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PubMed2004

Chemical modification of glycerinated stalks shows tyrosine residues essential for spasmoneme contraction of Vorticella sp.

Chemical modification of glycerinated stalks of Vorticella with TNM is used to investigate the role of tyrosine residues in the Ca(2+)-induced contraction of the spasmoneme. Tetranitromethane (TNM) is often employed as a specific reagent for the nitration of tyrosine residues in a protein at neutral and slightly alkaline pHs although TNM can also oxidize cysteine residues in the acidic and neutral pH range. Prior incubation with Ca(2+) of stalks to be treated with TNM can protect the spasmoneme from irreversible denaturation. On the other hand, TNM treatment in the absence of free Ca(2+) causes an irreversible denaturation of the spasmoneme. It was revealed by us that an isolated Ca(2+)-binding protein called spasmin could not bind with Ca(2+) after TNM treatment, even if the treatment was performed in the presence of Ca(2+). In an additional experiment, we confirmed that the chemical modification of cysteine residues in the spasmoneme with N-7-dimethyl-amino-4methyl- coumarinyl- maleimide (DACM) has no effect on the contractibility. These results suggest that tyrosine residues in spasmin are essential for spasmoneme contraction and are protected from TNM in the presence of Ca(2+) when spasmin binds with its receptor protein in the spasmoneme.

Animal Structures↗
PubMed1976

[Use of bifunctional reagents in the study of histones topography in chromatin].

The ability of bifunctional reagents--diimidoesters of sebacinic acid and tetranitromethane--to cross-link the histones in rat thymus chromatin was studied. The histone polymers formed were extracted with 0,8 N H2SO4 and analyzed by polyacrylamide gel electrophoresis. The results obtained suggest that in chromatin some histone fractions are localized in immediate proximity to each other. This phenomenon may probably account for the formation of chromatin structure and its functioning in the cell.

Animals↗
PubMed1999

Peroxynitrite inactivation of tyrosine hydroxylase: mediation by sulfhydryl oxidation, not tyrosine nitration.

Tyrosine hydroxylase (TH) is the initial and rate-limiting enzyme in the biosynthesis of dopamine (DA). TH activity is significantly diminished in Parkinson's disease (PD) and by the neurotoxic amphetamines, thereby accentuating the reductions in DA associated with these conditions. Reactive oxygen and nitrogen species have been implicated in the damage to DA neurons seen in PD and in reaction to amphetamine drugs of abuse, so we investigated the hypothesis that peroxynitrite (ONOO(-)) could interfere with TH catalytic function. ONOO(-) caused a concentration-dependent inactivation of TH. The inactivation was associated with tyrosine nitration (maximum of four tyrosine residues nitrated per TH monomer) and extensive sulfhydryl oxidation. Tetranitromethane, which causes sulfhydryl oxidation at pH 6 and 8 but which nitrates tyrosines only at pH 8, inactivated TH equally at either pH. Bicarbonate protected TH from ONOO(-)-induced inactivation and sulfhydryl oxidation but increased significantly tyrosine nitration. PNU-101033 blocked ONOO(-)-induced tyrosine nitration in TH but could not prevent enzyme inactivation or sulfhydryl oxidation. Together, these results indicate that the inactivation of TH by ONOO(-) is mediated by sulfhydryl oxidation. The coincident nitration of tyrosine residues appears to exert little influence over TH catalytic function.

Bicarbonates↗
PubMed1975

Interactions of bovine neurophysins with neurohypophyseal hormones. On the role of tyrosine-49.

Reaction of tetranitromethane with the lone tyrosine residue of bovine neurophysin I and II, tyrosine-49, gave nitro derivatives of these proteins which were obtained in a highly purified form by preparative electrophoresis. Equilibrium dialysis experiments indicated clearly that oxytocin binding remained essentially unaffected by the chemical modification of tyrosine-49. However, in the case of (8-lysine)vasopressin, the nitrated protein was found to bind only 1 hormone molecule in contrast to the 2 vasopressin molecules bound by the native protein. Ultraviolet absorption difference spectroscopy measurements between 250 nm and 300 nm indicated that upon binding of (2-phenylalanine, 8-lysine)vasopressin, tyrosine-49 of native neurophysin undergoes a change of microenvironment from less to more polar surroundings. Studies of the nitrotyrosyl-49 chromophore of neurophysin by ab sorption spectroscopy in the absence and presence of oxytocin or (8-lysine)vasopressin confirmed this finding. Since dimethylsulfoxide solvent perturbation studies suggested that in the Cys(Me)-Phe-Ile-NH2-neurophysin I complex, tyrosine-49 is more exposed to solvent than in neurophysin I alone, it is concluded that this residue is unmasked by conformational changes upon complex formation.

