766 Biol. Pharm. Bull. 29(4) (2006)

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1 766 Biol. Pharm. Bull. 29(4) (2006) Vol. 29, No. 4 Characterization of the Radical-Scavenging Reaction of 2-O-Substituted Ascorbic Acid Derivatives, AA-2G, AA-2P, and AA-2S: A Kinetic and Stoichiometric Study Jun TAKEBAYASHI, Akihiro TAI, Eiichi GOHDA, and Itaru YAMAMOTO* Department of Immunochemistry, Faculty of Pharmaceutical Sciences, Okayama University; Okayama , Japan. Received June 13, 2005; accepted December 13, 2005 The aim of this study was to characterize the antioxidant activity of three ascorbic acid (AA) derivatives O- substituted at the C-2 position of AA: ascorbic acid 2-glucoside (AA-2G), ascorbic acid 2-phosphate (AA-2P), and ascorbic acid 2-sulfate (AA-2S). The radical-scavenging activities of these AA derivatives and some common low molecular-weight antioxidants such as uric acid or glutathione against 1,1-diphenyl-picrylhydrazyl (DPPH) radical, 2,2 -azinobis(3-ethylbenzothiazoline-6-sulfonic acid) radical cation (ABTS ), or galvinoxyl radical were kinetically and stoichiometrically evaluated under ph-controlled conditions. Those AA derivatives slowly and continuously reacted with DPPH radical and ABTS, but not with galvinoxyl radical. They effectively reacted with DPPH radical under acidic conditions and with ABTS under neutral conditions. In contrast, AA immediately quenched all species of radicals tested at all ph values investigated. The reactivity of Trolox, a water-soluble vitamin E analogue, was comparable to that of AA in terms of kinetics and stoichiometrics. Uric acid and glutathione exhibited long-lasting radical-scavenging activity against these radicals under certain ph conditions. The radical-scavenging profiles of AA derivatives were closer to those of uric acid and glutathione rather than to that of AA. The number of radicals scavenged by one molecule of AA derivatives, uric acid, or glutathione was equal to or greater than that by AA or Trolox under the appropriate conditions. These data suggest the potential usage of AA derivatives as radical scavengers. Key words ascorbic acid derivative; long-lasting radical scavenging; kinetic study; stoichiometric study; 1,1-diphenyl-picrylhydrazyl (DPPH) radical; 2,2 -azinobis(3-ethylbenzothiazoline-6-sulfonic acid) radical cation (ABTS ) L-Ascorbic acid (AA), known as vitamin C, plays an important role in many biological processes, 1,2) including antioxidant activity. 3) However, AA decomposes upon exposure to heat, UV light, metal ions, and oxidants. These susceptibilities have led to the development of stable derivatives. The stabilization of AA is focused on the chemical modification of a 2,3-enediol moiety in the five-membered lactone ring, since the 2,3-enediol moiety is the reason for the instability of AA. L-Ascorbic acid 2-phosphate (AA-2P), 4) L-ascorbic acid 2-sulfate (AA-2S), 5) 2-O-octadecylascorbic acid (CV- 3611), 6) and L-ascorbyl 2,6-dipalmitate 7) have been reported to be stable AA derivatives that are O-substituted at the C-2 position of AA. We have also succeeded in synthesizing a 2- O-substituted stable AA derivative, 2-O-a-D-glucopyranosyl- L-ascorbic acid (ascorbic acid 2-glucoside, AA-2G). 8 11) AA-2G has been approved by the Japanese government as a quasidrug principal ingredient in skin care products and as a food additive and is currently being used as a medical additive in commercial cosmetics. Of these 2-O-substituted AA derivatives, AA-2G, AA-2P, and L-ascorbyl 2,6-dipalmitate exhibit inherent biological activity such as antiscorbutic activity after their enzymatic hydrolysis to free AA in vivo ) On the other hand, AA-2S and CV-3611 do not exert such activity because they cannot be enzymatically converted into AA. 19,20) In any event, AA derivatives themselves have been considered to have no activity. Recently, however, some 2-O-substituted AA derivatives per se have been found to have radical-scavenging activity. CV-3611 