Evidence for two lipoic acid residues per lipoate acetyltransferase chain in the pyruvate dehydrogenase multienzyme complex of Escherichia coli.

The reaction of two maleimides, N-ethylmaleimide and bis-(N-maleimidomethyl) ether, with the pyruvate dehydrogenase multienzyme complex of Escherichia coli in the presence of the substrate, pyruvate, was examined. In both cases, the reaction was demonstrated to be almost exclusively with the lipoate acetyltransferase component, and evidence is presented to show that the most likely sites of reaction are the lipoic acid residues covalently bound to this component. With both reagents the stoicheiometry of the reaction was measured: 2 mol of reagent reacted with each polypeptide chain of lipoate acetyltransferase, implying that each chain bears two functionally active lipolic acid residues. This observation can be reconciled with previous determinations of the lipoic acid content of the complex by allowing for the variability of the subunit polypeptide-chain ratio that can be demonstrated for this multimeric enzyme.     

Effect of bridging the two essential thiols of myosin on its spectral and actin-binding properties.

The circular dichroic and fluorescent spectral properties of the myosin head (subfragment I (SFI)) modified by covalently bridging the two essential thiol groups have been examined. CD spectra of SFI with the two thiols linked through reaction with a bifunctional reagent, N, N'- p-phenylenedimaleimide, show enhancement of the 282-nm minimum similar to that observed for the long-lived kinetic intermediate (Mg**MgADP-Pi) formed during the ATP cleavage reaction. No significant perturbation of the CD band at 282 nm is seen on blocking both thiol groups with the monofunctional reagent N-ethylmaleimide. The fluorescence emission maximum also shifts to lower wavelengths following covalent bridging (from 343 to 340 nm), but no change in fluorescent intensity has been detected. Formation of the covalent bridge completely inhibits interaction of the modified protein with F-actin. These results suggest that the local conformational state of the polypeptide chain formed on bridging the two thiol groups exhibits certain similarities with the state produced following binding of MgATP to native myosin.     

Action of spermine on phosphate transport in liver mitochondria

Spermine, at concentrations similar to those normally present in the cytosol of liver cells, facilitates the transport of phosphate into mitochondria and thus its accumulation within the matrix space. Both mersalyl and N-ethylmaleimide (NEM) inhibit phosphate influx either in the absence or in the presence of spermine. These inhibitors also inhibit, but only partially, the efflux from mitochondria of phosphate generated within the matrix space by the hydrolysis of ATP induced by carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) or the valinomycin-K+ system. The inhibition of phosphate efflux by both mersalyl and NEM is almost completely removed, unlike that of phosphate influx, by spermine. The possibility that spermine may induce phosphate efflux by damaging mitochondrial membranes and consequently inducing an unspecific permeability to phosphate is excluded by the full restoration of transmembrane potential once FCCP has been removed by albumin. Since spermine does not react with either thiol groups or thiol group reagents, the simplest explanation of the reported results is that the pathway of phosphate efflux is distinct from that of phosphate influx.     

Reactive thiol groups in rat liver acid phosphatase

Dimeric rat liver acid phosphatase P1 of Mr 92,000 is inactivated by p-chloromercuribenzoate and fluorescein mercuriacetate (FMA). The enzyme is protected against the mercurials by the substrate analogue Pi. The reaction with FMA is accompanied by changes in absorbance at 495 nm and in fluorescence emission at 520 nm that are characteristic of reaction of this compound with thiol groups. Titration of P1 with FMA monitored by spectrophotometry or by fluorimetry indicated that equivalence is reached at an FMA/P1 ratio of 3. Since FMA can act as a bifunctional reagent, it is likely that P1 contains either 3 or 6 reactive thiol groups per molecule. Analysis of FMA inactivation/modification data by a statistical method suggests that of 6 reactive thiol groups, 2 are essential so that there are probably 3 thiol groups per subunit, one of which is located at the active site. If the total thiol number is 3, analysis suggests 1 essential thiol per subunit.     

