Kinetics of uptake and deacetylation of N-acetylcysteine by human erythrocytes

Overproduction of reactive oxygen species associated with several diseases including sickle cell anaemia reduces the concentration of glutathione, a principal cellular antioxidant. Glutathione depletion in sickle erythrocytes increases their conversion to irreversible sickle cells that promote vaso-occlusion. Therapeutically, N-acetylcysteine partially restores glutathione concentrations but its mode of action is controversial. Following glutathione depletion, glutathione synthesis is limited by the supply of cysteine and it has been assumed that deacetylation of N-acetylcysteine within erythrocytes provides cysteine to accelerate glutathione production. To determine whether this is the case we studied the kinetics of transport and deacetylation of N-acetylcysteine. Uptake of N-acetylcysteine had a first order rate constant of 2.40+/-0.070min(-1) and only saturated above 10mM. Inhibition experiments showed that 56% of N-acetylcysteine transport was via the anion exchange protein. Deacetylation, measured using (1)H NMR, had a K(m) of 1.49+/-0.16mM and V(max) of 2.61+/-0.08micromolL(-1)min(-1). Oral doses of N-acetylcysteine increase glutathione concentrations in sickle erythrocytes at plasma N-acetylcysteine concentrations of approximately 10microM. At this concentration, calculated rates of N-acetylcysteine uptake and deacetylation were approximately 5% of the rate required to maintain normal glutathione production. We concluded that on oral administration, intracellular deacetylation of N-acetylcysteine supplies little of the cysteine required for accelerated glutathione production. Instead, N-acetylcysteine acts by freeing bound cysteine in the plasma that then enters the erythrocytes. To be effective, intracellular cysteine precursors must be designed to enter erythrocytes rapidly and employ enzymes with high activity within erythrocytes to liberate the cysteine.     

The metabolism of benzyl isothiocyanate and its cysteine conjugate.

1. The corresponding cysteine conjugate was formed when the GSH (reduced glutathione) or cysteinylglycine conjugates of benzyl isothiocyanate were incubated with rat liver or kidney homogenates. When the cysteine conjugate of benzyl isothiocyanate was similarly incubated in the presence of acetyl-CoA, the corresponding N-acetylcysteine conjugate (mercapturic acid) was formed. 2. The non-enzymic reaction of GSH with benzyl isothiocyanate was rapid and was catalysed by rat liver cytosol. 3. The mercapturic acid was excreted in the urine of rats dosed with benzyl isothiocyanate or its GSH, cysteinyl-glycine or cysteine conjugate, and was isolated as the dicyclohexylamine salt. 4. An oral dose of the cysteine conjugate of [14C]benzyl isothiocyanate was rapidly absorbed and excreted by rats and dogs. After 3 days, rats had excreted a mean of 92.4 and 5.6% of the dose in the urine and faeces respectively, and dogs had excreted a mean of 86.3 and 13.2% respectively. 5. After an oral dose of the cystein conjugate of [C]benzyl isothiocyanate, the major 14C-labelled metabolite in rat urine was the corresponding mercapturic acid (62% of the dose), whereas in dog urine it was hippuric acid (40% of the dose). 5. Mercapturic acid biosynthesis may be an important route of metabolism of certain isothiocyanates in some mammalian species.     

Activation of hydrazine derivatives to free radicals in the perfused rat liver: a spin-trapping study

Spin-trapping techniques and electron spin resonance spectroscopy have been used to study bioactivations of hydrazine and its derivatives by isolated perfused rat livers. Using phenylbutylnitrone (PBN) as the stable spin trap, it was found that the liver perfusion of hydrazine, acetylhydrazine and isoniazid resulted in the formation of the same carbon-centered radical which was shown to be the acetyl radical. The identity of the acetyl radical was confirmed after organic extraction of the liver perfusates, by comparing its coupling constants with those of the in vitro metal ion- or horseradish peroxidase-catalyzed oxidation products of the hydrazines in the same solvents. The liver perfusion of iproniazid formed the isopropyl radical which was previously observed to result from peroxidase-catalyzed oxidation.     