has been shown to have antioxidant activity without being converted into AA. 6) We have also found that AA-2G exerts radical-scavenging activity against 1,1-diphenyl-2- picrylhydrazyl (DPPH) radical 21,22) and 2,2 -azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical cation (ABTS ) 23) without enzymatic hydrolysis. The characteristics of the radical-scavenging reaction of AA-2G are: 1) AA- 2G scavenges these radicals more slowly than AA 22,23) ; 2) the quantity of radicals scavenged by AA-2G is greater than that by AA over a long-term period 22,23) ; and 3) the reactivity of AA-2G is affected by the ph of the solution ) AA-2G exhibits DPPH radical-scavenging activity under ph-uncontrolled conditions, but AA-2P and AA-2S do not, 21) when AA-2G, AA-2P, and AA-2S were employed in the available form of free acid, magnesium salt, and potassium salt, respectively (Fig. 1). AA-2G shows the DPPH radical-scavenging activity under acidic conditions but it shows little activity under neutral conditions, suggesting that the reactivity of AA-2G is affected by the dissociation degree of the acidic hydroxyl group at the C-3 position of the AA moiety. Since AA-2P and AA-2S are salt forms, their activity might be underestimated under ph-uncontrolled conditions. In this study, we assessed the radical-scavenging activities of AA-2G, AA-2P, and AA-2S under ph-controlled conditions. Experiments were performed with attention to the following proposals and points of view: 1) the activities of some antioxidants vary with the assay method, and thus using multiple methods is recommended ) 2) Niki et al. reported that there are two types of antioxidant scavenging radicals quickly and quenching many radicals, and proposed assess- Fig. 1. Chemical Structures of AA and 2-O-Substituted AA Derivatives To whom correspondence should be addressed. iyamamoto@pheasant.pharm.okayama-u.ac.jp 2006 Pharmaceutical Society of Japan

2 April Fig. 2. Chemical Structures of DPPH Radical, ABTS, and Galvinoxyl Radical ing reactivity based on both reaction rate and stoichiometry. 24) 3) Comparative studies with common antioxidants are essential to clarify the biological significance of the activity of AA derivatives, because the radicals used in this study do not exist in vivo. Therefore we kinetically and stoichiometrically evaluated the reactivity of AA-2G, AA-2P, and AA-2S against three radicals, the DPPH radical, ABTS, and galvinoxyl radical (Fig. 2) and compared the radical-scavenging properties of three AA derivatives with those of AA, uric acid, Trolox (a water-soluble vitamin E analogue), and glutathione. Our data show the function of these AA derivatives per se, which has been nearly ignored before, and we propose the possible usage of AA derivatives without converting into AA in vivo as radical scavengers. MATERIALS AND METHODS Chemicals AA and glutathione (reduced form) were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan). AA-2G was a gift from Hayashibara Biochemical Laboratories, Inc. (Okayama, Japan). AA-2P, AA- 2S, uric acid, ABTS, and horseradish peroxidase (HRP; type VI-A, essentially salt-free 1310 units/mg solid) were obtained from Sigma Chemical Co. (St. Louis, MO, U.S.A.). DPPH was obtained from Aldrich Chemical Co. (Milwaukee, WI, U.S.A.). Galvinoxyl was obtained from Tokyo Kasei Kogyo, Co. (Tokyo, Japan). 6-Hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox) was obtained from Calbiochem Co. (San Diego, CA, U.S.A.). H 2 O 2 (30%) was obtained from Nacalai Tesque, Inc. (Kyoto, Japan). Reagents were used without further purification. All water used was Milli-Q grade. Methods To examine the kinetic and stoichiometric analysis of the radical-scavenging activity of AA-2G, AA-2P, and AA-2S per se, a spectrophotometric method was used. In the experiments presented below, we confirmed that these derivatives did not degrade to AA (data not shown). DPPH Radical-Scavenging Assay The radical-scavenging activity of the AA derivatives, AA, and