Scavenging of hypochlorous acid by lipoic acid.

2244 Short communications 7. Distlerath LM and Guengerich FP, Characterization of a human liver cytochrome P-450 involved in the oxidation of debrisoquine and other drugs by using antibodies raised to the analogous rat enzyme. Proc Nad Acad Sci USA 81: 7348-7352, 1984. _ 8. Anthonv L. Koshakuii R and Wood AJJ. Multiole pathways oi propranofol’s metabolism are inhibited-by debrisoquin. Clin Pharmacol Ther 46: 297-300, 1989. 9. Boobis AR, Murray S, Hampden CE and Davis DS, Genetic polymorphism in drug oxidation: in vitro studies of human debrisoquine 4-hydroxylase and bufuralol 1‘-hydroxylase activities. Biochem Pharmacol 34: 65-71, 1985. 10. Matsunaga E, Zanger UM, Hardwick JP, Gelboin HV, Meyer US and Gonzalez FJ, The CYP2D gene subfamily: analysis of the molecular basis of the debrisoquine 4-hydroxylase deficiency in DA rats. Biochemistry 28: 7349-7355, 1989. Biochemical Phormocology. Printed in Great Britain. 861-865, 1991. Vol. 42. No. 11. pp. 2244-2246. 161, 1987. 17. Kahn GC, Rubenfield M, Davies DS, Murray S and Boobis AR, Sex and strain differences in hepatic debrisoquine 4-hydroxylase activity of the rat. Drug Metab Dispos 13: 510-515, 11. Omura T and Sato R, The carbon monoxide-binding pigment of liver microsomes. J Biof Chem 239: 23702378, 1964. 12. Yamaoka K, Tanigawara Y, Nakagawa T and Uno T, A pharmacokinetics analysis program (MULTI) for microcomnuter. J Pharmacobio Dvn 4: 879-885.1981. 13. Masubuchi Y, Fujita S, Chiba M, i(agimoto N, timeda S and Suzuki T, Impairment of debrisoquine 4hydroxylase and related monooxygenase activities in the rat following treatment with propranolol. Biochem Pharmacol41: 14. Lowry OH, Rosebrough NJ, Farr AL and Randall RJ, Protein measurement with the Folin phenol reagent. J Biol Chem 193: 265-275, 1951. 15. Al-Dabbagh SG, Idle JR and Smith RL, Animal modelling of human polymorphic drug oxidationmetabolism of debrisoquine and phenacetin in rat inbred strains. J Pharm Pharmacol33: 161-164, 1981. 16. Gonzalez FJ, Matsunaga T, Nagata K, Meyer US, Nebert DW. Pastewka J. Kozak CG. Gillette J. Gelboin HV and Hgrdwick JP,’ Debrisoqiine 4-hydkoxylase: characterization of a new P450 gene subfamily, regulation, chromosomal mapping, and molecular analysis of the DA rat polymorphism. DNA 6: 149- 1985. 18. Dannan GA, Guengerich FP and Waxman DJ, Hormonal regulation of rat liver microsomal enzymesRole of gonadal steroids in programming, maintenance, and suppression of A4-steroid 5a-reductase, flavincontain&g monooxygenase, and sex-specific cytochrome P-450. J Biol Chem 261: 10728-10735. 1986. 19. Kato R, Yamazoe Y, Shimada M, Murayama N and Kamataki T, Effect of growth hormone and ectopic transplantation of pituitary gland on sex-specific forms of cytochrome P-450 and testosterone and drug oxidations in rat liver. J Biochem 100: 895-902, 1986. 1991. 0 KM-2952/91 $3.00 + 0.00 1991. Pergamon Press plc Scavenging of hypochlorous acid by lipoic acid (Received 9 April 1991; accepted 3 August 1991) Upon stimulation, neutrophils are able to produce the reactive oxygen species, superoxide (O,‘-) and hydrogen peroxide. Moreover, neutrophils release the enzyme myeloperoxidase. This enzyme catalyses the conversion of chloride (Cl-) to the powerful oxidant hypochlorous acid (HOC]*) [l, 21. The reactive oxygen species and HOC1 contribute to the bactericidal action of neutrophils. However, the damaging effect of these products is not limited to bacteria, also the surrounding tissue is vulnerable. An important target for HOC1 is the ru,-antiproteinase (KY,AP). (Y,-AP is the most important inhibitor of elastase [l, 21. HOC1 oxidizes a critical methionine residue of LY,AP to a sulphoxide derivative with the consecutive loss of activity of the inhibitory protein [3]. In addition, activated neutrophilsexcrete elastase. A resulting imbalance between elastase and anti-elastase activity in the respiratory tract may cause the enzymatic destruction of the elastic fibers in the lung, a process believed to