- Email: [email protected]
Photodynamic action of actinomycin D: an EPR spin trapping study
Biochimica et Biophysica Acta 1527 (2001) 1^3
www.bba-direct.com
Rapid report
Photodynamic action of actinomycin D: an EPR spin trapping study Jing-Xi Pan a , Yang Liu b , Su-Ping Zhang c , Tie-Cheng Tu a , Si-De Yao Nian-Yun Lin a a
a;
*,
Laboratory of Radiation Chemistry, Shanghai Institute of Nuclear Research, Academia Sinica, P.O. Box 800-204, Shanghai 201800, PR China b Beijing Institute of Chemistry, Academia Sinica, Beijing 100080, PR China c Institute of Plant Physiology, Academia Sinica, Shanghai 200032, PR China Received 19 February 2001; received in revised form 20 April 2001; accepted 2 May 2001
Abstract Actinomycin D is one of the most widely studied anticancer antibiotic that binds to both double-stranded and single-stranded DNA, and this binding greatly enhances the DNA photosensitization. By use of electron paramagnetic resonance spin trapping techniques, both superoxide radical anion and the radical anion of actinomycin D were identified as important intermediates in the photodynamic process. A mechanism of electron transfer from a DNA base to excited actinomycin D was proposed. These novel findings may shed new light on future application of this drug in photodynamic therapy or cleavage of DNA in unique and controllable ways. ß 2001 Elsevier Science B.V. All rights reserved. Keywords : Actinomycin D; EPR; Photodynamic process
Actinomycin D (AMD, Fig. 1) is one of the most widely studied anticancer antibiotic that generates a wide variety of biochemical and pharmacological e¡ects [1,2]. The pharmacological action of AMD can be traced to its interactions with DNA, and in particular to its ability to inhibit DNA transcription [3^5]. For over 20 years, the inhibition of DNA transcription has been attributed to double-stranded DNA (dsDNA) binding, which occurs by intercalation [6,7]. More recently, however, AMD was shown to bind with high a¤nity to single-stranded DNA (ssDNA) [8], and sequence speci¢city and hemi-intercalation model was proposed [9]. The drug^ssDNA interactions were further con¢rmed by several researchers [10^ 12], who showed that AMD was highly e¡ective in inhibiting human immunode¢ciency virus (HIV) reverse transcriptase as well as DNA polymerases requiring a ssDNA template. In view of the interactions of AMD with both dsDNA and ssDNA and the fact that AMD^DNA complex formation results in the red shift of AMD absorption spectrum in the visible range (440 to ca. 460 nm for dsDNA) [9,13], it should be interesting and of signi¢cance to study the photosensitizing properties of AMD, especially in DNA complexes.
* Corresponding author. Fax: +86-21-5955-3021; E-mail : [email protected]
Although £uorescence quenching of AMD by dsDNA was observed in the literature [13,14], and a reversible electron transfer mechanism was proposed, no enough evidence was provided. We report herein direct detection of active intermediates formed from the irradiation of AMD, especially in ssDNA complex, using electron paramagnetic resonance (EPR) spin trapping techniques. Both superoxc3 ide radical anion (Oc3 2 ) and AMD radical anion (AMD ) were identi¢ed as important intermediates in the interaction of AMD with DNA under visible light irradiation. These novel ¢ndings indicate that the photodynamic action of AMD on DNA is initiated by electron transfer from DNA bases to excited AMD in DNA complex. Subsequent formation of a DNA base radical cation would lead to DNA damage and then to strand scission [15,16]. These results may shed new light on future application of this drug in photodynamic therapy or cleavage of DNA in unique and controllable ways [15,17]. The generation of Oc3 2 by AMD under light irradiation was detected by the spin trap 5-(diethoxyphosphoryl)-5methyl-1-pyrroline-N-oxide (DEPMPO). DEPMPO is a new e¤cient nitrone for the in vitro and in vivo spin trapping of oxygen-centered radicals [18^20]. A marked advantage of this nitrone for trapping Oc3 2 over the most commonly used spin trap 5,5-dimethylpyrroline-N-oxide (DMPO) was that the DEPMPO superoxide adduct (DEPMPO-OOH) was much more stable than that of
0304-4165 / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 1 6 5 ( 0 1 ) 0 0 1 5 2 - 0
BBAGEN 20372 13-6-01
2
J.-X. Pan et al. / Biochimica et Biophysica Acta 1527 (2001) 1^3
Fig. 1. Structure of actinomycin D. Thr, threonine; D-Val, Pro, proline; Sar, sarcosine; MeVal, N-methylvaline.