Metabolism of a glutathione conjugate of 2-hydroxyoestradiol-17β in the adult male rat

Adult male rats with cannulated or ligated bile ducts were given S-(2-hydroxyoestradiol-1-yl)[(35)S]glutathione, S-(2-hydroxy[6,7-(3)H(2)]oestradiol-1-yl)glutathione or S-(2-hydroxyoestradiol-1-yl)[glycine-(3)H]glutathione by intraperitoneal injection. The recovery of radioactivity in the bile of bile duct-cannulated rats was 33-86% and in the urine of bile duct-ligated rats was 54-105%. Oestrogen thioether derivatives of glutathione, cysteinylglycine, cysteine and N-acetylcysteine were isolated from bile; only the N-acetylcysteine derivatives could be identified in the urine. The steroid moiety was characterized by microchemical tests before and after treatment with Raney nickel: 2-hydroxyoestradiol-17beta was released from the glutathione conjugate, and 2-hydroxyoestrone and 2-hydroxyoestrone 3-methyl ether from the other conjugates. From intact rats the recovery of administered radioactivity was about 15% in the urine and 5% in the faeces over a period of several days and the radioactivity appeared to be largely protein-bound. The results demonstrate that injected oestrogen-glutathione conjugate undergoes conversion into N-acetylcysteine derivatives in vivo. Oestrogen-glutathione conjugates formed in the intact rat may be excreted in an apparently non-steroidal, possibly protein-bound form, which would not be detected by current analytical techniques.     

Production of the neuromodulator H2S by cystathionine beta-synthase via the condensation of cysteine and homocysteine

Hydrogen sulfide (H2S) has been observed in relatively high concentrations in the mammalian brain and has been shown to act as a neuromodulator. However, there is confusion in the literature regarding the actual source of H2S production. Reactions catalyzed by the cystathionine beta-synthase enzyme (CBS) are one possible source for the production of H2S. Here we show that the CBS enzyme can efficiently produce H2S via a beta-replacement reaction in which cysteine is condensed with homocysteine to form cystathionine and H2S. The production of H2S by this reaction is at least 50 times more efficient than that produced by hydrolysis of cysteine alone via beta-elimination. Kinetic studies demonstrate that the Km and Kcat for cysteine is 3-fold higher and 2-fold lower, respectively, than that for serine. Consistent with these data, in vitro reconstitution studies show that at physiologically relevant concentrations of serine, homocysteine, and cysteine, about 5% of the cystathionine formed is from cysteine. We also show that AdoMet stimulates this H2S producing reaction but that there is no evidence for stimulation by calcium and calmodulin as reported previously. In summary, these results confirm the ability of CBS to produce H2S, but show in contrast to prior reports that the major mechanism is via beta-replacement and not cysteine hydrolysis. In addition, these studies provide a biochemical explanation for the previously inexplicable homocysteine-lowering effects of N-acetylcysteine treatments in humans.     

The effect of reducing agents on proteolytic enzymes and oxidation of alpha 1-proteinase inhibitor

