Reduction of lapachones and their reaction with L-cysteine and mercaptoethanol on glassy carbon electrodes A.M. Oliveira-Brett a, * , M.O.F. Goulart b , F.C. Abreu a,b,c a Depto. de Quimica, Fac. de Ciencias e Tecnologia, Universidade de Coimbra, 3004-535, Coimbra, Portugal b Depto. Quimica/CCEN, Universidade Federal de Alagoas, 57072-970, Maceio ´, AL, Brazil c Depto. Quimica Fundamental, Universidade Federal de Pernambuco, Recife-PE, Brazil Received 1 June 2001; received in revised form 27 November 2001; accepted 30 November 2001 Abstract The electrochemical reduction of h-lapachone and its 3-sulphonic salt was studied by cyclic, square wave and differential pulse voltammetry in aqueous media using a glassy carbon electrode. These compounds have a wide range of biological activities, including antibacterial, cytotoxic, antifungal, trypanocidal and anticancer action. The reduction of h-lapachone in the presence of L-cysteine and 2- mercaptoethanol was studied and the results, together with others already published, suggest that the anticancer mechanism of h-lapachones can be explained via interaction with topoisomerase. D 2002 Elsevier Science B.V. All rights reserved. Keywords: h-Lapachone; Cytotoxic action; Electrochemical reduction; Topoisomerase 1. Introduction h-Lapachone is a naturally occurring quinone, easily synthesized from lapachol or lomatiol [1]. It has been shown to have a wide range of biological activities, including anticancer activity [2,3]. Its mechanism of action as anti- bacterial, cytotoxic and trypanocidal is related to its redox cycling nature, generating reactive oxygen species, which can damage DNA [4]. However, the clinical efficacy of this drug remains to be explored, and such studies await eluci- dation of its mechanism of action. The inhibitory effect of h- lapachone on human DNA topoisomerase II was investi- gated [5]. It is suspected that h-lapachone binds directly to the enzyme to prevent DNA unwinding by topoisomerase I. It was also suggested that the cytotoxic actions of naphtho- quinones derive, in part, from alkylation of exposed thiol residues on topoisomerase II-DNA complexes [6]. The majority of the above-mentioned biological activities of h- lapachone are dependent on bioreduction. The present work aims to obtain electrochemical data to help in the elucidation of the mechanism of the anticancer action of h-lapachones. 2. Experimental Calf thymus DNA (sodium salt, Type I) was obtained from Sigma and L-cysteine and 2-mercaptoethanol from Aldrich. h-Lapachone (1), 3-sulphonic acid (2) and a- lapachone (3) used as a model, were synthesized according to the literature [1]. In an aqueous ethanolic phosphate buffer pH 7.0, 1 and 3 were prepared, while in aqueous buffer solutions, 2 was prepared. The experimental con- ditions were described elsewhere [7]. The electrochemical DNA-biosensor was prepared as described [8]. 3. Results and discussions Cyclic voltammograms of h-lapachones at a glassy car- bon electrode show a reversible process involving the same number of electrons and protons (Fig. 1), and the reduction potential of h-lapachone is pH-dependent (slope 59 mV per pH unit) [7]. 1567-5394/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S1567-5394(02)00011-7 * Corresponding author. Tel.: +351-239-835295; fax: +351-239- 835295. E-mail address: brett@ci.uc.pt (A.M. Oliveira-Brett). www.elsevier.com/locate/bioelechem Bioelectrochemistry 56 (2002) 53 – 55