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PubMed1978

Purification and properties of rat liver mitochondrial glutathione peroxidase.

Glutathione peroxidase (glutathione:hydrogen peroxide oxidoreductase, EC 1.11.1.9) was purified from rat liver mitochondria. The enzyme was shown to be pure by polyacrylamide-gel electrophoresis and to contain multiple forms that differed in charge. Selenium was specifically associated with the enzyme. The enzyme was inhibited by iodoacetic acid and iodoacetamide in an unusual pattern of reduction by sulfhydryl compounds and pH dependency. The mitochondrial and cytoplasmic forms of the enzyme were compared, and an explanation of the inhibition patterns is offered.

Alkylating Agents↗
PubMed1980

Reactivity of one thiol group in the dimeric protein, 4-aminobutyrate aminotransferase.

The enzyme 4-aminobutyrate aminotransferase (4-aminobutyrate:2-oxoglutarate aminotransferase, EC 2.6.1.19) from pig brain is inactivated by incubation with 5-5'-dithiobis-2-nitrobenzoic acid at pH 7.4. The reaction of one SH group per dimer brings about 95% loss of aminotransferase activity. The reaction with 5-5'-dithiobis-2-nitrobenzoic acid under pseudo-first-order conditions proceeds at a slow rate (Kobs = 0.05 min-1) and the substrate 4-aminobutyrate has no effect on the rate of inactivation. The inactivation of the enzyme cannot be related to dissociation of the cofactor (pyridoxal-P and pyridoxamine-P) from the catalytic site. One thiol group of the enzyme reacts with N-iodoacetylaminoethyl-5-naphthylamine-1-sulfonic acid at pH 7.4. The reaction of approximately one SH group per dimer causes 95% loss of aminotransferase activity. The absorption and fluorescence properties of the enzyme tagged with the chromophore were used to gain information on the microenvironment of the reactive SH group critically connected with catalytic activity. The fluorescent properties, emission maximum (470 nm), fluorescence lifetime (19 ns) and degree of exposure to the collisional quencher acrylamide (Kq = 1.8 . 10(8) M-1 . s-1) indicate that the chromophore is shielded by the protein matrix.

4-Aminobutyrate Transaminase↗
PubMed1980

Kinetics of thymidylate synthase inhibition by disulfides.

The inactivation of thymidylate synthase (5,10-methylene-tetrahydrofolate: dUMP C-methyltransferase, EC 2.1.1.45) by a number of disulfides has been examined and found to be a second-order process. The apparent second-order rate constant was strongly influenced by the chemical structure of the disulfide. The data suggest that negatively charged functional groups decrease the reactivity of the disulfides and that positively charged groups enhance the reactivity. GSSG did not react with non-catalytic SH groups, since the number of SH groups of both GSSG-treated and untreated thymidylate synthase was the same. Several sulfhydryl compounds were tested for their ability to reactivate thymidylate synthase that had been inhibited by 2,2'-dithiodipyridine. Complete reactivation was obtained with either dithiothreitol or 2-mercaptoethanol. Reactivation by 2-mercaptoethanol was a second-order process.

Disulfides↗
PubMed1989

Ajoene prevents fat digestion by human gastric lipase in vitro.

Human gastric lipase (HGL) is a sulfhydryl enzyme which has been shown by Gargouri et al. (Gargouri, Y., Moreau, H., Piéroni, G. and Verger, R. (1988) J. Biol. Chem. 263, 2159-2162) to be inhibited by hydrophobic disulfides. Since HGL is involved in the digestion and absorption of dietary fats we have investigated in vitro the ability of ajoene, a natural disulfide to inactivate HGL. Ajoene is derived from ethanolic garlic extracts. The finding that ajoene inactivates HGL is consistent with the fact that it is reactive towards sulfhydryl compounds and also corroborates previous reports on the ability of garlic to lower triacylglycerol blood levels. These data may explain the age-old Mediterranean and Oriental belief in the 'blood-thinning' effects of garlic on a molecular and physiological basis.

Dietary Fats↗
PubMed1978

Three classes of sulfhydryl group in bovine alpha-crystallin according to reactivity to various reagents.

alpha-Crystallin from calf eye lens is found to have 30 sulfhydryl groups per 800 000 daltons by modification with 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) and 4,4'-dithiopyridine in 6 M urea. These -SH groups can be divided into three different classes in native alpha-crystallin from their reactivity with DTNB, 4,4'-dithiopyridine, iodoacetamide and ethylenimine. Results obtained with these reagents point to the presence of 7 +/- 1 thiol groups (Class I) which are likely to be surface exposed, with a concomitant 40--45% quenching of tryptophan fluorescence in DTNB-modified alpha-crystallin. Another 10 +/- 1 thiol groups (Class II) must be in a hydrophobic environment since they react only with the hydrophobic reagents, causing a further decrease in fluorescence intensity. Class III, 13 +/- 2 thiol groups, is inaccesible to any of these reagents. Introduction of up to 15 negatively charged thionitrobenzoate groups or seven positively charged aminoethyl groups in the alpha-crystallin molecule at pH 7.5--8.0 did not change the state of aggregation as judged from the sedimentation coefficients.