We have studied the effect of the mucolytic agent N-acetylcysteine and dithiothreitol on the oxidation of alpha 1-PI by hydrogen peroxide, and their effect on porcine pancreatic elastase and leukocyte elastase. In addition, the effect of S-(carboxymethyl)cysteine (= carbocisteine, a mucolytic agent which does not have reducing properties) was studied in vitro and in patients with chronic obstructive bronchitis. Following addition of 59.6mM N-acetylcysteine, the amidolytic activity of leukocyte elastase was decreased by 55.3% and that of porcine pancreatic elastase by 57.0%. Dithiothreitol (5.7 mM) caused the loss of 97.4% and 67.6% of amidolytic activity of leukocyte elastase and porcine pancreatic elastase respectively whereas S-(carboxymethyl)cysteine had no effect. Similar results were found for the effect on elastolytic activity. Oxidation of alpha 1-PI by 8.6mM H2O2 resulted in partial loss of inhibitory function (mean 68.7% activity of native alpha 1-PI). N-Acetylcysteine and dithiothreitol prevented oxidation of alpha 1-PI when pre-incubated with H2O2 or incubated with alpha 1-PI and H2O2 simultaneously (94.5% and 94.4% activity of native alpha 1-PI for N-acetylcysteine; 78.3% and 87.6% activity for dithiothreitol - p less than 0.025). S-(Carboxymethyl)cysteine, when pre-incubated with H2O2 or incubated concurrently with alpha 1-PI and H2O2, caused a further decrease in the porcine pancreatic elastase inhibitory capacity of alpha 1-PI (53.1% and 63.0% respectively - p less than 0.025). None of the agents reversed oxidative inactivation once it had occurred. S-(Carboxymethyl)cysteine had no effect on alpha 1-PI function in sputum at the dose used.     

Studies of isoniazid metabolism in isolated rat hepatocytes by mass fragmentography

Isoniazid metabolism in isolated rat hepatocytes was studied by mass fragmentography using single ion monitoring. Isoniazid and its metabolites were determined as the trimethylsilylated derivatives of acetylisoniazid and diacetylhydrazine and of the benzaldehyde hydrazones of isoniazid and acetylhydrazine. Deuterated analogues served as internal standards. Hydrazine was quantitated as benzalazine using ¹⁵N-labeled hydrazine as an internal standard. The method is well suited for the microanalysis of isoniazid metabolites in specificity and reliability to demonstrate the overall pathway of isoniazid metabolism, from which it was clarified that the greater part of hydrazine, a hazardous metabolite of isoniazid, was formed through the direct hydrolysis of isoniazid itself as expected.     

Nuclear magnetic resonance studies of the solution chemistry of metal complexes. 26. Mixed ligand complexes of cadmium, nitrilotriacetic acid, glutathione, and related ligands

The complexation of glutathione and related ligands by the nitrilotriacetic acid complex of Cd2+ (Cd(NTA)-) has been investigated by 1H NMR as a model for the coordination chemistry of Cd2+ and GSH in biological systems. Related ligands included glycine, glutamic acid, cysteine, N-acetylcysteine, penicillamine, N-acetylpenicillamine, mercaptosuccinic acid, and the S-methyl derivative of glutathione. The nature of the complexes formed was deduced from 1H NMR spectra of Cd(NTA)- and the ligands. Mixed ligand complexes (Cd(NTA)L) and single ligand complexes (CdLx) are formed with the thiol ligands, whereas only mixed ligand complexes form with glycine, glutamic acid and S-methylglutathione. Formation constants of the mixed and the single ligand complexes were determined from NMR data. The results indicate that formation constants for binding of a thiolate donor group by Cd2+, either as the free ion or in a coordinately unsaturated complex, are in the range 10(5)-10(6).     

Differentiation of Sclerotinia minor depends on thiol redox state and oxidative stress

Sclerotial differentiation in Sclerotinia minor is associated with oxidative stress and thiol redox state. The significance of oxidative stress to sclerotial differentiation was revealed by the higher oxidative stress of S. minor compared with a nonsclerotiogenic counterpart. The effect of thiol redox state on sclerotial differentiation was shown by the antioxidant action of the thiol (-SH) group of N-acetylcysteine and cysteine and by an unknown (not antioxidant) role of glutathione (GSH) on S. minor. The nonantioxidant role of GSH was indicated by the differentiation-inhibiting and differentiation-noninhibiting actions of the GSH biosynthesis inhibitor L-buthionine-S,R-sulfoximine and the GSH biosynthesis inducer L-2-oxo-thiazolidine-4-carboxylate, respectively, and by the increase of oxidative stress they caused during the transition from the undifferentiated to differentiated state of S. minor. Moreover, N-acetylcysteine can be used as a potent nontoxic fungicide against this phytopathogenic fungus by acting as a growth-inhibiting cytotoxic oxidant and by sustaining the fungus in the undifferentiated hyphal stage, which is vulnerable to degradation by soil microorganisms.     