other common antioxidants against DPPH radical was assessed as described in our previous paper 22) with a slight modification. Briefly, DPPH (100 m M) was mixed with an antioxidant (20 m M) in 60% ethanol/40% citric acid-sodium citrate buffer (10 mm, ph 3 6). The reaction was carried out under an atmosphere of argon at 25 C. Changes in the absorbance at 524 nm due to the scavenging of DPPH radical were measured with a spectrophotometer (Shimadzu UV-1200). ABTS -Scavenging Assay The radical-scavenging activity of the AA derivatives, AA and other common antioxidants against ABTS was assessed as described in our previous paper. 23) Briefly, ABTS (100 m M) generated by an ABTS/H 2 O 2 /HRP system was mixed with an antioxidant (20 m M) in citric acid-sodium citrate buffer (50 mm, ph 3 6). The reaction was carried out under an atmosphere of argon at 25 C. Changes in the absorbance at 730 nm due to the scavenging of ABTS were measured with a spectrophotometer. Galvinoxyl Radical-Scavenging Assay The radicalscavenging activity of the AA derivatives, AA, and other common antioxidants against galvinoxyl radical was assessed using a modification of the method of Shi et al. 28) Galvinoxyl (100 m M) was mixed with an antioxidant (20 m M) in 60% ethanol/40% citric acid-sodium citrate buffer (10 mm, ph 3 6). The reaction was carried out under an atmosphere of argon at 25 C. Changes in the absorbance at 432 nm due to the scavenging of galvinoxyl radical were measured with a spectrophotometer after 10-fold dilution with 60% ethanol/ 40% buffer. Stoichiometric Studies The number of radicals scavenged by each antioxidant was calculated using the following equation: RSA(n) (DA 120 /A 0 ) [radical]/[antioxidant] where RSA(n) is radical-scavenging activity factor n in moles of radicals scavenged by each mol of antioxidant; DA 120 the absorbance difference between the reaction solution and control at 120 min; A 0 the initial absorbance of the control; [radical] radical concentration; and [antioxidant] the AA derivative, AA, or other common antioxidant concentration. RESULTS DPPH Radical-Scavenging Activity The time-course study of the DPPH radical-scavenging reaction of 2-O-substituted AA derivatives and other antioxidants was carried out in 60% ethanol/40% citrate buffer, ph 3, for 2 h (Fig. 3), since AA-2G reacted with DPPH radical more effectively at ph 3 than at ph 6, as shown in our previous paper. 22) The antioxidants were divided into two groups: those that scavenged DPPH radical rapidly and those that quenched DPPH radical slowly and continuously. The former are AA and Trolox; the latter are AA-2G, AA-2P, AA-2S, uric acid, and glutathione. The reactions for AA and Trolox were completed within 10 min. On the other hand, the reactions for the AA derivatives, uric acid, and glutathione were not completed in 2 h and did not show full saturation for another several hours (data not shown). The slow-reacting antioxidants could be further divided into two groups based on the reaction profile: AA-2G and uric acid showed a biphasic reaction trend, with a relatively fast initial phase followed by a continued slow reaction; while AA-2P, AA-2S, and glutathione exhibited only a continued slow reaction. The AA derivatives had slow and long-lasting DPPH radical-scavenging activity. Therefore the stoichiometry of the long-lasting reaction of each antioxidant was studied at ph values of 3 6 (Fig. 4). The stoichiometric factor RSA(n) was defined as the number of radicals consumed per molecule of each antioxidant in a 2-h reaction. The RSA(n) values of AA and Trolox in the ph range of 3 to 5 were ca. 2, while