D-valine ;
DMPO, with a 15-fold longer half-life (13 min in aqueous solution at pH 7). In addition, spontaneous decomposition of the -OOH adduct to the -OH adduct was not observed [20,21]. When an oxygen-saturated aqueous solution (20% dimethyl sulfoxide, DMSO) containing AMD (0.2 mM), DEPMPO (10 mM) and diethylenetriaminepentaacetic acid (DTPA, 4 mM)1 was irradiated with visible light (V s 400 nm) for 10 min, a typical EPR spectrum of DEPMPO-OOH (KN = 13.17, KLÿH = 10.97, KQÿH = 0.91 (1H) and 0.40 (6H), KP = 50.08 G) identical to the literature [21,22] was obtained (Fig. 2A). The formation of this adduct was completely inhibited by superoxide dismutase (SOD, 80 units/ml), and no signal was obtained in the control experiments without light or oxygen (data not shown). All the results unambiguously support this spectrum corresponding with the DEPMPO-OOH adduct. When deoxyguanosine mononucleotide (dGMP, 0.5 mM) was added to the system previous to irradiation, the EPR intensity of DEPMPO-OOH was greatly enhanced (Fig. 2B), while the addition of the other three mononucleotides (dAMP, dCMP and TMP) had negligible e¡ect on it, up to concentrations of 2 mM (data not shown). When ssDNA2 (3 mM, a 15:1 ratio of bases to AMD) instead of dGMP was added, similar results were observed (Fig. 2C). In order to study the photosensitizing properties of AMD when intercalated into dsDNA, a 35:1 ratio of base pairs to AMD (50 WM) was used to make sure that all AMD were bound [23]. Surprisingly, however, no EPR signal was detected, even after 40 min illumination. The enhancement e¡ect of dGMP and ssDNA on the formation of Oc3 2 suggest that they might serve as electron donors in AMD photodynamic process. Thus AMDc3 , which can be generated by electron transfer from electron donors to excited AMD (AMD*), might be the precursor c3 of Oc3 2 . To con¢rm the O2 formation mechanism from 1 In the Oc3 2 trapping experiment, DTPA was added to chelate transition-metal ions to inhibit the formation of c OH. 2
SsDNA was prepared by heating dsDNA in neutral aqueous solution at 90³C for 10 min followed by chilling in an ice-salt bath. DsDNA was prepared by dissolving calf thymus DNA in 10 mM sodium phosphate bu¡er (pH 7.4) and stirring over night.
Fig. 2. (A) EPR spectrum of DEPMPO-OOH adduct generated from irradiation of an oxygen-saturated aqueous solution (20% DMSO, pH 7.4) containing AMD (0.2 mM), DEPMPO (10 mM) and DTPA (4 mM). (B) As in A but in the presence of 0.5 mM dGMP. (C) As in A but in the presence of 3 mM ssDNA. Instrumental settings : microwave power, 10 mW; modulation amplitude, 1 G; time constant, 0.064 s; scan time, 2 min; scan range, 160 G; receiver gain, 5U104 .