be central in the development of certain types of emphysema [l-3]. Lipoate is an 8-carbon fatty acid with, in its reduced form, two thiol groups on the 6th and 8th carbon atom. Oxidized lipoate contains an intramolecular disulfide bridge in a S-membered ring. Lipoate has been shown to be a * Abbreviations: HOCl, hypochlorous acid; (u,-AP, cul-antiproteinase; GSH, glutathione; GSMe, S-methyl glutathione; GSSG, oxidized glutathione. potent antioxidant [4,5]. In the present study the ability of liooate to scavenge HOC1 was determined. The scavekging activity is -compared to that of the potent scavengers N-acetylcysteine and GSH. Materials and Methods N-Acetylcysteine, GSH, GSSG, S-methyl glutathione (GSMe), N-t-BOC-L-alanine p-nitrophenol ester, (Y,antiproteinase (cY,-AP, code A 9024) and elastase (code E 0258) were obtained from the Sigma Chemical Co. (St Louis, MO, U.S.A.). Reduced lipoate (dihydrolipoate) and oxidized lipoate were gifts from Asta Pharma A.G. (Frankfurt am Main, Germany). Elastase activity was determined according to the assay described by Wasil et al. [2], with minor modifications. All reagents were dissolved in phosphate buffer used (19 mM KH2P04-KOH) pH 7.4, containing 140 mM NaCI, unless otherwise noted. Twenty micrograms of (u,-AP (unless otherwise noted) were mixed with the compound under investigation (&lOOpM) and preincubated it 25”. After 5 min, HOC1 was added (50 pM unless otherwise noted). The final volume was 100 pL. The concentration of the test comoounds c&lOOuM\ and HOC1 rusuallv 5OuM) indidated in ‘the te’xt ‘and the figures: refeis to ’ thd concentration in the 100 pL incubation mixture. After an additional 5 min, 200 pL buffer containing 5 pg elastase was added. Again after 5 min. 700 pL buffer and 50 pL of Short communications a 10 mM solution of N-t-BOC-L-alaninep-nitrophenol ester in methanol was added. Immediately after the addition of N-t-BOC-L-alanine p-nitrophenol ester, the increase in absorption at 410nm was determined, which represents the activity of elastase. In some experiments, the test ~rn~und and HOC1 were mixed before rxi-AP was added. In other experiments, a+4P and HOC1 were mixed before the test compound was added. None of the test compounds affected the elastase activity directly, nor the ability of oiAP to inhibit elastase (data not shown). All reactions were carried out at 25”. Dete~ination of elastase activity was reproducible within 5%. HOC1 was determined by adding iodate free potassium iodide to a solution of HOC1 in 10% acetic acid, and titrating the liberated iodine with thiosulfate PI. Results As shown in Fig. 1, a,-AP inhibited elastase activity in a dose-dependent manner. Twenty micrograms of a,-AP gave a complete inhibition of proteolytic activity of 5 @g elastase. In the follo~ng experiments 20 pg of at-AP was used. Prein~bation of (u,-AP with HOC1 destroyed the ability of (u,-AP to inactivate elastase. At a HOCl concentration of SOgM, no in~bition of elastase activity by (u,-AP was observed (Fig. 2). This indicates that at this concentration HOC1 inactivated all q-AP. The concentration of 50 i.cM HOC1 was used in the experiments where the scavenging of HOC1 by the compounds was determined. O- 100 0 u :fantiproteasZ(pg) 20 [HOCU ($W 60 Fig. 2. Concentration-dependent inactivation of cu,antiproteinase ((r,-AP) by HOCI. Twenty mi~o~ams of at-AP were pr~i~~b~te~ with several ~ncentr~tions of HOCl. After 5 min. 5 UII elastase was added. The inactivation of LY,-AP rest&s in an increase in elastase activity. Elastase activity was measured according to the method described in Materials and Methods, and it is expressed as percentage of that in the absence of tui-AP. BP 42tll-t GSH (Fig. 3) and Wacetylcysteine (Fig. 4) prevented the inactivation of 2Opg (u,-AP by 50lM HOC1 very efficiently. At a concentration of 80 JJM of either GSH or N-acetylcysteine almost no activity of elastase could be detected, indicating that at this concentration both GSH and ~-acetyl~ysteine almost completely protect against the