Molecular and enzymatic characterisation of Schistosoma mansoni thioredoxin Heather M. Alger a , Ahmed A. Sayed a , Miguel J. Stadecker b , David L. Williams a, * a Department of Biological Sciences, Illinois State University, Normal, IL 61790-4120, USA b Department of Pathology, Tufts University School of Medicine, Boston, MA 02111, USA Received 15 April 2002; received in revised form 29 May 2002; accepted 29 May 2002 Abstract Defense against oxidative damage can be mediated through glutathione and/or thioredoxin utilising systems. Here, we report the identification and characterisation of a thioredoxin from Schistosoma mansoni. The predicted protein has similarity to previously charac- terised thioredoxins including conservation of the redox active site. Recombinant six-histidine tagged schistosome thioredoxin had insulin reduction activity and supported the enzymatic function of thioredoxin reductase and thioredoxin peroxidase. By Western blotting, all mammalian stages of the schistosome lifecycle expresses thioredoxin. Thioredoxin is present in egg secretory products and antibodies against the recombinant protein produce the circumoval precipitin reaction. This is the first identification of defined antigen producing this reaction. Furthermore, thioredoxin is a novel egg immunogen as it elicits an antibody response in schistosome-infected mice. The most significant IgG production against thioredoxin occurs after parasite oviposition commences. These observations suggest that thioredoxin participates in processes vital to the parasite and may facilitate the passage and survival of eggs across inflamed host tissues. q 2002 Australian Society of Parasitology Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Schistosoma mansoni; Thioredoxin; Redox system; Oxidative stress; Circumoval precipitin reaction; Egg secretory antigen 1. Introduction Schistosomiasis is an important parasitic disease, currently infecting 250 million people and causing signifi- cant morbidity and mortality. Although chemotherapy has become more widely available for schistosomiasis control, in many countries the prevalence of disease has remained the same or even increased (Savioli et al., 1997). Adult Schistosoma mansoni live in the hepatic portal system of their human host for many years without being eliminated by the host’s immune system despite eliciting a complex host-specific immune response (Capron and Capron, 1986). Schistosomes are exposed to reactive oxygen compounds through their own respiratory processes and as a result of the host immune response. Therefore, schistosomes must possess adequate mechanisms of reactive oxygen detoxifi- cation. Antioxidant proteins are expressed at higher levels in parasite developmental stages more resistant to oxidative stress (Mkoji et al., 1988; LoVerde, 1998). Since schisto- somes lack catalase and have relatively low levels of glutathione peroxidase (Mkoji et al., 1988; Mei and LoVerde, 1997), two enzymes capable of detoxifying hydrogen peroxide, we have proposed that thioredoxin peroxidase plays a major role in antioxidant defenses in S. mansoni (Kwatia et al., 2000). Thioredoxin peroxidase is a member of a recently identified family of antioxidants, the peroxiredoxins, involved in the detoxification of hydrogen peroxide and other hydroperoxides (McGonigle et al., 1998). To catalyse the reduction of hydroperoxides thiore- doxin peroxidase requires reducing equivalents from thior- edoxin (Kwatia et al., 2000). Thioredoxin is in turn maintained in the reduced state by thioredoxin reductase and nicotinamide adenine dinucleotide phosphate (NADPH). Therefore, for effective functioning of thiore- doxin peroxidase the entire thioredoxin-dependent redox cycle must be active in schistosomes. Thioredoxins are small (,12 kDa) proteins with a redox active disulfide TrpCysGlyProCys motif. In the oxidised state, the cysteine residues form a disulfide that is subse- quently reduced by thioredoxin reductase and NADPH to the dithiol form. In addition to their antioxidant properties, thioredoxins are involved in a variety of cellular redox reac- tions, protein folding, transcription regulation and as growth factors for a variety of cells (reviewed in Powis and Montfort, 2001). International Journal for Parasitology 32 (2002) 1285–1292 0020-7519/02/$20.00 q 2002 Australian Society of Parasitology Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S0020-7519(02)00108-X www.parasitology-online.com * Corresponding author. Tel.: 11-309-438-2608; fax: 11-309-438-3722. E-mail address: dlwilli@ilstu.edu (D.L. Williams).