Thiols as myeloperoxidase-oxidase substrates.

Nine low-Mr thiols were compared with regard to their ability to function as myeloperoxidase-oxidase substrates under conditions where no auto-oxidation of the thiols could be observed. The methyl and ethyl esters of cysteine were found to be about twice as active as cysteamine at pH 7.0, in terms of increased O2 consumption. Cysteine itself was poorly active, whereas glutathione, N-acetylcysteine and penicillamine were completely inactive as myeloperoxidase-oxidase substrates under these conditions. The structure-activity relationships indicated that both a free thiol and free amino group were required for peroxidase-oxidase activity, and also that a free carboxy group abolished activity. In analogy with cysteamine, the activities of both cysteine esters were inhibited by superoxide dismutase (less than 5 micrograms/ml) and by catalase and not by the hydroxyl-radical scavenger mannitol. In contrast with cysteamine, the activities of both cysteine esters were stimulated more than 2-fold by high concentrations (greater than 5 micrograms/ml) of superoxide dismutase. The activities of both cysteine esters exhibited broad pH optima at pH 7. A mechanism for the myeloperoxidase-oxidase oxidation of the cysteine esters is proposed, which is partly different from that previously proposed for cysteamine.     

Formation of difluorothionoacetyl-protein adducts by S-(1,1,2,2-tetrafluoroethyl)-L-cysteine metabolites: nucleophilic catalysis of stable lysyl adduct formation by histidine and tyrosine

19F NMR spectroscopy was used in conjunction with isotopic labeling to demonstrate that difluorothionoacetyl-protein adducts are formed by metabolites of the nephrotoxic cysteine conjugate S-(1,1,2,2-tetrafluoroethyl)-L-cysteine (TFEC). To determine which amino acid residues can be involved in adduct formation, the reactivity of TFEC metabolites with a variety of N-acetyl amino acids was also investigated. An N alpha-acetyl-N epsilon-(difluorothionoacetyl)lysine (DFTAL) adduct was isolated and characterized by 19F and 13C NMR spectroscopy and mass spectrometry. N alpha-Acetylhistidine and N-acetyltyrosine were found to act as nucleophilic catalysts to facilitate the formation of both the protein and DFTAL adducts. Adduct formation was greatly reduced when lysyl-modified protein was used as the substrate, indicating that lysyl residues are primary sites of adduct formation. However N alpha-acetyllysine, at concentrations of greater than 100-fold in excess compared to protein lysyl residues, was not effective in preventing binding of metabolites to protein. Therefore, nucleophilic catalysis at the surface of the protein may be an important mechanism for the binding of TFEC metabolites to specific lysyl residues in protein. TFEC metabolites were very reactive with the thiol nucleophiles glutathione and N-acetylcysteine. However, the predicted difluorodithioesters could not be isolated. Both stable difluorothioacetamide and less stable difluorodithioester protein adducts may play a role in TFEC-mediated nephrotoxicity.     