Amino Acids↗
PubMed1975

Effect of modification on physico-chemical and biological properties of haptoglobin. Acetylation, iodination and nitration.

Human haptoglobin (Hp) type 2-1 was modified with N-acetylimidazole, iodine or tetranitromethane (TNM), and the ability of the obtained derivatives to form with haemoglobin (Hb) complexes with peroxidase activity, was estimated. At low reagent to protein molar ratios, 11 tyrosine residues were nitrated, 12 acetylated and 13 iodinated. The biological activity of NO2-Hp and I-Hp amounted to 40% of the activity of native Hp whereas the activity of Ac-Hp only to 16%. The derivatives modified at high ratios of N-acetylimidazole or iodine lost the ability to bind with Hb. Deacylation. of tyrosines and partial liberation of acetylated xi-amino groups resulted in partial recovery of the activity. As demonstrated by polyacrylamide-gel electrophoresis, the modification of Hp with high excess of TNM or iodine induced polymer formation

Acetylation↗
PubMed1976

Effect of selective nitration of ovine lutropin on the subunit association and biological activity of the hormone.

Different nitrated lutropin (LH) subunits were tested for their abilities to recombine with their counterpart subunits. (A detailed description of the preparations tested is presented in the preceding paper (Burleigh, B. D., Liu, W.K., and Ward, D.N. (1976) J. Biol. Chem. 251, 308-315).) The results indicated that the fully nitrated beta subunit of ovine LH (oLHBeta) (diatyrosyl oHbeta) can combine with the alpha subunit of ovine LH (oLHalpha) as wen which tyrosine residues alpha21, alpha41, alpha93 are modified) recombined partially with oLHbeta yielding about 40% as recombined product under specified recombination conditions. The fully nitrated oLHalpha (pentaatyrosyl oLH) combined poorly with oLHbeta and only 20 to 25% was obtained as the recombined product. The various nitrated LH and recombined LH derivatives containing separately nitrated subunits were tested for their potency in an in vitro radioligand assay and their in vivo biological potency in the ovarian ascorbic acid depletion assay. In the radioligand assay, the monoatyrosyl oLH and the oLHalpha + diatyrosyl oLHbeta were the most active derivatives and had about the same competitive inhibtion curves as that of oLHalpha + oLHbeta. Diatyrosyl oLH had intermediate native potency and the triatyrosyl oLH, tetraatyrosyl oLH + OLHbeta, and pentaatyrosyl oLHalpha + oLHbeta were the least active derivatives in this series. No tetranitromethane-modified LH derivative in our study was completely devoid of the specific binding activity in the radioligand assay. At a dosage of 1000-fold excess over the 125I-labeled oH, all the preparations tested in this series completely inhibited the specific binding of 125I-labeled oLH to the hormone receptors in rat testis preparations. In the in vivo ovarian ascorbic acid depletion assay, both the monoatyrosyl oLH and oLHalpha + diatyrosyl oLHbeta retained about 50% of its original activity; the diatyrosyl oLH and triatyrosyl oLH had about 10% of the activity and the recombined products of tetraatyrosyl oHalpha + oLHbeta and pentaatyrosyl oLHalpha + oLHbeta showed virtually no activity. Based on the results obtained from this paper and the preceding paper, the role of the individual tyrosine residues in oLH is assessed.

Animals↗
PubMed1992

Kinetic studies on the formation and decomposition of compounds II and III. Reactions of lignin peroxidase with H2O2.