AMDc3 , a spin elimination method usually used in the detection of photosensitizer radical anion [24] was employed in our experiments. If AMDc3 was generated in anaerobic conditions, the spin elimination of the 2,2,6,6tetramethyl-4-piperidone-N-oxyl radical (4-oxo-TEMPO) should occur. Fig. 3 shows that 4-oxo-TEMPO in nitrogen-saturated solutions (20% DMSO) was degraded when exposed to light in the presence of AMD and dGMP (Fig. 3a) or ssDNA (Fig. 3b). Moreover, it can be seen that dGMP is more powerful than ssDNA in inducing spin elimination of 4-oxo-TEMPO. As in the case of benzoporphyrin [24], it is assumed that the spin elimination of 4oxo-TEMPO is caused by the reaction of 4-oxo-TEMPO with AMDc3 as shown in Eq. 1.
1
c3 can be Our ¢ndings indicate that both Oc3 2 and AMD produced in the photodynamic process of AMD. There are two possible pathways for AMDc3 formation: (i) electron transfer from AMD in the ground state to AMD* when there are no other electron donors; (ii) electron
BBAGEN 20372 13-6-01
J.-X. Pan et al. / Biochimica et Biophysica Acta 1527 (2001) 1^3
3
Scheme 1.
quent radical anion of actinomycin D is trapped by O2 to form superoxide. This work was supported by the National Natural Science Foundation of China (No. 39830090). References
Fig. 3. Spin elimination of 4-oxo-TEMPO (2U1036 M) by AMD (0.2 mM) as a function of illumination time using s 400 nm light in an N2 saturated solution (20% DMSO, pH 7.4) containing (a) 0.5 mM dGMP, (b) 3 mM ssDNA. Inset: a typical EPR spectrum of TEMPO. Scan time, 2 min; scan range, 100 G; receiver gain, 5U104 .
transfer from electron donors (i.e., dGMP or ssDNA) to AMD*. The subsequent AMDc3 will reduce O2 to form Oc3 2 . Although previous data indicate that electron transfer from any of the DNA bases to the excited singlet state of AMD is exothermic, it cannot be concluded in this experiment whether the singlet state or the triplet state of AMD functioned as the precursor of AMDc3 . The photodynamic process of AMD on ssDNA can be described as Scheme 1. The experiments on Oc3 2 production and spin elimination of 4-oxo-TEMPO all indicate that dGMP is a more powerful electron donor than the other three mononucleotides, which is consistent with the fact that guanine (G) is the most easily oxidizable base in DNA [16]. In respect with these results and the fact that the sites in ssDNA proper for AMD binding usually contain Gs, we postulate that the primarily generated DNA radical cations (DNAc ) are mostly centered on Gs. In summary, we provide direct evidence for the photodynamic action of actinomycin D for the ¢rst time. Both Oc3 2 and the radical anion of actinomycin D are important intermediates in this photodynamic process. The photodynamic action on DNA is initiated by electron transfer from a base to the excited state of this drug. The subse-
[1] M.J. Waring, Annu. Rev. Biochem. 50 (1981) 159^192. [2] I.H. Goldberg, T.A. Beerman, R. Poon, Cancer 5: A Comprehensive Treatise, Becker, New York, 1977, pp. 427^456. [3] R.J. White, D.R. Phillips, Biochemistry 27 (1988) 9122^9132. [4] H.M. Sobell, Proc. Natl. Acad. Sci. USA 82 (1982) 5328^5331. [5] I.H. Goldberg, P.A. Friedman, Annu. Rev. Biochem. 40 (1971) 775^ 810. [6] S. Kumitori, F. Takusagawa, J. Mol. Biol. 225 (1992) 445. [7] W.D. Wilson, R.L. Jones, G. Zon, E.V. Scott, D.L. Banville, L.G. Marzilli, J. Am. Chem. Soc. 108 (1986) 7113. [8] R.M. Wadkins, T.M. Jovin, Biochemistry 30 (1991) 9469^9478. [9] R.M. Wadkins, E.A. Jares-Erijman, R. Klement, A. Rudiger, T.M. Jovin, J. Mol. Biol. 262 (1996) 53^68. [10] S. Gabbara, W.R. Davis, L. Hupe, D. Hupe, J.A. Peliska, Biochemistry 38 (1999) 13070^13076. [11] W.R. Davis, S. Gabbara, D. Hupe, J.A. Peliska, Biochemistry 37 (1998) 14213^14221. [12] R.L. Rill, K.H. Hecker, Biochemistry 35 (1996) 3525^3533. [13] L. Chinsky, P.Y. Turpin, Biochim. Biophys. Acta 475 (1977) 54^63. [14] L. Kittler, G. Lober, F.A. Gollmick, H. Berg, Bioelectrochem. Bioenerg. 