Dietary cysteine alleviates sucrose-induced oxidative stress and insulin resistance

Diets that promote oxidative stress favor impairment in glucose homeostasis. In this context, increasing the cysteine intake may be beneficial by maintaining glutathione status. We have investigated the effects of dietary cysteine on oxidative stress and glucose homeostasis in rats fed a high-sucrose (HS) diet. Rats were assigned for 6 weeks to a standard diet or to HS diets in which the protein source was either an alpha-lactalbumin-rich whey concentrate (a cysteine-rich protein) or the total milk proteins alone or supplemented with 5.8 or 20 g N-acetylcysteine per kilogram of food. Increasing the cysteine intake prevented HS-induced oxidative stress, as assessed by blood and tissue glutathione and carbonyl levels. At the same time, the HS-induced glucose intolerance, impaired postprandial glycemic control, and decrease in muscle and liver insulin-induced activation of insulin receptor substrate 1 and Akt were prevented by increasing the level of dietary cysteine, a major original finding. Of great interest was the observation that all beneficial effects of cysteine supplementation were duplicated by the consumption of a cysteine-rich protein. These data show that increasing the cysteine intake limits HS-induced impairment of glucose homeostasis and suggest that these effects are mediated by a reduction in oxidative stress.     

The reaction of N-acetylneuraminate lyase with chloropyruvate (Short Communication)

Chloropyruvate, like bromopyruvate, rapidly inactivates N-acetylneuraminate lyase at pH7.2. At 5 degrees C, 0.5mm-chloropyruvate reacted with the enzyme about ten times as fast as bromopyruvate. In contrast, at pH6.0 and 9 degrees C, chloropyruvate reacted with N-acetylcysteine seven times more slowly than bromopyruvate. A brief (2min) incubation of the enzyme with 1.0mm-chloro[(14)C]pyruvate gave an inactive enzyme in which 4.5 cysteine residues were alkylated per molecule of enzyme. This corresponded to the number of [(14)C]pyruvate residues (3.7) bound to the enzyme by borohydride reduction of the [(14)C]-pyruvate complex, and confirmed the previous suggestion that there is one ;essential' cysteine residue per active site. It is suggested that, for this enzyme, chloropyruvate can be selectively used to alkylate the active-site residues, whereas bromopyruvate cannot. The apparent molecular weight of the enzyme prepared by two slightly different methods was approx. 100000 or 250000. The latter value has been used to calculate the number of pyruvate residues bound to the enzyme.     

Effect of rifampin, phenobarbital pretreatment, and acetylator phenotype on acetylisoniazid metabolism in the rabbit

The metabolism of [14C]acetylisoniazid was studied in male New Zealand White rabbits. Pretreatment of the rabbits with the microsomal enzyme inducers rifampin and phenobarbital had little effect on acetylisoniazid metabolism. Rifampin appears to produce some inhibition of acetylation of the metabolite acetylhydrazine to diacetylhydrazine. Acetylation phenotype was an important factor. Covalent binding of 14C to hepatic protein increased as the acetylation rate decreased. In plasma and urine acetylhydrazine levels were negatively correlated with acetylation rate and diacetylhydrazine levels were positively correlated as one would expect. It was concluded that in the rabbit covalent binding to hepatic protein was more dependent on the acetylation rate than on induction of microsomal oxidase.     

Effect of a cysteine prodrug, L-2-oxothiazolidine-4-carboxylic acid, on the metabolism and toxicity of bromobenzene: an acute study

The effect of a cysteine prodrug, L-2-oxothiazolidine-4-carboxylic acid (OTCA), on certain aspects of the metabolism and toxicity of bromobenzene administered acutely to mice was investigated by (i) characterizing the influence of OTCA on the metabolic profile of low and high bromobenzene dose at 0-6, 6-12, and 12-24 h, (ii) determining the effective doses range and administration time for OTCA, as well as the optimum period for urine sampling; and (iii) measuring the efficacy of OTCA for protection against bromobenzene induced toxicity. Coadministration of OTCA and bromobenzene enhanced the urinary excretion of mercapturic acid and phenolic metabolites, during 6-12 h, by approximately 152 and 193%, respectively. Maximum efficacy was observed when OTCA (16.0 mmol/kg) was administered concomitantly with bromobenzene (4.0 mmol/kg). Finally, OTCA administration was found to afford substantial protection against elevation of plasma transaminases used as indices of bromobenzene-induced hepatotoxicity. N-acetylcysteine, another cysteine prodrug, had essentially similar effects on the metabolism and toxicity of bromobenzene. Thus, administration of cysteine prodrugs enhances the urinary excretion of several metabolites of bromobenzene and affords protection against bromobenzene-induced hepatotoxicity.     