3 768 Vol. 29, No. 4 Fig. 3. Time Course of DPPH Radical-Scavenging Reaction of 2-O-Substituted AA Derivatives and Common Antioxidants AA derivatives (or common antioxidants, 20 m M) and DPPH radical (100 m M) were incubated in 60% ethanol/40% citrate buffer (10 mm, ph 3) under an atmosphere of argon at 25 C. Changes in absorbance at 524 nm due to scavenging of DPPH radical were monitored at the indicated times. Values are mean S.D. of three separate experiments. Fig. 4. Effects of ph on RSA(n) of 2-O-Substituted AA Derivatives and Common Antioxidants against DPPH Radical The reaction mixture contained AA derivatives (20 m M) or common antioxidants (20 m M) and DPPH radical (100 m M) in 5.0 ml of 60% ethanol/40% citrate buffer (10 mm, ph 3 6). The reaction was carried out under an atmosphere of argon at 25 C for 2 h. The number of DPPH radicals scavenged per molecule of AA derivatives or common antioxidants is expressed as RSA(n). Results are mean S.D. (n 6). Fig. 5. Time Course of ABTS -Scavenging Reaction of 2-O-Substituted AA Derivatives and Common Antioxidants AA derivatives (or common antioxidants, 20 m M) and ABTS (100 m M) were incubated in citrate buffer (50 mm, ph 6) under an atmosphere of argon at 25 C. Changes in absorbance at 730 nm due to scavenging of ABTS were monitored at the indicated times. Values are mean S.D. of three separate experiments. the value at ph 6 was slightly less than 2. The RSA(n) of AA-2G in the ph range of 3 to 4 was greater than that of AA. The scavenging ability of AA-2G sharply decreased with increasing ph and disappeared at ph 6. AA-2P and AA- 2S showed a profile similar to AA-2G although there were slight differences; the RSA(n) of AA-2P and AA-2S at ph 3 were 1.3 and 1.5, respectively, which did not exceed that of AA, and the reactivity of AA-2P remained to some extent at ph 6. The RSA(n) of uric acid was greater than that of AA under acidic conditions but was reduced to the same level of that of AA and Trolox at ph 6. Unlike the case of the other slow-reacting antioxidants, the RSA(n) value of glutathione at ph 6 was greater than that at ph 3. Glucose, which is a partial structure of AA-2G, did not react with DPPH radical at all (data not shown). ABTS -Scavenging Activity The reaction of 2-O-substituted AA derivatives and other antioxidants toward ABTS was measured in citrate buffer, ph 6, for 2 h (Fig. 5), since AA-2G was found to react with ABTS more effectively at ph 6 than at ph 3. 23) The reaction of AA and Trolox occurred rapidly and was completed within 10 min. On the other hand, the reaction of the three AA derivatives, uric acid, and glutathione proceeded continuously for 1 2 h. The reaction of AA-2P and glutathione as well as AA-2G and uric acid occurred via a relatively rapid step followed by a slow step, but that of AA-2S showed only a slow step.

4 April Fig. 6. Effects of ph on RSA(n) of 2-O-Substituted AA Derivatives and Common Antioxidants against ABTS The reaction mixture contained AA derivatives (20 m M) or common antioxidants (20 m M) and ABTS (100 m M) in 5.0 ml of citrate buffer (50 mm, ph 3 6). The reaction was carried out under an atmosphere of argon at 25 C for 2 h. The number of ABTS scavenged per molecule of AA derivatives or common antioxidants is expressed as RSA(n). Results are mean S.D. (n 6). Fig. 7. Time Course of Galvinoxyl Radical-Scavenging Reaction of 2-O-Substituted AA Derivatives and Common Antioxidants AA derivatives (or common antioxidants, 20 m M) and galvinoxyl radical (100 m M) were incubated in 60% ethanol/40% citrate buffer (10 mm, ph 6) under an atmosphere of argon at 25 C. Changes in absorbance at 432 nm due to scavenging of galvinoxyl radical were monitored at the indicated times. Values are mean S.D. of three separate experiments. The AA derivatives reacted with ABTS continuously. Therefore the effects of ph on the long-lasting ABTS scavenging activity of each antioxidant were investigated stoichiometrically, as shown in Fig. 6. The RSA(n) values of AA and Trolox were ca. 2 at all ph values examined. The reactivity of AA-2P, AA-2S, and glutathione against ABTS tended to increase with increasing ph. On the other hand, there were no clear relationships between ph and the RSA(n) of AA-2G and uric acid. The initial reaction rate of AA-2G increased with increasing