Effects of Ellman’s reagent and other thiol compounds on ion transport and ATPase activity in anaerobically grown Escherichia coli cells

It was found that modification of thiol (SH-) groups of membrane proteins by Ellman’s reagent (5,5′-dithiol-bis-(2-nitrobenzoic) acid) results in inhibition of proton efflux and K⁺ influx in anaerobically grown (pH 7.5) wild-type strains of Escherichia coli and causes disturbances in K⁺-dependent, N,N′-dicyclohexylcarbodiimide-sensitive ATPase activity and molecular hydrogen production. No such effects were observed after substitution of the cysteine residue in the b-subunit of F₀ of proton F₀F₁-ATPase for alanine. Moreover, the redox potential (RP) decreased as a result of H₂ release during glucose fermentation and formate utilization was partly restored in the presence of Ellman’s reagent. Similar changes were established when another specific SH-reagent, succinimidyl-6(β-maleimidopropionamido)hexanoate, was used. Another thiol reagent, N-ethylmaleimide, did not exert such effects despite its inhibitory action on ion transport and ATPase activity. The data obtained provide conclusive evidence in favor of essential role of thiol groups and the cysteine residue in the b-subunit of F₀ of F₀F₁-ATPase in proton-potassium exchange and H₂ production in E. coli cells. The results also point to a possible involvement of SH-groups in the TrkA system of K⁺ uptake and an involvement of hydrogenases 3 or 4 in the interactions of these integral proteins with each other.     

Purification and characterization of the bifunctional thymidylate synthetase-dihydrofolate reductase from methotrexate-resistant Leishmania tropica

Thymidylate synthetase (TS) and dihydrofolate reductase (DHFR) in Leishmania tropica exist as a bifunctional protein. By use of a methotrexate-resistant strain, which overproduces the bifunctional enzyme, the protein was purified 80-fold to apparent homogeneity in two steps. The native protein has an apparent molecular weight of 110 000 and consists of two subunits with identical size and charge. Available data indicate that each of the subunits possesses TS and DHFR. The TS of the bifunctional protein forms a covalent 5-fluoro-2'-deoxyuridylate (FdUMP)-(+/-)-5,10-methylenetetrahydrofolate-enzyme complex in which 2 mol of FdUMP is bound per mole of enzyme. In contrast, titration of DHFR with methotrexate indicated that only 1 mol of the inhibitor is bound per mole of dimeric enzyme. Both TS and DHFR activities of the bifunctional enzyme were inactivated by the sulfhydryl reagent N-ethylmaleimide. Substrates of the individual enzymes afforded protection against inactivation, indicating that each enzyme requires at least one cysteine for catalytic activity. Kinetic evidence indicates that most, if not all, of the 7,8-dihydrofolate produced by TS is channeled to DHFR faster than it is released into the medium. Although the mechanism of channeling is unknown, the possibility that the two enzymes share a common folate binding site has been ruled out.     

Evidence for a role of a vicinal dithiol in the transport of gamma-butyrobetaine in Agrobacterium sp

An Agrobacterium sp. isolated from soil is able to use gamma-butyrobetaine as its sole source of carbon and nitrogen. The involvement of thiol groups for active transport of gamma-butyrobetaine was investigated by use of the thiol alkylating reagent N-ethylmaleimide (NEM) and the dithiol specific reagent phenylarsine oxide (PAO). Both reagents strongly inhibited gamma-butyrobetaine uptake, but also induced the release of the accumulated substrate, suggesting that the transport system either contains a dithiol-dependent protein or that a small thiol-containing molecule is implicated in the uptake phenomenon.     

Water permeability in human erythrocytes: identification of membrane proteins involved in water transport

The water permeability of human erythrocytes has been monitored by nuclear magnetic resonance (NMR) before and after treatment of the cells with various sulfhydryl reagents. Preincubation of the cells with N-ethylmaleimide (NEM), a non-inhibitory sulfhydryl reagent, results in a faster and more sensitive inhibition of water exchange by mercurials. The inhibition of water exchange by p-chloromercuribenzene sulfonate (PCMBS) was maximal at a binding of approximately 10 nmol PCMBS per mg protein when non-specific sulfhydryl groups are blocked by NEM. Inhibition by PCMBS has been correlated with the binding of 203Hg to erythrocyte membrane proteins. A significant binding of label to band 3 and the polypeptides in band 4.5 occurs, with approximately 1 mol of mercurial bound per mol of protein. Inhibition of water transport by sulfhydryl reagents does not induce major morphological changes in the cells as assessed by freeze-fracture and scanning electron microscopy.     