inactivation of (u,-AP by HOCl. The protective effect was not due to the regeneration of rui-AP after it has been inactivated by HOCl. This can be concluded from the experiment where a,-AP was first inactivated by 5OpM HOCL Addition of lC@&4 GSH or N-acetvicvsteine to the inactivated cr,-AP did not restore the inhibibitoryeffect of a(,-AP on elastase (data not shown). Therefore, the most likely mechanism by which GSH and N-acetylcysteine protect cq-AP is that they scavenge HOC1 before it can react with or-AP. The effect of GSMe was comparable to that of GSH, although GSMe was slightly less potent (Fig. 3). GSSG was not able to protect efficiently against the inactivation of ar-AP by HOC1 (Fig. 3). Preincubation of 50pM HOCl with 20 FM GSSG before the addition of q-AP did protect against the inactivation of (wt-AP (data not shown). Apparently GSSG scavenges HOCl, but the rate of the reaction of HOC1 with GSSG is much lower than the rate of the reaction of HOC1 with the methionine residue of aj-AP. The reduced form of lipoate, dihydro~~ate, appeared to be a very effective scavenger of HOC1 (Fig. 4), its potency is comparable to that of GSH and N-acetylcysteine. 30 Fig. 1. Dose-dependent inhibition of elastase by cyrantiproteinase (a,-AP). Five micrograms of elastase were used. Elastase activity was measured according to the method described in Materials and Methods, and it is expressed as percentage of that in the absence of a,-AP. 0 2245 Corme”,tion (pM) Fig. 3. Concentration-dependent protection of of-antipr&einase ((u,-AP) by GSH(+), &Me (0) and GSSG (Cl) against the inactivation bv HOCI. The comoounds here mixed with 20 fig o,-AP’before 50~M HO-Cl was added. Elastase activity was measured according to the method described in Materials and Methods, and it is expressed as percentage of that in the absence of or-AP. 0 100 Conce?mtion (FM) Fig. 4. Concentration-dependent protection of q-antinroteinase (at-AP) by reduced i&ate (Cl), oxidized l&ate (+) and ~~a~tyl~y~eine (0) against the inactivation by HOCl. The compounds were mixed with 2Opg (u,-AP before 50~M HOC1 was added. Elastase activity was measured according to the method described in Materials and Methods, and it is expressed as percentage of that in the absence of o-r-AP. 2246 Short communications To our surprise, the oxidized form of lipoate was equally effective in scavenging HOC1 as dihydrolipoate (Fig. 4). In contrast to oxidized glutathione, oxidized lipoate appears to compete with the methionine residue of (u,-AP in the reaction with HOCI. Discussion Data from literature and also from this study indicate that several thiol-containing compounds, like cysteamine [7], N-acetylcysteine [8, this study] and GSH [this study] are effective scavenaers of HOCl. Therefore, it did not come as a surprise That reduce lipoate, a molecule that contains two thiol groups, is aiso-a potent scavenger of HOCI. Moreover. it was found that GSMe was a good scavenger. That a compound with a methylated thiol gioup can act as a scavenger of HOC1 is also not surprising, because it is known that a methionine residue (which also contains a methylated thiol group) in cu,-AP is attacked by HOC! on its sulphur atom (31. In addition, we found that GSSG is only a poor scavenger of HOCI. GSSG reacts with HOCI, but the reaction rate is too slow to provide an efficient protection of the methionine residue of o,-AP. Unexpectedly, we found that oxidized lipoate is a potent scavenger of HOCI. In the past, the antioxidant activity of lipoate has been ascribed to its reduced form. Dihydrolipoate is able to elevate the concentration of GSH by reducing GSSG [4]. In addition, it has been stated that dihydroli~ate may protect against lipid peroxidation in an interaction with vitamin E [S]. The oxidized form of hpoate has no protective effect-on the orocess of lioid oeroxidation 14.51. The findings of this Study indicate thit, with respecitd the scavenging<f HOCI, oxidized lipoate, as well as reduced lipoate, acts as a potent antioxidant. This is