The present study characterizes the serial reactions of H2O2 with compounds I and II of lignin peroxidase isozyme H1. These two reactions constitute part of the pathway leading to formation of the oxy complex (compound III) from the ferric enzyme. Compounds II and III are the only complexes observed; no compound III* is observed. Compound III* is proposed to be an adduct of compound III with H2O2, formed from the complexation of compound III with H2O2 (Wariishi, H., and Gold, M. H. (1990) J. Biol. Chem. 265, 2070-2077). We provide evidence that demonstrates that the spectral data, on which the formation of compound III* is based, are merely an artifact caused by enzyme instability and, therefore, rule out the existence of compound III*. The reactions of compounds II and III with H2O2 are pH-dependent, similar to that observed for reactions of compounds I and II with the reducing substrate veratryl alcohol. The spontaneous decay of the compound III of lignin peroxidase results in the reduction of ferric cytochrome c. The reduction is inhibited by superoxide dismutase, indicating that superoxide is released during the decay. Therefore, the lignin peroxidase compound III decays to the ferric enzyme through the dissociation of superoxide. This mechanism is identical with that observed with oxymyoglobin and oxyhemoglobin but different from that for horseradish peroxidase. Compound III is capable of reacting with small molecules, such as tetranitromethane (a superoxide scavenger) and fluoride (a ligand for the ferric enzyme), resulting in ferric enzyme and fluoride complex formation, respectively.

Anisoles↗
PubMed2004

Photodegradation mechanism of two dyes: the influence of adsorption behavior on the novel TiO2 particles.

The relationship between adsorption behavior and photocatalytic mechanism of the two dyes was investigated. Adsorption isotherms showed that the adsorption of cationic pink FG was Langmuir type behavior, while the reactive brilliant red k-2G was Freundlich type behavior. The increasing pH favored the adsorption of FG but have little effect on the photodegradation. The increasing pH favored the adsorption and the photodegradation of k-2G. The presence of scavenger of hvb+ and OH* radical potassium iodide inhibited the degradation of k-2G, free radicals scavenger tetranitromethane inhibited the photodegradation of FG. These results indicated that the photodegradation of FG mainly via free radicals in solution, and the photodegradation of k-2G was mainly on the catalysts surface or near the interface of solid and solution by react with hvb+ and surface-bound OH*. The different effect of SO4(2-), HCO3- on the adsorption and photodegradation of two dyes confirmed these results.

Adsorption↗
PubMed1991

Regulation of apolipoprotein E secretion by high density lipoprotein 3 in mouse macrophages.

Recent reports from this laboratory indicate that exposure of cholesterol-loaded macrophages to high density lipoprotein 3 (HDL3) stimulates not only cholesterol efflux, but also results in a two- to threefold increase in apoE accumulation in the media (Dory, L., 1989. J. Lipid Res. 30: 809-816). The present experiments demonstrate that the effect of HDL3, and to a lesser extent HDL2, on apoE secretion is specific, concentration-dependent, and may require interaction with the HDL receptor. Very low density lipoproteins (VLDL) and low-density lipoproteins (LDL) fail to specifically stimulate apoE secretion by cholesterol-loaded macrophages. The effect of HLD3 is maximal at 25-50 micrograms/ml (0.26-0.52 microM) and can be totally abolished by mild nitrosylation (with 3 mM tetranitromethane (TNM)). Data are also presented to indicate that the increased rate of apoE secretion in the presence of HDL3 is not due to a "protective" effect of this lipoprotein on possible proteolytic degradation or cellular reuptake of apoE secreted into the media. The stimulatory effect of HDL on apoE secretion can be clearly dissociated from cholesterol efflux; HDL stimulates apoE secretion from oxysterol-treated cells in the absence of measurable cholesterol efflux, while TNM-HDL promotes substantial cholesterol efflux from cholesterol-loaded cells but has no effect on apoE secretion. The kinetics of apoE synthesis and secretion, determined in short-term labeling studies, demonstrate that under all experimental conditions examined a substantial portion of cellular apoE is not secreted. Furthermore, in cholesterol-loaded cells HDL3 increases apoE secretion essentially by diversion of a greater portion of cellular apoE pool for secretion. While HDL3 has no effect on the rate of apoE synthesis, cellular apoE turns over two-fold faster in cells incubated in the presence of HDL3 than in its absence (t 1/2 = 11 +/- 2 and 22 +/- 4 min, respectively), an observation corresponding well with the changes in the rates of apoE secretion under similar conditions. The HDL3-mediated increase in apoE secretion by cholesterol-loaded macrophages suggests another mechanism by which HDL exerts a protective effect in the development of atherosclerosis; increased contribution to the metabolic pool of apoE by peripheral tissues may lead to a more effective clearance of peripheral cholesterol by the liver (reverse cholesterol transport).

Animals↗
PubMed1990

Chemical modification reveals involvement of tyrosine in ligand binding to dopamine D1 and D2 receptors.