The effect of salts on catecholamine fluxes and adenosine triphosphatase activity in storage vesicles from the adrenal medulla

1. Influx and efflux of catecholamine and adenosine triphosphatase activity in storage vesicles from the adrenal medulla were studied with dl-[(14)C]adrenaline in different media. 2. The lowest values for flux and adenosine triphosphatase activity were observed in sucrose media in which an ATP-dependent influx of catecholamine compensated for an efflux of the same magnitude. Efflux in the presence or absence of ATP was similar. 3. In media containing sodium succinate or glutarate adenosine triphosphatase activity was higher and the ATP-dependent influx of catecholamine was about twice that observed in iso-osmotic sucrose medium. In the presence of ATP influx and efflux of catecholamine were balanced; in its absence there was a net release of catecholamine, since efflux was more than twice the influx. Efflux in the presence or absence of ATP was similar. 4. In media containing sodium or potassium chloride and in the presence of ATP influx and adenosine triphosphatase activity were further enhanced, but in the absence of ATP there was no further increase in influx, since catecholamine was released with or without ATP at the same rate. Efflux was therefore twice as high in the presence of ATP as in its absence. 5. Sodium nitrate suppressed the ATP-dependent influx nearly completely, but caused a greatly enhanced efflux, which was twice as high in the presence of ATP as in its absence. 6. The extinction of vesicular suspensions remained unchanged in the presence of ATP under conditions where the catecholamine efflux was balanced by the influx. Under conditions where the efflux was not compensated by influx, the extinction of the suspensions decreased in the presence of ATP more than in its absence.     

Protein determination using bicinchoninic acid in the presence of sulfhydryl reagents

The bicinchoninic acid (BCA) copper reagent, developed for quantification of proteins, was found to react with thiol reagents in a linear and reproducible manner. The reactivity with thiols closely matched the extinction coefficient determined for the Cu(I)-BCA complex [6.6 X 10(3) liters (mol Cu.cm)-1], suggesting that the reaction is quantitative. This reaction interferes with the accurate determination of protein concentrations. A method was developed for determining protein concentrations in the presence of thiol reagents using the BCA protein reagent. The procedure involves preincubation of the protein solution with iodoacetamide prior to addition of the BCA protein reagent. Iodoacetamide does not react with the BCA reagent by itself. In the presence of a 10-fold molar excess of iodoacetamide over thiol equivalents, the reaction of the thiol with the BCA reagent is prevented. The method is simple and allows the assay of solutions of proteins which have been stabilized by the addition of thiol reagents.     

The thiol groups of the Folch-Pi protein from bovine white matter. Exposure, reactivity and significance.

The number and the reactivity of accessible thiol groups of the Folch-Pi apoprotein and proteolipid (50% of myelin proteins) were studied, by using a specific thiol-disulphide interchange reaction, in connection with the known solubility of this protein in organic and aqueous solvents. The high reactivity of 2,2'-dipyridyl disulphide towards thiol groups leads to the titration of 4.8 mol of SH groups/mol of protein (Mr 30000) in alkaline and acidic chloroform/methanol (2:1, v/v). Unlike previous findings, this value was consistently found from batch to batch and remained stable with time. In the proteolipid 1 mol of SH groups/mol was not accessible as compared with the apoprotein. In aqueous solvents, a similar number of 4.4 mol of SH groups/mol was also found. For the first time, kinetic studies carried out in chloroform/methanol discriminated between two classes of thiol groups. The reaction of 2 mol of SH groups/mol was characterized by apparent second-order rate constants whose values were 5-10-fold higher than those of the other class. Kinetic studies and cyanylation experiments in aqueous solvents also indicated the high reactivity of these thiol groups with Ellman's reagent. Together with kinetic results, studies on the stoichiometry of the interchange reaction of equimolar solutions of protein and disulphide indicate that these highly reactive thiol groups are near to each other in the amino acid sequence. The location of the thiol groups at the boundary between hydrophilic and hydrophobic domains of the Folch-Pi protein is suggested in connection with their possible structural and biological significance.     