of special importance because lipoate is used therapeutically in its oxidized form. The most striking result in this study was the difference between GSSG, not a potent scavenger of HOCI, and oxidized hpoate, a very good scavenger of HOCI. The explanation for this differ&ce may be Found in the nature of the disulfide bridge in both molecules. Dihvdroliooate contains an intramol&dar S-S bridge. Because this bridge is fixed in a 5membered ring, some ring strain exists. It has been reported that the angle between both suiphur molecules is energetically not optimal [9]. In GSSG no ring strain exists because it is a gexible molecule. The difference of the S-S bridges probably explains the difference between GSSG and oxidized lipoate in their ability to scavenge HOC]. By scavenging HOCI, oxidized lipoate is probably converted into a sulphoxide. The sulphoxide of dihydrolipoate is also formed in the reaction of dihydroli~ate with hydrogen peroxide [lo]. In summary, it was found that reduced GSH, N* Corresponding author. acetylcysteine and GSMe are potent scavengers of HOCl, while GSSG is not. Surprisingly, not only reduced but also oxidized lipoate is an effective scavenger. The difference in scavenging effect between oxidized GSH and oxidiied lipoate is probably caused by the difference in reactivity of the disulfide bridge in both molecules. The present results indicate that lipoate might be effective in the treatment of emphysema caused by the destruction of (x1antiproteinase. Department of Ph~~&o&he~~ F~~~ of Chemistry Vrije ~niversiteit De Boelelaan 1083 1081 HV Amsterdam The Netherlands GUIDO R. M. M. HAENEN’ AALT BAST 1. Clark RA, Stone PJ, Hag AE, Galore JD and Franzblau C, Myeloperoxidase-catalyzed inactivation of I%,proteinase inhibitor by human neutrophils. J Biol Chem 256: 334&3353, 1981. 2. Wasil M, Halliwell B, Hutchison DCS and Baum H, The antioxidant action of human extracellular fluids. Biochem J 243: 219-223, 1987. 3. Matheson NR and Travis J. Differential effects of oxidizing agents on human plasma cyt-proteinase inhibito; and human neutrophil myeloperoxidase. J Biochem 24: 1941-1945, 1985. 4. Bast A and Haenen GRMM, Interplay between lipoic acid and glutathione in the protection against microsomal lipid peroxidation. Biochim Siophys Acra 963: 558-561, 1988. 5. Scholich II, Murphy ME and Sies H, Antioxidant activity of dihydrolipoate against microsomaf lipid peroxidation and its dependence on o-tocopherol. Biochim Biophys Acta 1001: 265-261, 1989. 6. Vogel AI, Textbookof QuantitativeInorganicAnalysis, 3rd edn, pp. 364-365, Longmans, London, 1961. 7. Aruoma 01, Ha&well B, Hoey BM and Butler .I, The antioxidant action of taurine, hv~~u~ne and their metabolic precursors. Biochem Ji56: 251-255,1988. 8. Aruoma 01. HaItiwell B. Hoev BM and Butler J. The antioxidant ‘action of N-acety&ysteine. Free Rad. Biol Med 6: 593-597, 1989. 9. Schmidt U, Grafen P, Altland K and Goedde HW, Biochemistry and chemistry of lipoic acids. In: Advances inwEnzymology(Ed. Nord l??), Vol. 32, pp. 423-467. Interscience Publishers. New York. 1969. 10. Calvin M, Chemical and photo~he~~l reactions of thioctic acid and related disulfides. Fed Proc 13: 697711, 1954. 3iochmdcal Pharmacoiogy, Vol. 42. No. 11. * m. . 2246-2249. 1991. Printed in Great Britain. @06-2952191 S3.00 + 0.00 Press ptc @ 1991. Pergmon Interspecies differences in in vitro etoposide plasma protein binding (Received 20 May 1991; accepted 31 July 1991) Preclinical studies in laboratory animals are used to define initial pharmacologic and toxicologic endpoints of anticancer drugs. Phase I protocols of anticancer drugs initiate doses in humans equal to or less than one-tenth the mouse lethal dose (LD,~), and titrate the dose upwards until dose-limiting toxicity is observed. This procedure is based on previous observations that a quantitative relationship exists between toxic doses of anticancer drugs in animals and humans [l]. Despite these similarities, often several dose escalations are required to reach the maximum