The effect of tyrosine-alkylating agents on the ligand-binding properties of bovine striatal dopamine D1 and D2 receptors was investigated. The tyrosine-alkylating agents, p-nitrobenzenesulphonylfluoride (pNBSF) and tetranitromethane (TNM) caused a time-and dose-dependent loss of the binding of [3H]SCH-23390 and [3H]spiroperidol, ligands specific for dopamine D1 and D2 receptors, respectively. The two dopamine receptors, however, showed a differential sensitivity to inactivation by these agents. The mechanism of inhibition of the two receptors appears to be complex as treatment of membranes with pNBSF and TNM resulted in a decrease of both the Kd and the Bmax of ligand binding. Spiroperidol almost completely protected the TNM-induced inhibition of [3H]spiroperidol binding to dopamine D2 receptors whereas SCH-23390 afforded only partial protection of the [3H]SCH-23390 binding by TNM suggesting that more than one tyrosine groups may be involved in the D1 receptor binding activity.

Alkylating Agents↗
PubMed1975

Nicotinamide adenine dinucleotide phosphate-specific glutamate dehydrogenase of Neurospora. III. Inactivation by nitration of a tyrosine residue involved in coenzyme binding.

Neurospora glutamate dehydrogenase (NADP-specific) is rapidly inactivated upon reaction with tetranitromethane. This inactivation is completely prevented by the presence of coenzyme (NADP) or nicotinamide mononucleotide (NMN) but not by substrate. NADH, or 2'-monophosphoadenosine-5'-diphosphoribose. Amino acid analysis indicates that the primary effect of modification is nitration of a single residue of tyrosine per polypeptide chain. We have identified the reactive tyrosine by isolation of a single, uniquely labeled peptide after hydrolysis with trypsin followed by cleavage with cyanogen bromide. The modified residue proved to be tyrosine-168 in the linear sequence. This residue is not present in the part of the sequence that had been previously implicated as involved in the binding of the adenylate portion of the coenzyme. Both NMN and 2-monophosphoadenosine-5'-diphosphoribose act as competitive inhibitors of NADP in the oxidation of glutamate with Ki values of 4.65 x 10(-4) M and 4.30 x 10(-4) M, respectively. Thus, the specific protection afforded by NADP and NMN, but not by 2'-monophosphoadenosine-5'-diphosphoribose, indicates that tyrosine-168 is involved in binding the nicotinamide portion of the coenzyme.

Amino Acid Sequence↗
PubMed1985

Structure and function of human C5a anaphylatoxin. Selective modification of tyrosine 23 alters biological activity but not antigenicity.

Reaction of either human C5a or its des-Arg74 derivative (des-Arg74-C5a) with tetranitromethane under nondenaturing conditions results in selective nitration of only 1 of the 2 tyrosine residues found in these glycopolypeptides. This reactive tyrosyl residue was identified as that found in position 23 of the sequence. Nitrotyrosyl23-C5a and -des-Arg74-C5a were separated from their respective unmodified precursors by cation-exchange fast protein liquid chromatography. These purified derivatives served as reagents for the subsequent preparation of both aminotyrosyl23-C5a and -des-Arg74-C5a as well as photoreactive analogs of C5a. Radioimmunoassays performed with C5a derivatives serving as competing ligands and a murine antihuman C5a monoclonal antibody employed as first antibody demonstrated that selective modification of tyrosine23 did not produce a detectible alteration in the antigenic properties of C5a. By contrast, either nitro- or aminotyrosyl23-C5a was approximately 3-fold less active than native C5a in both bioassays and competitive ligand-receptor binding assays. Additionally, photoreactive derivatives prepared by coupling either N-succinimidyl-6-(4'-azido-2'-nitrophenylamino)-hexanoate or p-nitrophenyl-2-diazo-3,3,3-trifluoropropionate to aminotyrosyl23-C5a at pH 5.0 failed to interact with the neutrophil C5a receptor. These observations suggest that the tyrosyl23 residue of C5a may participate importantly in the binding interactions of this chemotactic factor and its granulocyte receptor.

Affinity Labels↗
PubMed1989

Investigation of the relationship between tyrosyl residues and the adenosine 5'-monophosphate binding site of rabbit liver fructose-1,6-biphosphatase as studied by chemical modification and nuclear magnetic resonance spectroscopy.