Protein kinase C mediated regulation of calcium channels in PC-12 pheochromocytoma cells

Depolarization of PC-12 pheochromocytoma cells with K+ produces an immediate increase in catecholamine release. The stimulation of release is blocked by Co2+, removal of extracellular Ca2+ or by dihydropyridine drugs such as nitrendipine. Release is enhanced by other dihydropyridines such as BAY K8644. Release is accompanied by a voltage dependent uptake of 45Ca2+ which is also blocked by Co2+ or nitrendipine and enhanced by BAY K8644. The phorbol ester phorbol 12-myristate-13-acetate (TPA) in the range 10(-9)-10(-6) M produced little effect by itself but augmented the K+ evoked release of catecholamine. An analog of TPA which does not activate protein kinase C was ineffective. In contrast, TPA in the same concentration range blocked influx of 45Ca2+ induced by 70 mM K+ or 70 mM K+/BAY K8644. 45Ca2+ influx produced by A23187 was not blocked by TPA. The results suggest a system by which protein kinase C may regulate the output of transmitters from secretory cells.     

Reaction of thiol groups of gizzard myosin heavy chains with 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole

Chicken gizzard myosin treated with 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) resulted in a 65% inhibition of the K(+)-ATPase (myosin ATP phosphohydrolase (actin translocating), EC 3.6.1.32) activity and 3.5 mol of the reagent was bound per 4.7 x 10(5) g protein. The labeling was limited to the heavy chain region and none of the light chains were lost. MgATP had no effect on the inactivation or labeling pattern. Thiolysis of NBD-myosin with dithiothreitol restored the K(+)-ATPase activity and concurrently, 1 mol of the NBD group was removed from the heavy chain region. Cysteine residues were modified in NBD-myosin at sites other than the active site when the enzyme activity was inhibited. There was a difference in the extent of NBD-Cl modification of gizzard myosin at 0.6 M KCl (6 S elongated state) when compared to that at 0.15 M KCl (10 S folded state). This was also seen in the heavy meromyosin-like chymotryptic fragments and tryptic fragments of NBD-myosin. The reagent NBD-Cl can detect changes in the conformation of gizzard myosin by way of its reaction with thiol groups of the heavy chain region.     

Active accumulation of tetracycline by Escherichia coli

1. At low concentrations of tetracycline (10mug/ml) net accumulation of the drug by Escherichia coli cells ceased after 7-10min. 2. At higher concentrations of tetracycline (>30mug/ml) the period of net accumulation of the drug was significantly extended. 3. The efflux of tetracycline from E. coli cells transferred from medium containing 10mug of tetracycline/ml to drug-free medium was a rapid temperature-dependent process and was accelerated by 2,4-dinitrophenol. 4. As the concentration of tetracycline in the preloading phase was increased, the rate of subsequent efflux of the drug progressively declined. The efflux of drug from cells preloaded in medium containing 200mug of tetracycline/ml was negligible, although efflux was readily provoked by 2,4-dinitrophenol, by N-ethylmaleimide or by omission of glucose from the medium. 5. The initial rate of uptake of tetracycline by E. coli cells was linearly proportional to the concentration of tetracycline in the medium up to the maximum concentration of drug obtainable under the experimental conditions used (400mug/ml, 0.83mm). 6. Although N-ethylmaleimide strongly inhibited the accumulation of tetracycline by E. coli, no evidence was obtained for the direct involvement of thiol groups in the transport process. It was concluded that N-ethylmaleimide inhibited accumulation by interruption of the energy supply of the cells. 7. Osmotic shock of E. coli cells did not significantly affect the influx of tetracycline, but promoted both efflux of tetracycline and cell lysis in cells treated with a high concentration of tetracycline. 8. A study of the distribution of tetracycline among the subcellular fractions of penicillin-induced spheroplasts preincubated with various concentrations of tetracycline indicated that 60-70% of the accumulated tetracycline was in the high-speed supernatant fraction. Sephadex chromatography showed that the tetracycline of this fraction was present as the free drug. Sephadex chromatography of a detergent extract of the membrane fraction, however, indicated that a significant proportion of the tetracycline radioactivity of this fraction was apparently bound to some macromolecular component. 9. Cellulose phosphate paper chromatography of cold-acid extracts of spheroplasts preloaded with tetracycline indicated that the accumulated drug was chemically unchanged. 10. Membrane preparations isolated from osmotically lysed penicillin-induced spheroplasts showed a temperature-dependent binding of tetracycline that was not energy-dependent and was not inhibited by N-ethylmaleimide. The binding process was stimulated by omitting Mg(2+) from the medium, but conversely was profoundly inhibited by EDTA. 11. The relevance of these findings to the probable mechanism of active tetracycline accumulation by E. coli is discussed.     

A reactive nucleophile proximal to vicinal thiols is an evolutionarily conserved feature in the mechanism of Arg aminoacyl-tRNA protein transferase.

