Acrylamide quenching of the fluorescence of glyceraldehyde-3-phosphate dehydrogenase: reversible and irreversible effects

Mechanistic Analysis of Fluorescence Quenching of Reduced Nicotinamide Adenine Dinucleotide by Oxamate in Lactate Dehydrogenase Ternary Complexes

Fluorescence of Reduced Nicotinamide Adenine Dinucleotide (NADH) is extensively employed in studies of oxidoreductases. A substantial amount of static and kinetic work has focused on the binding of pyruvate or substrate mimic oxamate to the binary complex of lactate dehydrogenase (LDH)-NADH where substantial fluorescence quenching is typically observed. However, the quenching mechanism is not well understood limiting structural interpretation. Based on time-dependent density functional theory (TDDFT) computations with cam-B3LYP functional in conjunction with the analysis of previous experimental results, we propose that bound oxamate acts as an electron acceptor in the quenching of fluorescence of NADH in the ternary complex, where a charge transfer (CT) state characterized by excitation from the highest occupied molecular orbital (HOMO) of the nicotinamide moiety of NADH to the lowest unoccupied molecular orbital (LUMO) of oxamate exists close to the locally excited (LE) state involving only the nicotinamide moiety. Efficient quenching in the encounter complex like in pig heart LDH requires that oxamate forms a salt bridge with Arg-171 and hydrogen bonds with His-195, Thr-246 and Asn-140. Further structural rearrangement and loop closure, which also brings about another hydrogen bond between oxamate and Arg-109, will increase the rate of fluorescence quenching as well.

Identification of alkylation-sensitive target chaperone proteins and their reactivity with natural products containing Michael acceptor

Molecular chaperones have a crucial role in the folding of nascent polypeptides in endoplasmic reticulum. Some of them are known to be sensitive to the modification by electrophilic metabolites of organic pro-toxicants. In order to identify chaperone proteins sensitive to alkyators, ER extract was subjected to alkylation by 4-acetamido-4'-maleimidyl-stilbene-2,2'-disulfonate (AMS), and subsequent SDS-PAGE analyses. Protein spots, with molecular mass of 160, 100, 57 and 36 kDa, were found to be sensitive to AMS alkylation, and one abundant chaperon protein was identified to be protein disulfide isomerase (PDI) in comparison with the purified PDI. To see the reactivity of PDI with cysteine alkylators, the reduced form (PDIred) of PDI was incubated with various alkylators containing Michael acceptor structure for 30 min at 38 degrees C at pH 6.3, and the remaining activity was determined by the insulin reduction assay. Iodoacetamide or N-ethylmaleimide at 0.1 mM remarkably inactivated PDIred with N-ethylmaleimide being more potent than iodoacetamide. A partial inactivation of PDIoxid was expressed by iodoacetamide, but not N-ethylmaleimide (NEM) at pH 6.3. Of Michael acceptor compounds tested, 1,4-benzoquinone (IC50, 15 microM) was the most potent, followed by 4-hydroxy-2-nonenal and 1,4-naphthoquinone. In contrast, 1,2-naphthoquinone, devoid of a remarkable inactivation action, was effective to cause the oxidative conversion of PDIred to PDIoxid. Thus, the action of Michael acceptor compounds differed greatly depending on their structure. Based on these, it is proposed that PDI, one of chaperone proteins in ER, could be susceptible to endogenous or xenobiotic Michael acceptor compounds in vivo system.

Effect of Four-α-Helix Bundle Cavity Size on Volatile Anesthetic Binding Energetics

Currently, it is thought that inhalational anesthetics cause anesthesia by binding to ligand-gated ion channels. This is being investigated using four-alpha-helix bundles, small water-soluble analogues of the transmembrane domains of the "natural" receptor proteins. The study presented here specifically investigates how multiple alanine-to-valine substitutions (which each decrease the volume of the internal binding cavity by 38 A(3)) affect structure, stability, and anesthetic binding affinity of the four-alpha-helix bundles. Structure remains essentially unchanged when up to four alanine residues are changed to valine. However, stability increases as the number of these substitutions is increased. Anesthetic binding affinities are also affected. Halothane binds to the four-alpha-helix bundle variants with 0, 1, and 2 substitutions with equivalent affinities but binds to the variants with 3 and 4 more tightly. The same order of binding affinities was observed for chloroform, although for a particular variant, chloroform was bound less tightly. The observed differences in binding affinities may be explained in terms of a modulation of van der Waals and hydrophobic interactions between ligand and receptor. These, in turn, could result from increased four-alpha-helix bundle binding cavity hydrophobicity, a decrease in cavity size, or improved ligand/receptor shape complementarity.