7 (1980) 503^511. [15] B. Armitage, Chem. Rev. 98 (1998) 1171^1200. [16] C.J. Burrows, J.G. Muller, Chem. Rev. 98 (1998) 1109^1251. [17] G. Pratviel, J. Bernadou, B. Meunier, Angew. Chem. Int. Ed. Engl. 34 (1995) 746. [18] K.J. Liu, M. Miyake, T. Panz, H. Swarts, Free Radic. Biol. Med. 26 (1999) 714^721. [19] G.S. Timmins, K.J. Liu, E.J.H. Bechara, Y. Kotake, H.M. Swarts, Free Radic. Biol. Med. 27 (1999) 329^333. [20] C. Frejaville, H. Karoui, B. Tuccio, F.L. Moigne, M. Culcasi, S. Pietri, R. Lauricella, P. Tordo, J. Med. Chem. 38 (1995) 258^265. [21] B. Tuccio, R. Lauricella, C. Frejaville, J.-C. Bouteiller, P. Tordo, J. Chem. Soc. Perkin Trans. 2 (1995) 295^298. [22] V. Roubaud, S. Sankarapaudi, P. Kuppusamy, P. Tordo, J.L. Zweier, Anal. Biochem. 247 (1997) 404^411. [23] W. Muller, D.M. Crothers, J. Mol. Biol. 35 (1968) 251^290. [24] C. Hadjur, G. Wagnieres, P. Monnier, H. Bergh, Photochem. Photobiol. 65 (1997) 818^827.
BBAGEN 20372 13-6-01
www.bba-direct.com
Rapid report
Photodynamic action of actinomycin D: an EPR spin trapping study Jing-Xi Pan a , Yang Liu b , Su-Ping Zhang c , Tie-Cheng Tu a , Si-De Yao Nian-Yun Lin a a
a;
*,
Laboratory of Radiation Chemistry, Shanghai Institute of Nuclear Research, Academia Sinica, P.O. Box 800-204, Shanghai 201800, PR China b Beijing Institute of Chemistry, Academia Sinica, Beijing 100080, PR China c Institute of Plant Physiology, Academia Sinica, Shanghai 200032, PR China Received 19 February 2001; received in revised form 20 April 2001; accepted 2 May 2001
Abstract Actinomycin D is one of the most widely studied anticancer antibiotic that binds to both double-stranded and single-stranded DNA, and this binding greatly enhances the DNA photosensitization. By use of electron paramagnetic resonance spin trapping techniques, both superoxide radical anion and the radical anion of actinomycin D were identified as important intermediates in the photodynamic process. A mechanism of electron transfer from a DNA base to excited actinomycin D was proposed. These novel findings may shed new light on future application of this drug in photodynamic therapy or cleavage of DNA in unique and controllable ways. ß 2001 Elsevier Science B.V. All rights reserved. Keywords : Actinomycin D; EPR; Photodynamic process
Actinomycin D (AMD, Fig. 1) is one of the most widely studied anticancer antibiotic that generates a wide variety of biochemical and pharmacological e¡ects [1,2]. The pharmacological action of AMD can be traced to its interactions with DNA, and in particular to its ability to inhibit DNA transcription [3^5]. For over 20 years, the inhibition of DNA transcription has been attributed to double-stranded DNA (dsDNA) binding, which occurs by intercalation [6,7]. More recently, however, AMD was shown to bind with high a¤nity to single-stranded DNA (ssDNA) [8], and sequence speci¢city and hemi-intercalation model was proposed [9]. The drug^ssDNA interactions were further con¢rmed by several researchers [10^ 12], who showed that AMD was highly e¡ective in inhibiting human immunode¢ciency virus (HIV) reverse transcriptase as well as DNA polymerases requiring a ssDNA template. In view of the interactions of AMD with both dsDNA and ssDNA and the fact that AMD^DNA complex formation results in the red shift of AMD absorption spectrum in the visible range (440 to ca. 460 nm for dsDNA) [9,13], it should be interesting and of signi¢cance to study the photosensitizing properties of AMD, especially in DNA complexes.
