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M Koob

Publications and source records attributed to M Koob.

23 records · Page 2Linked to original sources
PubMed1990

A mechanism of haloalkene-induced renal carcinogenesis.

Several halogenated alkenes are nephrotoxic; some others induce renal tubular adenocarcinomas in rodents after lifelong administration. A bioactivation mechanism accounting for the organ-selective tumor induction has been elucidated: conjugation of the parent compounds with glutathione (GSH), catalyzed by hepatic GSH S-transferases, results in the formation of haloalkyl and halovinyl glutathione S-conjugates. Formation of S-conjugates (identified by NMR and mass spectrometry) could be demonstrated with trichloroethene, tetrachloroethene, hexachlorobutadiene, perfluoropropene, trichlorotrifluoropropene, and dichloroacetylene in incubations with rat liver microsomes and in the isolated perfused rat liver. The GSH conjugates formed are eliminated from the rat liver with the bile and may be translocated to the kidney, intact or after metabolism to the corresponding cysteine S-conjugates that are metabolized in the kidney by renal tubular cysteine conjugate beta-lyase (beta-lyase) to reactive intermediates, most likely thioacylchlorides and thioketenes. Interaction of these potent electrophiles with DNA [demonstrated for intermediates formed from S-(1,2,3,4,4-pentachlorobutadienyl)-L-cysteine] causes mutagenicity in bacteria, genotoxicity in cultured renal cells, and cytotoxicity in kidney cells. As an alternative to beta-lyase-catalyzed cleavage, the cysteine S-conjugates may be acetylated to the corresponding mercapturic acids, which have been identified in urine. The ability of the kidney to concentrate GSH and cysteine S-conjugates and the intensive metabolism of GSH S-conjugates to cysteine S-conjugates in this organ are evidently responsible for the organotropic carcinogenicity.

Acetylcysteine↗
PubMed1988

Conferring operator specificity on restriction endonucleases.

Mapping and manipulation of very large genomes, including the human genome, would be facilitated by the availability of a DNA cleavage method with very high site specificity. Therefore, a general method was devised that extends the effective recognition sequences well beyond the present 8-base pair limit by combining the specificity of the restriction endonuclease with that of another sequence-specific protein that binds tightly to DNA. It was shown that the tightly binding lac or lambda repressor protects a restriction site within the operator from specific modification methylases, M.Hha I or M.Hph I, while all other similar sites are methylated and thus rendered uncleavable. A plasmid containing a symmetric lac operator was specifically cleaved by Hha I, only at the site within the operator, after M.Hha I methylation in the presence of the lac repressor, whereas the remaining 31 Hha I sites on this plasmid were methylated and thus not cleaved. Analogous results were obtained with the Hae II site within the lac operator, which was similarly protected by the lac repressor, and with the Hph I site within the phage lambda oL operator, which was protected by lambda repressor from M.Hph I methylation.

Bacteriophage lambda↗
PubMedDate not supplied

Metabolism of hexafluoropropene. Evidence for bioactivation by glutathione conjugate formation in the kidney.

We investigated the metabolism of hexafluoropropene, a nephrotoxic fluoroalkene, in rat liver and kidney subcellular fractions and in rats in vivo. Incubation of hexafluoropropene (1 mM) with microsomes and cytosol in the presence of glutathione (GSH) yielded S-(1,2,3,3,3-pentafluoropropenyl)glutathione (PPFG) and S-(1,1,2,3,3,3-hexafluoropropyl)glutathione (HFPG) as identified by thermospray mass spectrometry and 1H-NMR. In liver microsomes, PFPG formation was predominant (240 nmol/min/mg protein) over HFPG (36 nmol/min/mg), whereas in cytosol, HFPG was the only hexafluoropropene metabolite (136 nmol/min/mg) detectable. In kidney microsomes, GSH-conjugate formation could not be detected; in kidney cytosol, HFPG was exclusively formed (46 nmol/min/mg). Hexafluoropropene inhalation (800 ppm for 1 hr) in rats fitted with a biliary cannula resulted in the biliary elimination of PFPG without detectable formation of HFPG; the exclusively formed urinary metabolite, identified by GC/MS, was N-acetyl-S-(1,1,2,3,3,3-hexafluoropropy)-L-cysteine. The results show that hexafluoropropene is metabolized to two different GSH-conjugates in rat liver and kidney. The data also suggest that hexafluoropropene metabolites formed in the liver and eliminated with bile are not translocated to the kidney and that intrarenal bioactivation by GSH-conjugation may be responsible for hexafluoropropene-induced nephrotoxicity.

Animals↗