Ascorbate abolishes auxotrophy caused by the lack of superoxide dismutase in Saccharomyces cerevisiae. Yeast can be a biosensor for antioxidants

Yeast (Saccharomyces cerevisiae) mutants lacking cytoplasmic superoxide dismutase (CuZnSOD) show Lys and Met auxotrophy under aerobic conditions. This metabolic defect can be ameliorated by exogenous ascorbate as well as other antioxidants (glutathione, cysteine and N-acetylcysteine). Restoration of growth of CuZnSOD- yeast mutants on media devoid of Met and/or Lys may therefore be a simple and useful means to detect and quantify antioxidants. The protective effect of antioxidants is oxygen-dependent: the lower the oxygen content of the atmosphere, the lower antioxidant concentrations are required to restore prototrophy. Therefore, the sensitivity of the test can be augmented by growing the yeast under lowered partial oxygen pressure. While 6 mM, 10 mM and 30 mM ascorbate was necessary to restore the growth in the absence of Met, in the absence of Lys, and in the absence of Lys and Met, respectively, under 21% oxygen, 3 mM and 6 mM ascorbate was sufficient for growth restoration in the absence of Lys and in the absence of Lys and Met, respectively, under 3% oxygen. The protective effects of cysteine and N-acetylcysteine peaked at 0.5 mM and 6 mM, respectively, disappearing at higher concentrations of these compounds, pointing to the detection of not only protective but also toxic cellular effects of the compounds studied by the test proposed.     

Amino acid-specific ADP-ribosylation. Sensitivity to hydroxylamine of [cysteine(ADP-ribose)]protein and [arginine(ADP-ribose)]protein linkages

Hydroxylamine stability has been used to classify (ADP-ribose)protein bonds into sensitive and resistant linkages, with the former representing (ADP-ribose)glutamate, and the latter, (ADP-ribose)arginine. Recently, it was shown that cysteine also serves as an ADP-ribose acceptor. The hydroxylamine stability of [cysteine([32P]ADP-ribose)]protein and [arginine([32P] ADP-ribose)]protein bonds was compared. In transducin, pertussis toxin catalyzes the ADP-ribosylation of a cysteine residue, whereas choleragen (cholera toxin) modifies an arginine moiety. The (ADP-ribose)cysteine bond formed by pertussis toxin was more stable to hydroxylamine than was the (ADP-ribose)arginine bond formed by choleragen. The (ADP-ribose)cysteine bond apparently represents a third class of ADP-ribose bonds. Pertussis toxin ADP-ribosylates the inhibitory guanyl nucleotide-binding regulatory protein (Gi) of adenylate cyclase, whereas choleragen modifies the stimulatory guanyl nucleotide-binding regulatory protein (Gs). These (ADP-ribose)protein linkages are identical in stability to those formed in transducin by the two toxins, consistent with the probability that cysteine and arginine are modified in Gi and Gs, respectively. Bonds exhibiting differences in hydroxylamine-stability were found in membranes from various non-intoxicated mammalian cells following incubation with [32P]NAD, which may reflect the presence of endogenous NAD:protein-ADP-ribosyl-transferases.     