* Corresponding author. Fax: +86-21-5955-3021; E-mail : [email protected]
Although £uorescence quenching of AMD by dsDNA was observed in the literature [13,14], and a reversible electron transfer mechanism was proposed, no enough evidence was provided. We report herein direct detection of active intermediates formed from the irradiation of AMD, especially in ssDNA complex, using electron paramagnetic resonance (EPR) spin trapping techniques. Both superoxc3 ide radical anion (Oc3 2 ) and AMD radical anion (AMD ) were identi¢ed as important intermediates in the interaction of AMD with DNA under visible light irradiation. These novel ¢ndings indicate that the photodynamic action of AMD on DNA is initiated by electron transfer from DNA bases to excited AMD in DNA complex. Subsequent formation of a DNA base radical cation would lead to DNA damage and then to strand scission [15,16]. These results may shed new light on future application of this drug in photodynamic therapy or cleavage of DNA in unique and controllable ways [15,17]. The generation of Oc3 2 by AMD under light irradiation was detected by the spin trap 5-(diethoxyphosphoryl)-5methyl-1-pyrroline-N-oxide (DEPMPO). DEPMPO is a new e¤cient nitrone for the in vitro and in vivo spin trapping of oxygen-centered radicals [18^20]. A marked advantage of this nitrone for trapping Oc3 2 over the most commonly used spin trap 5,5-dimethylpyrroline-N-oxide (DMPO) was that the DEPMPO superoxide adduct (DEPMPO-OOH) was much more stable than that of
0304-4165 / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 1 6 5 ( 0 1 ) 0 0 1 5 2 - 0
BBAGEN 20372 13-6-01
2
J.-X. Pan et al. / Biochimica et Biophysica Acta 1527 (2001) 1^3
Fig. 1. Structure of actinomycin D. Thr, threonine; D-Val, Pro, proline; Sar, sarcosine; MeVal, N-methylvaline.
D-valine ;
DMPO, with a 15-fold longer half-life (13 min in aqueous solution at pH 7). In addition, spontaneous decomposition of the -OOH adduct to the -OH adduct was not observed [20,21]. When an oxygen-saturated aqueous solution (20% dimethyl sulfoxide, DMSO) containing AMD (0.2 mM), DEPMPO (10 mM) and diethylenetriaminepentaacetic acid (DTPA, 4 mM)1 was irradiated with visible light (V s 400 nm) for 10 min, a typical EPR spectrum of DEPMPO-OOH (KN = 13.17, KLÿH = 10.97, KQÿH = 0.91 (1H) and 0.40 (6H), KP = 50.08 G) identical to the literature [21,22] was obtained (Fig. 2A). The formation of this adduct was completely inhibited by superoxide dismutase (SOD, 80 units/ml), and no signal was obtained in the control experiments without light or oxygen (data not shown). All the results unambiguously support this spectrum corresponding with the DEPMPO-OOH adduct. When deoxyguanosine mononucleotide (dGMP, 0.5 mM) was added to the system previous to irradiation, the EPR intensity of DEPMPO-OOH was greatly enhanced (Fig. 2B), while the addition of the other three mononucleotides (dAMP, dCMP and TMP) had negligible e¡ect on it, up to concentrations of 2 mM (data not shown). When ssDNA2 (3 mM, a 15:1 ratio of bases to AMD) instead of dGMP was added, similar results were observed (Fig. 2C). In order to study the photosensitizing properties of AMD when intercalated into dsDNA, a 35:1 ratio of base pairs to AMD (50 WM) was used to make sure that all AMD were bound [23]. Surprisingly, however, no EPR signal was detected, even after 40 min illumination. The enhancement e¡ect of dGMP and ssDNA on the formation of Oc3 2 suggest that they might serve as electron donors in AMD photodynamic process. Thus AMDc3 , which can be generated by electron transfer from electron donors to excited AMD (AMD*), might be the precursor c3 of Oc3 2 . To con¢rm the O2 formation mechanism from 1 In the Oc3 2 trapping experiment, DTPA was added to chelate transition-metal ions to inhibit the formation of c OH. 2
SsDNA was prepared by heating dsDNA in neutral aqueous solution at 90³C for 10 min followed by chilling in an ice-salt bath. DsDNA was prepared by dissolving calf thymus DNA in 10 mM sodium phosphate bu¡er (pH 7.4) and stirring over night.