The effects of AMP, fructose 6-phosphate (Fru-6-P), fructose 2,6-bisphosphate (Fru-2,6-P2), and paramagnetic ions on the aromatic region of the proton nuclear magnetic resonance (NMR) spectrum of rabbit liver fructose-1,6-bisphosphatase have been investigated at 300 MHz. Two well resolved peaks in this region of the NMR spectrum are assigned to the protons from the aromatic ring of a tyrosyl residue of the enzyme by chemical modification with tetranitromethane and by nuclear Overhauser effects. Nitration of the tyrosyl residue causes desensitization of the enzyme to AMP inhibition as well as the loss of activity. In the presence of AMP during the modifications, 1 tyrosyl residue could be protected, presumably the one observed by NMR. Binding of AMP, an allosteric inhibitor of the enzyme, to rabbit liver fructose-1,6-bisphosphatase leads to an upfield shift of the tyrosyl proton signals in the NMR spectrum. No chemical shift or line broadening could be detected in the presence of the paramagnetic manganous ion, Fru-2,6-P2, or Fru-6-P. The negative intramolecular nuclear Overhauser effect from the ribose H2' proton to the adenine H8 proton of AMP suggested that AMP binds to the enzyme with an anti conformation about the glycosidic bond. The failure to observe intermolecular nuclear Overhauser effects between the tyrosyl residue and the protons of AMP indicates that the distances between them are greater than 4 A. On the basis of these observations, it is suggested that the AMP-related tyrosyl residue may be close to the AMP binding site, but it is not directly involved in ligand binding. Rather, the protection of this tyrosyl residue by AMP as observed by chemical modification experiments may well be due to a conformational change that results from covalent modification of the enzyme.

Adenosine Monophosphate↗
PubMed1989

Inactivation of the human placental serotonin transporter by tyrosyl group-specific reagents.

Treatment of human placental brush-border membrane vesicles with tyrosyl group-specific reagents, N-acetylimidazole, 7-chloro-4-nitrobenzo-2-oxa-1,3,-diazole and tetranitromethane, inhibited NaCl gradient-driven serotonin uptake in these vesicles without affecting vesicle integrity. The concentrations of these reagents causing 50% inhibition of serotonin uptake were 3.75 mM, 10 microM and 5 microM, respectively. The inhibition of N-acetylimidazole was reversible with hydroxylamine and the inhibition by 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole was reversible with 2-mercaptoethanol. Kinetic analysis of serotonin uptake in control and in N-acetylimidazole-treated membrane vesicles revealed that the treatment decreased the maximal velocity of the transport system with virtually no effect on the affinity of the transporter for serotonin. Similarly, the treatment did not change the affinity of the transporter for Na+. Even though serotonin uptake was reduced in treated vesicles compared with control vesicles at all concentrations of Na+, in both cases the dependence of the uptake rate on Na+ concentration was hyperbolic, indicating the involvement of one Na+ per transport of one serotonin molecule. The serotonin transporter could be protected from the N-acetylimidazole-induced inhibition by Na+. It is concluded that tyrosyl residues are essential for optimal transport function of the human placental serotonin transporter and that these critical tyrosyl residues are located at or near the Na+-binding site.

4-Chloro-7-nitrobenzofurazan↗
PubMed1987

Alpha-proton abstraction and carbanion formation in the mechanism of action of lysyl oxidase.

Tetranitromethane (TNM) was employed as an electrophilic reagent to probe for the lysyl oxidase-catalyzed processing of n-butylamine to an intermediate carbanion during the oxidation of this amine to n-butyraldehyde according to a prior description of the use of TNM to trap enzyme-generated carbanion intermediates (Christen, P. and Riordan, J. F. (1968) Biochemistry 7, 1531-1538). The addition of n-butylamine to assay mixtures containing lysyl oxidase and TNM markedly increased the background rate of nitroform release. The Km for n-butylamine was essentially the same whether determined from the rate of lysyl oxidase-catalyzed nitroform release or from the rate of n-butyraldehyde production in the absence of TNM, the latter assessed by measurements of the rate of H2O2 formation. The initial rate of substrate- and enzyme-dependent nitroform production was linearly related to functional active site content. These data are consistent with the enzyme-dependent abstraction of an alpha-proton from the substrate to form an intermediate enzyme-bound carbanion. Kinetic analyses of the oxidation of n-butylamine and 1,1-dideutero-n-butylamine by lysyl oxidase revealed kinetic isotope effects on Vmax and Vmax/Km parameters, consistent with a rate-contributing alpha-proton abstraction step. These and other available data are incorporated into a proposal for the mechanism of action of this enzyme.

