of thioacetamide and oxygenated thioacetamide species. This dead-end metabolite accounted for only ca. 12% of the metabolized thioacetamide. Neither acetonitrile nor acetamide was detected during thioacetamide degradation, but R. pickettii grew on both compounds as nitrogen and carbon sources. It is proposed that R. pickettii TA degrades thioamides via a mechanism involving consecutive oxygenations of the thioamide sulfur atom.Amide compounds are very common in biological systems, but thioamides are rare. Correspondingly, reports of amide metabolism are very common, whereas comparatively little has been reported on bacterial thioamide metabolism. Thioamides are found naturally in the copper-chelating compound methanobactin described in Methylosinus trichosporium OB3b (24). The antibiotic sulfinemycin, produced by Streptomyces albus NRRL 3384, has a primary thioamide S-oxide moiety (28). Thioacetamide has applications in leather, textile, paper, rubber, and petroleum industries (36), and 2,6-dichlorothiobenzamide (chlorthiamid) is used as a herbicide (20). Thioamide compounds such as 2-ethyl-4-pyridinecarbothioamide (ethionamide) are important second-line drugs in the treatment of multidrug-resistant Mycobacterium tuberculosis and M. leprae (34,35). In M. tuberculosis, oxidation of the thioamide sulfur is a necessary step in converting the prodrug ethionamide to its active form (5, 13).It is currently unclear how bacteria would metabolize thioamides in a manner that supports growth. There are two biochemically logical mechanisms by which thioamides could be metabolized to liberate ammonia and thus support growth as a nitrogen source. First, enzymes could directly hydrolyze the thioamide C-N bond. These reactions have been shown to be catalyzed by some peptidases, but thioamides are typically hydrolyzed slowly compared to structurally analogous amides (4,7,8,29). A second mechanism involves oxygenation of the sulfur ato...

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“…Additionally, the reaction of thio ligand II with copper(I) bromide in acetonitrile/chloroform also involved C-S bond cleavage with subsequent conversion of II into 2-bromo-1methylimidazole (N 2 C 4 H 5 Br). [21][22][23] Scheme 1. [13][14][15][16][17][18] Apart from copper, reactions of heterocyclic thioamides with metal carbonyls also involve cleavage of C-S bonds by splitting of sulfur, which subsequently acts as a tri-or tetradentate bridge to generate homo-or heteronuclear clusters.…”

New times, new trends for ethionamide: In vitro evaluation of drug-loaded thermally carbonized porous silicon microparticles

“…1) [7,8]. In vivo studies both in mammals and bacteria suggest that ETA-SO retains the biological activity of the parent drug [9], which is in line with the fact that ETA and many current antimycobacterial agents require some form of cellular activation corresponding to the oxidative, reductive, or hydrolytic unmasking of active groups [10][11][12]. However, other metabolites for ETA (with less therapeutic importance) have been identified, such as ETA-nitrite (2-ethyl-4-carboxypyridine), ETA-aldehyde (2-ethyl-4-aldehydopyridine), and ETA-OH (2-ethyl-4-hydroxymethylpyridine) [13][14][15][16].…”

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