Cyanide-resistant respiration in Taenia crassiceps metacestode (cysticerci) is explained by the H 2 O 2 -producing side-reaction of respiratory complex I with O 2 I. Patricia del Arenal a, * , M. Esther Rubio b , Jorge Ramı ´rez c , Juan L. Rendo ´n a , J. Edgardo Escamilla c a Departamento de Bioquı ´mica, Facultad de Medicina, Universidad Nacional Auto ´noma de Me ´xico, Apartado postal 70-159, Me ´xico 04510 D.F., Me ´xico b Departamento de Fisiologı ´a, Instituto Nacional de Cardiologı ´a ‘‘Ignacio Cha ´vez’’, Me ´xico 14080 D.F., Me ´xico c Instituto de Fisiologı ´a Celular, Universidad Nacional Auto ´noma de Me ´xico, Apartado postal 70-242, Me ´xico 04510 D.F., Me ´xico Received 9 August 2004; accepted 14 April 2005 Available online 14 June 2005 Abstract The nature of the cyanide-resistant respiration of Taenia crassiceps metacestode was studied. Mitochondrial respiration with NADH as substrate was partially inhibited by rotenone, cyanide and antimycin in decreasing order of effectiveness. In contrast, respiration with succinate or ascorbate plus N,N,NV ,NV -tetramethyl-p-phenylenediamine (TMPD) was more sensitive to antimycin and cyanide. The saturation kinetics for O 2 with NADH as substrate showed two components, which exhibited different oxygen affinities. The high-O 2 -affinity system (K m app = 1.5 AM) was abolished by low cyanide concentration; it corresponded to cytochrome aa 3 . The low-O 2 -affinity system (K m app = 120 AM) was resistant to cyanide. Similar O 2 saturation kinetics, using succinate or ascorbate – TMPD as electron donor, showed only the high- O 2 -affinity cyanide-sensitive component. Horse cytochrome c increased 2 – 3 times the rate of electron flow across the cyanide-sensitive pathway and the contribution of the cyanide-resistant route became negligible. Mitochondrial NADH respiration produced significant amounts of H 2 O 2 (at least 10% of the total O 2 uptake). Bovine catalase and horse heart cytochrome c prevented the production and/or accumulation of H 2 O 2 . Production of H 2 O 2 by endogenous respiration was detected in whole cysticerci using rhodamine as fluorescent sensor. Thus, the CN-resistant and low-O 2 -affinity respiration results mainly from a spurious reaction of the respiratory complex I with O 2 , producing H 2 O 2 . The meaning of this reaction in the microaerobic habitat of the parasite is discussed. D 2005 Elsevier Ireland Ltd. All rights reserved. Keywords: Helminth; Respiratory chain; Cytochrome o; Respiratory inhibitors; Hydrogen peroxide; Antioxidant enzymes 1. Introduction The respiratory systems of helminths are more diverse and complex than those of their hosts. Differences in the respiratory system exist among many helminth species and, in many cases, in different stages of their life cycles [1,2]. Parasites, such as Ascaris [3,4], Moniezia [5] and filaria [6,7] have been extensively studied. Parasites exhibit aerobic and anaerobic metabolisms with succinate as the product of the fumarate reductase reaction [8,9]. Nutrient sources, CO 2 and oxygen tension play an important role in habitat variations during the parasite’s alternate lifestyles. Two properties of the energy metabolism of helminths have been highlighted: the high resistance of respiration to the classical respiratory inhibitors, particularly cyanide [3,10], and the production of significant amounts of hydro- gen peroxide by their mitochondria [11,12]. A putative type o cytochrome has been frequently implicated in the inhibitor-resistant respiration. Hence, alternative respiratory chains with different terminal oxidases have been proposed [13,14]. Production of H 2 O 2 has also been reported as a by- 1383-5769/$ - see front matter D 2005 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.parint.2005.04.003 * Corresponding author. Tel.: +52 55 56232169; fax: +52 55 56162419. E-mail address: darenal@bq.unam.mx (I.P. del Arenal). Parasitology International 54 (2005) 185 – 193 www.elsevier.com/locate/parint