Fig. 2. (A) EPR spectrum of DEPMPO-OOH adduct generated from irradiation of an oxygen-saturated aqueous solution (20% DMSO, pH 7.4) containing AMD (0.2 mM), DEPMPO (10 mM) and DTPA (4 mM). (B) As in A but in the presence of 0.5 mM dGMP. (C) As in A but in the presence of 3 mM ssDNA. Instrumental settings : microwave power, 10 mW; modulation amplitude, 1 G; time constant, 0.064 s; scan time, 2 min; scan range, 160 G; receiver gain, 5U104 .
AMDc3 , a spin elimination method usually used in the detection of photosensitizer radical anion [24] was employed in our experiments. If AMDc3 was generated in anaerobic conditions, the spin elimination of the 2,2,6,6tetramethyl-4-piperidone-N-oxyl radical (4-oxo-TEMPO) should occur. Fig. 3 shows that 4-oxo-TEMPO in nitrogen-saturated solutions (20% DMSO) was degraded when exposed to light in the presence of AMD and dGMP (Fig. 3a) or ssDNA (Fig. 3b). Moreover, it can be seen that dGMP is more powerful than ssDNA in inducing spin elimination of 4-oxo-TEMPO. As in the case of benzoporphyrin [24], it is assumed that the spin elimination of 4oxo-TEMPO is caused by the reaction of 4-oxo-TEMPO with AMDc3 as shown in Eq. 1.
1
c3 can be Our ¢ndings indicate that both Oc3 2 and AMD produced in the photodynamic process of AMD. There are two possible pathways for AMDc3 formation: (i) electron transfer from AMD in the ground state to AMD* when there are no other electron donors; (ii) electron
BBAGEN 20372 13-6-01
J.-X. Pan et al. / Biochimica et Biophysica Acta 1527 (2001) 1^3
3
Scheme 1.
quent radical anion of actinomycin D is trapped by O2 to form superoxide. This work was supported by the National Natural Science Foundation of China (No. 39830090). References
Fig. 3. Spin elimination of 4-oxo-TEMPO (2U1036 M) by AMD (0.2 mM) as a function of illumination time using s 400 nm light in an N2 saturated solution (20% DMSO, pH 7.4) containing (a) 0.5 mM dGMP, (b) 3 mM ssDNA. Inset: a typical EPR spectrum of TEMPO. Scan time, 2 min; scan range, 100 G; receiver gain, 5U104 .