Aminoacyl-tRNA protein transferases post-translationally aminoacylate protein N-termini. At least in part, these enzymes function to allow a subset of cellular proteins to be targeted for protein degradation. A eukaryotic enzyme of this class, Arg aminoacyl-tRNA protein transferase, arginylates N-terminal Glu or Asp residues of proteins, allowing such proteins to be recognized by a specific ubiquitin-protein ligase. We showed previously that inorganic arsenite, a reagent expected to bind specifically to protein vicinal thiol groups, inhibited Arg aminoacyl-tRNA transferase activity in rabbit reticulocyte lysate (N. S. Klemperer and C. M. Pickart, 1989, J. Biol. Chem. 264, 19245-19252). We now report that a bifunctional arsenoxide reagent, p-[(bromoacetyl)-amino]phenylarsenoxide, is a potent and irreversible inactivator of the same enzyme (K0.5 = 11.5 microM). Bromoacetyl aniline, which lacks the arsenoxide moiety, has no effect. These results show that the transferase has a reactive nucleophile proximal to the site which binds arsenoxides. The related monofunctional arsenoxide reagent, p-aminophenylarsenoxide, is a reversible inhibitor whose potency (K0.5 = 7.7 microM) is 20-fold greater than that of inorganic arsenite. As expected for a mechanism in which p-aminophenylarsenoxide binds to vicinal thiol groups: (i) pretreatment of reticulocyte lysate with a thiol-blocking reagent prevents binding of the transferase to a phenylarsenoxide-Sepharose column; and (ii) inhibition by p-aminophenylarsenoxide is reversed by a competing chemical dithiol, but not by a monothiol reagent. Like the rabbit enzyme, Arg aminoacyl-tRNA protein transferase from the yeast Saccharomyces cerevisiae (expressed in Escherichia coli) is reversibly inhibited by the monofunctional phenylarsenoxide and irreversibly inactivated by the bifunctional phenylarsenoxide (but not by bromoacetylaniline). Thus, a reactive nucleophile proximal to vicinal thiol groups is a conserved feature of the activity of the transferase. We speculate that these groups are catalytic elements in the transferase active site.     

Effect of silver ions on transport and retention of phosphate by Escherichia coli.

Silver ions inhibited phosphate uptake and exchange in Escherichia coli and caused efflux of accumulated phosphate as well as of mannitol, succinate, glutamine, and proline. The effects of Ag+ were reversed by thiols and, to a lesser extent, by bromide. In the presence of N-ethylmaleimide and several uncouplers, Ag+ failed to cause phosphate efflux, but still inhibited exchange of intracellular and extracellular phosphate, indicating an interaction at more than one site. It is unlikely that Ag+ caused metabolite efflux by acting solely as an uncoupler, as an inhibitor of the respiratory chain, or as a thiol reagent.     

Reconstitution of the basal calcium transport in resealed human red blood cell ghosts

The (45)Ca(2+) influx into right-side-out resealed ghosts (RG) prepared from human red blood cells (RBC) was measured. The (45)Ca(2+) equilibration occurred with t(1/2)=2.5 min and the steady-state was reached after 17 min with the level of 22+/-2 micromol/L(packed cells) at 37 degrees C. The rate of the influx was 97+/-17 micromol/L(packed cells)h. The (45)Ca(2+) influx was saturated with [Ca(2+)](0) at 4 mmol/L and was optimal at pH 6.5 and 30 degrees C. Divalent cations (10(-4)-10(-6)mol/L), nifedipine (10(-5)-10(-4)mol/L), DIDS (up to 10(-4)mol/L), and quinidine (10(-4)-10(-3)mol/L), inhibited the (45)Ca(2+) influx while uncoupler (10(-6)-10(-5)mol/L) stimulated it. In contrast to intact RBC, vanadate inhibited the (45)Ca(2+) influx when added to the external medium, however, the stimulation was observed when vanadate was present in media during both lysis and resealing. PMA had no effect under conditions found to stimulate the Ca(2+) influx in intact RBC. The results show that the Ca(2+) influx into RG is a carrier-mediated process but without control by protein kinase C and that the influx and efflux of Ca(2+) are coupled via the H(+) homeostasis similarly as in intact RBC but with modified mechanism.     

