Affinity labeling of the reduced diphosphopyridine nucleotide inhibitory site of glutamate dehydrogenase by 6-[(4-bromo-2,3-dioxobutyl)thio]-6-deaminoadenosine 5'-diphosphate

Gliotoxin inactivates alcohol dehydrogenase by either covalent modification or free radical damage mediated by redox cycling

The fungal metabolite gliotoxin shows selective toxicity to cells of the immune system and has been implicated in the aetiology of invasive aspergillosis. The related toxin sporidesmin is the causative agent of facial eczema in sheep. The toxicity of these compounds has been related to their ability to redox cycle intracellularly and thus produce damaging free radicals. These toxins are also potentially capable of forming mixed disulphides with thiol groups on proteins by virtue of their bridged disulphide structure. We show here that gliotoxin can inactivate horse liver alcohol dehydrogenase by either oxidative damage or covalent modification of thiol groups on the enzyme. Either Cys-281 or Cys-282 is selectively modified. Neither of these residues are at the active site. Covalent modification occurs in the absence of reducing agents such as dithiothreitol. In the presence of dithiothreitol no protection is observed and the rate of inactivation is enhanced although as expected no covalent modification occurs. Gliotoxin can therefore inhibit alcohol dehydrogenase by either pathway and this will depend on the availability of reducing agents such as glutathione and/or how readily the reactive oxygen species generated are removed.

Three new potential cAMP affinity labels. Inactivation of human platelet low Km cAMP phosphodiesterase by 8-[(4-bromo-2,3-dioxobutyl)thio]adenosine 3',5'-cyclic monophosphate

Three new analogues of cAMP have been synthesized and characterized: 2-[(4-bromo-2,3-dioxobutyl)thio]adenosine 3',5'-cyclic monophosphate (2-BDB-TcAMP), 2-[(3-bromo-2-oxopropyl)thio]-adenosine 3',5'-cyclic monophosphate (2-BOP-tcAMP), and 8-[(4-bromo-2,3-dioxobutyl)thio]adenosine 3',5'-cyclic monophosphate (8-BDB-TcAMP). The bromoketo moiety has the ability to react with the nucleophilic side chains of several amino acids, while the dioxobutyl group can interact with arginine. These cAMP analogues were tested for their ability to inactivate the low Km (high affinity) cAMP phosphodiesterase from human platelets. The 2-BDB-TcAMP and 2-BOP-TcAMP were competitive inhibitors of cAMP hydrolysis by the phosphodiesterase with Ki values of 0.96 +/- 0.12 and 0.70 +/- 0.12 microM, respectively. However, 2-BDB-TcAMP and 2-BOP-TcAMP did not irreversibly inactivate the phosphodiesterase at pH values from 6.0 to 7.5 and at concentrations up to 10 mM. These results indicate that although the 2-substituted TcAMP analogues bind to the enzyme, there are no reactive amino acids in the vicinity of the 2-position of the cAMP binding site. In contrast, incubation of the platelet low Km cAMP phosphodiesterase with 8-BDB-TcAMP resulted in a time-dependent, irreversible inactivation of the enzyme with a second-order rate constant of 0.031 +/- 0.009 min-1 mM1. Addition of the substrates, cAMP and cGMP, and the product, AMP, to the reaction mixture resulted in marked decreases in the inactivation rate, suggesting that the inactivation was due to reaction at the active site of the phosphodiesterase.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of histidyl peptide labeled by 2-(4-bromo-2,3-dioxobutylthio)adenosine 5'-monophosphate in an ADP regulatory site of glutamate dehydrogenase

Bovine liver glutamate dehydrogenase reacts covalently with 2-(4-bromo-2,3-dioxobutylthio)adenosine 5'-monophosphate (2-BDB-TAMP) with incorporation of 1 mol reagent/mol enzyme subunit and loss of one of the two ADP sites of native enzyme [S. P. Batra and R. F. Colman, J. Biol. Chem. 261, 15565-15571 (1986)]. Incorporation of reagent is prevented specifically by ADP. The modified enzyme has now been digested with trypsin. The nucleotidyl peptide has been purified by chromatography on phenylboronate-agarose, followed by reverse-phase HPLC. On the basis of amino acid composition following acid hydrolysis, and gas-phase sequencing, the modified tryptic peptide was established as Ala-Gln-His-Ser-Gln-His-Arg, corresponding to amino acids 80-86 of the known glutamate dehydrogenase primary structure. The evidence presented indicates that the target amino acid attacked by 2-BDB-TAMP is histidine-82 and that this residue is located within the high-affinity ADP-activating site of glutamate dehydrogenase. In the course of this work, it was found that the positions of Gln84 and His85 had been reported as reversed in the revised sequence of bovine liver glutamate dehydrogenase [J. H. Julliard and E. L. Smith, J. Biol. Chem. 254, 3427-3438 (1979)]. Three additional corrections are here reported in the amino acid sequence of the native enzyme on the basis of gas-phase sequencing of other peptides purified by HPLC: Asp168 (not Asn); His221-Gly222 (not Gly-His); and Glu355 (not Gln).

Affinity labeling of NADP+-specific isocitrate dehydrogenase by a new fluorescent nucleotide analogue, 2-[(4-bromo-2,3-dioxobutyl)thio]-1,N6-ethenoadenosine 2',5'-bisphosphate

A new reactive fluorescent adenine nucleotide analogue has been synthesized and characterized: 2-[(4-bromo-2,3-dioxobutyl)thio]-1,N6-ethenoadenosine 2',5'-bisphosphate (BDB-T epsilon ADP). This compound reacts irreversibly with NADP+-specific isocitrate dehydrogenase. Biphasic kinetics of inactivation are observed that can be described in terms of a fast initial phase of inactivation resulting in partially active enzyme of 8-10% residual activity, followed by a slower phase leading to total inactivation. NADPH protects completely against the fast phase of the reaction, indicating that modification occurs at the coenzyme binding site, whereas isocitrate with MnSO4 protects totally against the slow phase of reaction. The inactivation rate constants for both phases exhibit nonlinear dependence on BDB-T epsilon ADP concentration, consistent with the formation of a reversible complex with the enzyme prior to irreversible modification. Covalent incorporation of BDB-T epsilon ADP is limited and specific; only 0.99 mol of reagent/mol of subunit is incorporated when the enzyme is 98% inactivated in the absence of ligands. A maximum incorporation of 0.5 mol of reagent/mol of subunit is obtained in the presence of isocitrate and MnSO4, while incorporation in the presence of NADPH equals the difference between the incorporation in the absence of ligands and that measured in the presence of isocitrate and MnSO4. It appears that 0.5 mol of reagent/mol of subunit is responsible for the fast phase of inactivation and the remaining incorporation causes the slow phase. Under all conditions used in this study, isocitrate dehydrogenase has been shown to exist as a dimer by analytical ultracentrifugation and by cross-linking with dimethyl suberimidate followed by analysis on polyacrylamide gels in the presence of sodium dodecyl sulfate. It is proposed that, in the fast phase of inactivation, 2-[(4-bromo-2,3-dioxobutyl)thio]-1,N6-ethenoadenosine 2',5'-bisphosphate reacts at the coenzyme binding site of one subunit of dimeric isocitrate dehydrogenase, causing complete inactivation of the modified subunit and substantial inactivation of the other subunit. This new reagent is likely to have general applicability as an affinity label for other NADP+ binding enzymes.