Amino Acid Oxidoreductases↗
PubMed1989

High density lipoprotein stimulates sterol translocation between intracellular and plasma membrane pools in human monocyte-derived macrophages.

Binding of high density lipoprotein (HDL) to its receptor on cultured fibroblasts and aortic endothelial cells was previously shown to facilitate sterol efflux by initiation of translocation of intracellular sterol to the plasma membrane. After cholesterol-loaded human monocyte-derived macrophages were incubated with either [3H]mevalonolactone or lipoprotein-associated [3H]cholesteryl ester to radiolabel intracellular pools of sterol, incubation with HDL3 led to stimulation of 3H-labeled sterol translocation from intracellular sites to the cell surface which preceeded maximum 3H-labeled sterol efflux. A similar pattern was demonstrated for macrophages that were preloaded with cholesterol derived from either low density lipoprotein (LDL), acetyl-LDL, or phospholipase C-modified LDL. However, in macrophages that were not loaded with cholesterol, HDL3 stimulated net movement of 3H-labeled sterol from the plasma membrane into intracellular compartments, the opposite direction from that seen for cholesterol-loaded cells. A similar influx pattern was found in nonloaded macrophages and fibroblasts that were labeled with trace amounts of exogenous [3H]cholesterol. Cholesterol translocation from intracellular pools to the cell surface of cholesterol-loaded macrophages appeared to be stimulated by receptor binding of HDL, since chemical modification of HDL with tetranitromethane (TNM), which abolishes its receptor binding, reduced its ability to stimulate 3H-labeled sterol translocation and efflux. In nonloaded cells, however, the ability of HDL3 to stimulate sterol efflux and movement of sterol from the plasma membrane into intracellular pools was unaffected by TNM modification. Thus, binding of HDL to its receptor on cholesterol-loaded macrophages appears to promote translocation of intracellular cholesterol to the plasma membrane followed by cholesterol efflux into the medium. However, in nonloaded macrophages, HDL stimulates sterol movement from the plasma membrane into intracellular pools by a receptor-independent process.

Body Fluids↗
PubMed1987

The mechanism of oxyperoxidase formation from ferryl peroxidase and hydrogen peroxide.

Formation of oxyperoxidase from the reaction of ferryl horseradish peroxidase with H2O2 is inhibited by a small amount of tetranitromethane (TNM), a powerful scavenger of superoxide anion radical. The inhibition by TNM, however, does not exceed 35% as the TNM concentration is increased above 5 microM. The stoichiometry of the reaction in the presence of TNM suggests the following equation for TNM-sensitive formation of oxyperoxidase. Ferryl peroxidase + H2O2----(ferric peroxidase + O2- + H+)----oxyperoxidase The kinetic study on the TNM-resistant formation of oxyperoxidase suggests that the displacement of the oxygen with H2O2 takes place at the sixth coordination position at maximal rates of 0.048 and 0.054 s-1 for peroxidases A and C, respectively, at 5 degrees C. The TNM-sensitive and -resistant reactions are concluded to occur in parallel, and both yield oxyperoxidase. In either mechanism, the protonated form of ferryl peroxidase is active and the pK alpha value is 7.1 for peroxidase A and 8.6 for peroxidase C. Oxyperoxidase decomposes spontaneously with a large activation energy (23.0 kcal/mol), and the reaction of ferryl peroxidase with H2O2 reaches a steady level of oxyperoxidase, which depends on pH and the concentration of H2O2.

Ferric Compounds↗
PubMed1987

Chemical modification of histidine, tyrosine, tryptophan and cysteine residues in carp (Cyprinus carpio) muscle enolase.

Enolase from carp (Cyprinus Carpio) muscle was modified by diethylpyrocarbonate, tetranitromethane, N-bromosuccinimide and 5,5'-dithiobis(2-nitrobenzoic acid). The extent and rate of modification and its effect on the enzyme activity were determined. Modification of histidine, tyrosine and tryptophan residues caused complete inactivation of the enzyme; Mg2+ as well as 2-phosphoglycerate markedly altered the rates of modification and inactivation. The above-mentioned amino acid residues seem to be essential for the functioning of muscle enolases. Modification of cysteine residues had no effect on the enolase activity.

Amino Acids↗