Animals↗
PubMed1979

Effect of sulfhydryl reagents and protease inhibitors on sodium dodecyl sulfate-heat induced dissociation of Ricinus communis agglutinin.

Ricinus communis agglutinin dissociated to lower molecular weight forms when heated in sodium dodecyl sulfate in the absence of reducing agents, while ricin was little affected by such treatment. The data suggest that strong noncovalent bonds hold together two A-B heterodimers in the Ricinus communis agglutinin tetramer. Protease inhibitors such as diisopropylfluorophosphate, phenylmethansefulonyl fluoride, and EDTA, did not prevent the sodium dodecyl sulfate-heat induced dissociation; however, sulfhydryl specific reagents (N-ethylmaleimide, 5,5'-dithiobis (2-nitrobenzoic acid) and p-chloromercuribenzoate) were effective. Titration of the lectins in sodium dodecyl sulfate indicated that ricin contains one sulfhydryl and Ricinus communis agglutinin four sulfhydryl groups, none of which react in the presence of 8 M urea. The sulfhydryl groups that could be titrated in the intact proteins in sodium dodecyl sulfate were on the A chains.

Lectins↗
PubMed1983

Evidence for a membrane sulfhydryl associated with resistance to melphalan in a murine L1210 leukemia line.

Murine L1210 leukemia cells, which display sensitivity or resistance to the chemotherapeutic alkylating agent, melphalan, are equivalently sensitive or resistant to the poorly permeable mercurial, p-chloromercuribenzenesulfonate. Cells of both lines do not differ in sensitivity to the sulfhydryl reagents, N-ethylamaleimide, iodoacetamide and iodoacetate or to the glutathione transferase substrates 1-chloro-2,4-dinitrobenzene and p-nitrobenzyl chloride. The results are interpreted in terms of the known biological reactivity of p-chloromercuribenzenesulfonate as a selective reagent for sulfhydryl groups of membrane proteins associated with monovalent cation permeability.

Animals↗
PubMed1983

Copper and thiol regulation of gonadotropin releasing hormone binding and luteinizing hormone release.

The effects of copper, sulfhydryl and disulfide reagents on gonadotropin, releasing hormone (GnRH) binding to pituitary membrane preparations and on luteinizing hormone (LH) release from pituitaries of immature female rats were studied. Copper ions reduced the specific binding of 125I-labeled [D-Ser(t-Bu)6, des-Gly10-ethylamide]GnRH (Buserelin) to pituitary membranes in a dose responsive manner. Fifty percent inhibition of the specific binding was obtained at 3 x 10(-5)M Cu+2. The decreased binding stems from a 8-fold decrease in the apparent affinity of Buserelin, in the presence of copper ions. Cupric sulfate was examined for its ability to affect basal and GnRH stimulated LH release. Copper stimulated basal LH release 18-fold and 8-fold at 10(-4)M and 10(-5)M, respectively. The LH release in response to Cu+2 was calcium dependent. The effect of Cu+2 on GnRH stimulated LH release was a combined effect of Cu+2 on GnRH binding. GnRH on LH release and Cu+2 on LH release. N-ethylmaleimide, hydrogen peroxide and dithiothreitol did not alter significantly the specific binding of 125I-labeled Buserelin. N-ethylmaleimide and hydrogen peroxide did not affect GnRH stimulated LH release, whereas dithiothreitol (1 mM) significantly inhibited GnRH stimulated LH release. The present findings may explain the induction of ovulation by copper and the reduction in serum LH after injection of reducing agents.

Animals↗
PubMed1986

Cationic glutathione S-transferase of human erythrocytes has unique kinetic characteristics among human glutathione S-transferases.

The cationic glutathione S-transferase (GST sigma) of human erythrocytes is activated when incubated with 1 mM N-ethylmaleimide or other sulfhydryl blocking agents. Other GST isoenzymes of human tissues were inhibited by these reagents under similar conditions. At higher concentrations of NEM, GST sigma was also inhibited. Dithiothreitol, 2-mercaptoethanol, and sodium borohydride also caused several fold activation of GST sigma but noe of the other human GST isoenzymes were activated by these reagents.

Cations↗