Post-translational protein modification by carotenoid cleavage products

Carotenoids are known to generate various aldehydes, known as carotenoid-derived aldehydes (CDAs), which could efficiently react with protein or DNA. In this in vitro model study, interaction between CDA and protein has been studied. Various proteins were incubated with CDA, and protein modification and adduct formation were confirmed by using matrix-assisted laser desorption and ionization time-of-flight, amino acid analysis, and measuring enzyme activity on modification with CDA. Using radiolabeled NaB((3) H)H(4) and Raney nickel as well as sulfhydryl assay (Ellman's reagent), we confirmed that CDA could conjugate with cysteine through a thioether linkage. The carbonyl assay using 2,4-dinitrophenylhydrazine revealed the possible involvement of Schiff's base reaction between CDA and lysine. The adducts formed between β-apo-8-carotenal (BA8C) and N-acetylcysteine and BA8C and N-acetyllysine were confirmed by HPLC and ESI-MS. Our results suggest that CDA could alter protein function by post-translational interaction with cysteine and lysine by thioether linkage and by schiff's based bonds, respectively. Thus, the formation of CDA adducts with proteins could alter functional properties of proteins responsible for maintaining cell homeostasis and thereby cause cellular toxicity. In view of these observations, further studies are required to understand the delicate balance between beneficial and/or harmful effects of carotenoids as a dietary supplement to slow age-related macular degeneration progression.     

Direct measurement of the rate of glutathione synthesis in 1-chloro-2,4-dinitrobenzene treated human erythrocytes

Cell glutathione scavenges free radicals, degrades peroxides, removes damaging electrophiles and maintains the redox state. The aim of this study was to develop an effective and efficient method to measure the rate of glutathione synthesis from its constituent amino acids in whole erythrocytes (RBCs). RBCs (10% haematocrit) were exposed to 0.3 mM 1-chloro-2,4-dinitrobenzene (CDNB) to lower their total glutathione content by 70% and then incubated with glucose, and N-acetylcysteine as a cysteine source. Over 3 h, glutathione levels increased at a constant rate of 1.2 micromol (L RBC)(-1)min(-1), almost 5 times faster than the rate of glutathione synthesis in RBCs with normal glutathione levels. Glutathione at concentrations normally found in RBCs is known to inhibit glutamate cysteine ligase (the major rate controlling enzyme for glutathione synthesis). The rate of glutathione recovery was substantially reduced in RBCs treated with buthionine sulfoximine, a specific inhibitor of glutamate cysteine ligase. Our results indicate that the measurement of glutathione recovery rate after CDNB treatment can be used to estimate de novo synthesis of glutathione. Application of this direct method for measuring glutathione synthesis will increase understanding of the interactions of effectors that determine glutathione levels in RBCs under various physiological and pathological conditions.     

Effects of N-acetylcysteine amide (NACA), a thiol antioxidant on radiation-induced cytotoxicity in Chinese hamster ovary cells

Ionizing radiation is known to cause tissue damage in biological systems, mainly due to its ability to produce reactive oxygen species (ROS) in cells. Many thiol antioxidants have been used previously as radioprotectors, but their application has been limited by their toxicity. In this investigation, we have explored the possible radioprotective effects of a newly synthesized thiol antioxidant, N-acetylcysteine amide (NACA), in comparison with N-acetylcysteine (NAC), a commonly used antioxidant. Protective effects of NACA and NAC were assessed using Chinese hamster ovary (CHO) cells, irradiated with 6 gray (Gy) radiation. Oxidative stress parameters, including levels of reduced glutathione (GSH), cysteine, malondialdehyde (MDA), and activities of antioxidant enzymes like glutathione peroxidase, glutathione reductase, and catalase, were measured. Results indicate that NACA was capable of restoring GSH levels in irradiated cells in a dose dependent manner. In addition, NACA prevented radiation-induced loss in cell viability. NACA further restored levels of malondialdehyde, caspase-3 activity, and antioxidant enzyme activities to control levels. Although NAC affected cells in a similar manner to NACA, its effects were not as significant. Further, NAC was also found to be cytotoxic to cells at higher concentrations, whereas NACA was non-toxic at similar concentrations. These results suggest that NACA may be able to attenuate radiation-induced cytotoxicity, possibly by its ability to provide thiols to cells.