The Effects of Electroshock Convulsions on Calcium Transport within Synaptic Terminals

Abstract: Calcium transport was assessed within synaptic terminals isolated from cerebral cortices of rats which experienced one maximal electroshock (ES) convulsion daily. No significant change in calcium content [(Ca₁)] of synaptosomes was present after 2 consecutive days of maximal convulsions. After 4 and 6 days of maximal seizures, (Ca₁) rose 20% and 37%, respectively. ¹⁵Ca²⁺ influx within synaptosomes in vitro increased after 6 days of ES convulsions (1.94 ± 0.4 μmol/g protein/min in ES convulsions versus 1.54 ± .03 μmol/g protein/min in controls). The higher rate of ⁴⁵Ca²⁺ influx in convulsed animals was accounted for by elevated internal sodium [(Na₁)] values. Maximal ⁴⁵Ca²⁺ efflux decreased after ES convulsions (0.48 μmol/g protei/min in ES convulsions versus 0.8 μmol/g protei/min in controls). The slower rate of ⁴⁵Ca²⁺ efflux after convulsions was also accounted for by elevated (Na₁). Our results suggest that (Ca₁) increased within synapses after in vivo ES convulsions secondary to a primary ionic event, namely, elevated (Na₁).     

Heme-apoprotein interaction in the modified oxyhemoglobin-bis(N-maleimidomethyl)ether and in oxyhemoglobin at high Cl-concentration detected by resonance Raman scattering

The dispersion of the depolarization ratio of oxidation and spin-marker lines of oxyhemoglobin-bis(N-maleimidomethyl)ether and oxyhemoglobin at high Cl- concentration (1 M) have been examined for different pH values in the neutral and alkaline regions. The oxidation marker line at 1375 cm-1 shows no pH-dependence in the physiological region for oxyHb-bis(N-maleimidomethyl)ether and a comparatively small variation for oxyHb at a Cl- concentration higher than 0.4 M. The spin-marker line at 1638 cm-1 exhibits a strong pH-dependence of depolarization ratio for high Cl- concentration, but a minor pH-induced variation for oxyHb-bis(N-maleimidomethyl)ether. Interpretation of these data yield the following conclusions: (1) The oxidation marker line monitors symmetry-lowering distortions of the heme group introduced by central coupling to the protein via the Fe-N bond, whereas the spin-marker line monitors peripheral coupling due to heme-protein interaction in the heme pocket. (2) At low Cl- concentrations (below 0.3 M) both types of coupling are present. These are induced by the salt bridge between His 146 beta and Asp94 beta and flexibility of the FG corner. (3) At high Cl- concentrations the salt bridge is missing, eliminating central coupling. (4) In oxyhemoglobin-bis(N-maleimidomethyl)ether, due to constraint of the bis(N-maleimidomethyl)ether bridging the FG corner and eliminating its flexibility and the missing salt bridge, both central and peripheral coupling are drastically reduced.     

Selective Inhibition of Auxin-Stimulated NADH Oxidase Activity and Elongation Growth of Soybean Hypocotyls by Thiol Reagents

The NADH oxidase activity of isolated vesicles of soybean (Glycine max cv Williams 82) plasma membranes and elongation growth of 1-cm-long hypocotyl segments were stimulated by auxins (indole-3-acetic acid or 2,4-dichlorophenoxyacetic acid [2,4-D]). The auxin-induced stimulations of both NADH oxidase and growth were prevented by the thiol reagents N-ethylmaleimide, p-chloromercuribenzoate, 5,5[prime]-dithiobis(2-nitrophenylbenzoic acid), dithiothreitol, and reduced glutathione. These same reagents largely were without effect on or stimulated slightly the basal levels of NADH oxidase and growth when assayed in the absence of auxins. In the presence of dithiothreitol or reduced glutathione, both 2,4-D and indole-3-acetic acid either failed to stimulate or inhibited the NADH oxidase activity. The rapidity of the response at a given concentration of thiol reagent and the degree of inhibition of the 2,4-D-induced NADH oxidase activity were dependent on order of reagent addition. If the thiol reagents were added first, auxin stimulations were prevented. If auxins were added first, the inhibitions by the thiol reagents were delayed or higher concentrations of thiol reagents were required to achieve inhibition. The results demonstrate a fundamental difference between the auxin-stimulated and the constitutive NADH oxidase activities of soybean plasma membranes that suggest an involvement of active-site thiols in the auxin-stimulated but not in the constitutive activity.