Effects of acrylamide on creatine kinase from rabbit muscle

The mechanism of inhibition of creatine kinase (CK) by acrylamide (Acr) has been examined (in vitro). Within the concentration range of 0 to 1 M, Acr markedly inhibited CK and depleted the protein thiols. Both inactivation and thiol depletion were time- and Acr concentration-dependent. Addition of dithiothreitol (DTT) did not reactivate CK inactivated by Acr. However, CK with 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) pre-blocked thiols can be reactivated by DTT after incubation with Acr. The transition-state analogue also had a significant protective effect on CK against Acr inhibition. We conclude that thiol alkylation is a critical event in inactivation of CK by Acr. Furthermore, Acr binding to CK changed its surface charge, which may be the same effect for the toxicity of Acr towards other proteins.

Effect of acrylamide on aldolase structure. I. Induction of intermediate states

Acrylamide is a fluorescence quencher frequently applied for analysis of protein fluorophores exposure with the silent assumption that it does not affect the native structure of protein. In this report, it is shown that quenching of tryptophan residues in aldolase is a time-dependent process. The Stern-Volmer constant increases from 1.32 to 2.01 M-1 during the first 100 s of incubation of aldolase with acrylamide. Two tryptophan residues/subunit are accessible to quenching after 100 s of aldolase interaction with acrylamide. Up to about 1.2 M acrylamide concentration enzyme inactivation is reversible. Independent analyses of the changes of enzyme activity, 1ANS fluorescence during its displacement from aldolase active-site, UV-difference spectra and near-UV CD spectra were carried out to monitor the transition of aldolase structure. From these measurements a stepwise transformation of aldolase molecules from native state (N) through intermediates: I1, T, I2, to denatured (D) state is concluded. The maxima of I1, T, I2 and D states populations occur at 0.2, 1.0, 2.0 and above 3.0 M of acrylamide concentration, respectively. Above 3.5 M, acrylamide aldolase molecules become irreversibly inactivated.

Effect of acrylamide on aldolase structure. II. Characterization of aldolase unfolding intermediates

Molecules of muscle aldolase A exposed to acrylamide change their conformation via I1, T, I2, D intermediates [1] and undergo a slow irreversible chemical modification of thiol groups. There is no direct correlation between activity loss and thiol groups modification. In the native enzyme two classes of Trp residues of 1. 8 ns and 4.9 ns fluorescence lifetime have been found. Acrylamide (0. 2-0.5 M) increases lifetime of longer-lived component, yet the transfer of aldolase molecules even from higher (1.0 M) perturbant concentration to a buffer, allows regain original Trp fluorescence lifetime. I1, detected at about 0.2 M acrylamide, represents low populated tetramers of preserved enzyme activity. T, of maximum population at about 0.7-1.0 M acrylamide, consists of meta-stable tetramers of partial enzymatic activity. These molecules are able to exchange their subunits with aldolase C in opposition to the native molecules. At transition point for I2 appearance (1.8 M acrylamide), aldolase becomes highly unstable: part of molecules dissociate into subunits which in the absence of perturbant are able to reassociate into active tetramers, the remaining part undergoes irreversible denaturation and aggregation. Some expansion of aldolase tetramers takes place prior to dissociation. D, observed above 3.0 M acrylamide, consists of irreversibly denatured enzyme molecules.

Glyceraldehyde-3-phosphate dehydrogenase as a biochemical marker of cytotoxicity by vinyl sulfones in cultured murine spleen lymphocytes

Recently, vinyl sulfones have been observed to selectively inhibit glyceraldehyde-3-phosphate dehydrogenase (GAPDH), which is an important ATP-generating enzyme in glycolysis. The possibility of using GAPDH as a biochemical parameter of cytotoxicity by vinyl sulfones was investigated using mouse lymphocytes. Incubation of lymphocyte GAPDH with ethylvinyl sulfone resulted in a pseudo-first-order loss of enzyme activity. The exposure of lymphocytes to ethylvinyl sulfone resulted in the decrease of GAPDH activity followed by ATP depletion and cell death, which were both dependent on the concentration of ethylvinyl sulfone. A further study on the time-dependent change indicated that cell death was preceded by ATP loss. Compared to ethylvinyl sulfone, divinyl sulfone was more than 8 times more potent in causing either ATP depletion or cell death.