ph, but that of uric acid was in the order of ph 4 ph 5 ph 3 ph 6 (data not shown). Therefore the AA derivatives scavenged ABTS more effectively under neutral conditions than under acidic conditions. The RSA(n) values of AA-2G, AA-2P, and AA-2S at ph 6 were 3.8, 4.3, and 1.6, respectively. The RSA(n) values of AA-2G, AA-2P, uric acid, and glutathione exceeded those of AA and Trolox in the ph range of 3 to 6, whereas the values of AA- 2S were close to those of AA and Trolox even at ph 5 and 6. Glucose did not react with ABTS at any ph examined (data not shown). Galvinoxyl Radical-Scavenging Activity The timecourse study of galvinoxyl radical-scavenging reaction of AA derivatives and other antioxidants was carried out in 60% ethanol/40% citrate buffer (ph 3 6) for 2 h. AA-2G, AA- 2P, and AA-2S did not quench galvinoxyl radical at any ph values investigated. The results at ph 6 only are shown in Fig. 7. AA and Trolox scavenged galvinoxyl radical at all ph ranges. Uric acid reacted with the radical at ph 4 to 6. Glutathione quenched the radical only at ph 6. The RSA(n) values at ph 6 of AA, Trolox, uric acid, and glutathione were 2.0, 1.7, 1.8, and 0.7, respectively. DISCUSSION Our present data indicat that AA-2G, AA-2P, and AA-2S as 2-O-substituted AA derivatives per se reacted with DPPH radical and ABTS slowly and continuously. The reaction rates of the AA derivatives were slower than those of AA and Trolox but were comparable to those of uric acid and glutathione (Figs. 3, 5). On the other hand, the reaction stoichiometries of the AA derivatives, uric acid, and glutathione for a long-term period were affected by ph changes and were comparable or superior to those of AA and Trolox under their optimal conditions (Figs. 4, 6). We also found that the hydroxyl radical-scavenging activity of the AA derivatives as measured using an electron spin resonance (ESR)-spin trapping method was much less than that of AA, and the halfmaximal scavenging concentration of AA, AA-2G, AA-2P, and AA-2S for 5 min against hydroxyl radical generated by the Fenton reaction was 5 m M, 0.8 mm, 4 mm, and 30 mm, respectively (unpublished data). Therefore the question arises of whether the slow and long-lasting radical-scavenging reaction of the AA derivatives against DPPH radical and ABTS is physiologically relevant, since in vivo it appears crucial to scavenge harmful radicals as rapidly as possible. However, we believe that the slow and long-lasting radical-scavenging

5 770 Vol. 29, No. 4 reaction of the AA derivatives per se is biologically relevant for the following three reasons. First, the profiles of the radical-scavenging reaction of the AA derivatives are similar to those of uric acid and glutathione, which act as antioxidants in vivo. 29,30) Second, Brand-Williams et al. 31) and van den Berg et al. 32) showed that slow-reacting radical scavengers as well as fast-reacting radical scavengers effectively inhibited lipid peroxidation. Third, AA-2G was reported to reduce oxidative stress in ischemic-reperfusion injury and has been studied as a useful means for organ transplantation ) We think that the radical-scavenging activity of AA-2G per se may contribute to the reduction of the oxidant stress because AA-2G appears to be metabolized only to a small extent to AA in the experiments carried out at 3 C or 4 C. Another question concerning the biological significance of AA derivatives arises from the fact that they did not scavenge the galvinoxyl radical at all, but other common antioxidants did (Fig. 7). Edaravone (MCI-186), a novel neuroprotective agent approved for the acute therapy of embolic stroke, 36) has been reported to scavenge DPPH radical 37,38) and hydroxyl radical 38,39) but not superoxide anion. 39) Thus effective antioxidants are not required to react with all types of radicals. Therefore we conclude that the AA derivatives seem to play a physiologic role as antioxidants. Our results also yielded helpful information on the radicalscavenging mechanisms of the AA derivatives. These derivatives scavenged DPPH radical at ph 3 to 5 but not at ph 6 (Fig. 4). The pk a values of AA-2G, AA-2P, and AA-2S were 3. 