Cysteine is the major low-molecular weight thiol in Giardia duodenalis

Abstract

Aerobic metabolism can result in the production of reduced molecular oxygen intermediates, i.e., superoxide (O2-), hydroxyl radical ( .OH), and hydrogen peroxide (H202) [1]. Defence mechanisms specifically employed for removal of these toxic radicals include superoxide dismutase [2], catalase [3], and the glutathione (GSH) cycling enzymes, GSH peroxidase [4] and GSH reductase [5]. GSH cycling has been regarded as one of the primary mechanisms of detoxification in organisms ranging from Escherichia coli to man. All organisms contain high levels of at least one thiol whether it be GSH, GSH analogues or another low-molecular weight thiol. The haemoflagellate Trypanosoma, for example, conjugates the majority (>70%) of its free intracellular GSH with spermidine to form trypanothione, which acts as a cofactor with low levels of GSH in redox cycling [6,7]. During stationary phase under anaerobic conditions, E. coli, which exhibits GSH metabolism aerobically and during log phase, converts the majority of its free GSH to glutathionylspermidine [8]. Anaerobes must detoxify both oxygen and