transfer from electron donors (i.e., dGMP or ssDNA) to AMD*. The subsequent AMDc3 will reduce O2 to form Oc3 2 . Although previous data indicate that electron transfer from any of the DNA bases to the excited singlet state of AMD is exothermic, it cannot be concluded in this experiment whether the singlet state or the triplet state of AMD functioned as the precursor of AMDc3 . The photodynamic process of AMD on ssDNA can be described as Scheme 1. The experiments on Oc3 2 production and spin elimination of 4-oxo-TEMPO all indicate that dGMP is a more powerful electron donor than the other three mononucleotides, which is consistent with the fact that guanine (G) is the most easily oxidizable base in DNA [16]. In respect with these results and the fact that the sites in ssDNA proper for AMD binding usually contain Gs, we postulate that the primarily generated DNA radical cations (DNAc ) are mostly centered on Gs. In summary, we provide direct evidence for the photodynamic action of actinomycin D for the ¢rst time. Both Oc3 2 and the radical anion of actinomycin D are important intermediates in this photodynamic process. The photodynamic action on DNA is initiated by electron transfer from a base to the excited state of this drug. The subse-
[1] M.J. Waring, Annu. Rev. Biochem. 50 (1981) 159^192. [2] I.H. Goldberg, T.A. Beerman, R. Poon, Cancer 5: A Comprehensive Treatise, Becker, New York, 1977, pp. 427^456. [3] R.J. White, D.R. Phillips, Biochemistry 27 (1988) 9122^9132. [4] H.M. Sobell, Proc. Natl. Acad. Sci. USA 82 (1982) 5328^5331. [5] I.H. Goldberg, P.A. Friedman, Annu. Rev. Biochem. 40 (1971) 775^ 810. [6] S. Kumitori, F. Takusagawa, J. Mol. Biol. 225 (1992) 445. [7] W.D. Wilson, R.L. Jones, G. Zon, E.V. Scott, D.L. Banville, L.G. Marzilli, J. Am. Chem. Soc. 108 (1986) 7113. [8] R.M. Wadkins, T.M. Jovin, Biochemistry 30 (1991) 9469^9478. [9] R.M. Wadkins, E.A. Jares-Erijman, R. Klement, A. Rudiger, T.M. Jovin, J. Mol. Biol. 262 (1996) 53^68. [10] S. Gabbara, W.R. Davis, L. Hupe, D. Hupe, J.A. Peliska, Biochemistry 38 (1999) 13070^13076. [11] W.R. Davis, S. Gabbara, D. Hupe, J.A. Peliska, Biochemistry 37 (1998) 14213^14221. [12] R.L. Rill, K.H. Hecker, Biochemistry 35 (1996) 3525^3533. [13] L. Chinsky, P.Y. Turpin, Biochim. Biophys. Acta 475 (1977) 54^63. [14] L. Kittler, G. Lober, F.A. Gollmick, H. Berg, Bioelectrochem. Bioenerg. 7 (1980) 503^511. [15] B. Armitage, Chem. Rev. 98 (1998) 1171^1200. [16] C.J. Burrows, J.G. Muller, Chem. Rev. 98 (1998) 1109^1251. [17] G. Pratviel, J. Bernadou, B. Meunier, Angew. Chem. Int. Ed. Engl. 34 (1995) 746. [18] K.J. Liu, M. Miyake, T. Panz, H. Swarts, Free Radic. Biol. Med. 26 (1999) 714^721. [19] G.S. Timmins, K.J. Liu, E.J.H. Bechara, Y. Kotake, H.M. Swarts, Free Radic. Biol. Med. 27 (1999) 329^333. [20] C. Frejaville, H. Karoui, B. Tuccio, F.L. Moigne, M. Culcasi, S. Pietri, R. Lauricella, P. Tordo, J. Med. Chem. 38 (1995) 258^265. [21] B. Tuccio, R. Lauricella, C. Frejaville, J.-C. Bouteiller, P. Tordo, J. Chem. Soc. Perkin Trans. 2 (1995) 295^298. [22] V. Roubaud, S. Sankarapaudi, P. Kuppusamy, P. Tordo, J.L. Zweier, Anal. Biochem. 247 (1997) 404^411. [23] W. Muller, D.M. Crothers, J. Mol. Biol. 35 (1968) 251^290. [24] C. Hadjur, G. Wagnieres, P. Monnier, H. Bergh, Photochem. Photobiol. 65 (1997) 818^827.
BBAGEN 20372 13-6-01