1,9) 3.4, 40) and 3.1, 40) respectively. Therefore the protonated forms but not deprotonated forms of these derivatives probably reacted with DPPH radical, suggesting that the reaction proceeds primarily via a hydrogen-transfer reaction. On the other hand, the AA derivatives scavenged ABTS at all ph ranges, especially under neutral conditions (Fig. 6). The deprotonated forms rather than protonated forms of these derivatives quenched ABTS, suggesting that the reaction proceeds primarily via an electron-transfer reaction. The reaction stoichiometry of the AA derivatives, however, indicated that the reaction proceeded via a more complex mechanism. If a reaction proceeds via simple hydrogen or electron transfer, the reaction stoichiometry corresponds approximately to the number of oxidizable OH groups. For example, one molecule of AA has two oxidizable OH groups and thereby scavenged two molecules of DPPH radical, ABTS, and galvinoxyl radical (Figs. 4, 6, 7). One molecule of each AA derivative has only one oxidizable OH group but scavenged more than one molecule of DPPH radical and ABTS under optimal conditions (Figs. 4, 6), suggesting that the radical-scavenging mechanisms are not simple. We detected a covalent adduct of DPPH radical with AA-2G in the reaction mixture with liquid chromatographymass spectrometry and found that this adduct could further quench DPPH radical (unpublished data). Thus a possible explanation of the DPPH radical-scavenging reaction of AA- 2G is: AA-2G DPPH (AA-2G) DPPH:H (AA-2G) DPPH (AA-2G) (DPPH) (AA-2G) (DPPH) DPPH? This well explains why the RSA(n) of AA-2G against the DPPH radical was ca. 3 at ph 3. The radical-scavenging reaction of AA-2G may be attributable not to its glucose moiety but to its AA moiety because glucose, known to be a hydroxyl radical scavenger, did not show any radical-scavenging activity in this experiment. The radical-scavenging reaction of AA-2P and AA-2S showed roughly the same tendency as that of AA-2G. Thus AA-2P and AA-2S could scavenge DPPH radical by a mechanism similar to that of AA-2G. Furthermore, the mechanism might be generalized to the radical-scavenging mechanisms of 2-O-substituted AA derivatives against other radical species. For a better understanding of their radical-scavenging mechanisms, it is necessary to analyze the reaction intermediates using ESR spectrometry or the reaction products by liquid chromatographymass spectrometry. The radical-scavenging activity of the AA derivatives was in the order of AA-2G AA-2P AA-2S against DPPH radical at ph 3 (Fig. 4) and AA-2P AA-2G AA-2S against ABTS at ph 6 (Fig. 6). The contribution of the substituent groups to the radical-scavenging activity of the AA derivatives is difficult to explain. In addition, DPPH radical and ABTS are not necessarily mimics of reactive oxygen species generated in vivo. Further research, especially in a biologic context is needed to assess their efficiency. In this study, we demonstrated that AA-2G, AA-2P, and AA-2S as 2-O-substituted AA derivatives had radical-scavenging activity against DPPH radical and ABTS without converting to AA. The AA derivatives reacted with the radicals slowly compared with AA. However, the reactions of these derivatives proceeded continuously and thereby could quench equal or more radicals than AA under optimal conditions. The slow and long-lasting scavenging reaction was also observed in the case of uric acid and glutathione, suggesting that the AA derivatives act as biologically relevant antioxidants. The greater than expected reaction stoichiometry based on the number of oxidizable OH groups indicated that the reaction of the AA derivatives with the radicals did not proceed via a simple hydrogen or electron transfer. 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