Functional and physicochemical characterization of the thioredoxin system in Trypanosoma brucei

Trypanosoma brucei, the causative agent of African sleeping sickness, possesses a single thioredoxin that has an unusually high pI value of 8.5 and lacks a conserved aspartyl residue claimed to be involved in catalysis in other thioredoxins. Despite these peculiarities, T. brucei thioredoxin behaves like classical thioredoxins. It is reduced by thioredoxin reductases from different species, serves as donor of reducing equivalents for the ribonucleotide reductase of the parasite, and catalyzes the reduction of protein disulfides. The redox potential of -267 mV was obtained from protein-protein redox equilibration with Escherichia coli thioredoxin. The pK value of T. brucei thioredoxin was determined by two different methods. Carboxamidomethylation of the reduced protein yielded a pK value of 7.4 and generated mono-alkylated protein. The thiolate absorption at 240 nm resulted in a pK of 7.6 and, based on the extinction coefficient of 11.6 mM(-1) cm(-1), there are two (or three) cysteines titrating with very similar pK values. A thioredoxin reductase has not yet been detected in any organism of the order Kinetoplastida. T. brucei thioredoxin is spontaneously reduced by trypanothione (bis(glutathionyl) spermidine). Obviously, a specific thioredoxin reductase is not required as thioredoxin reduction can be conducted by the parasite-specific trypanothione/trypanothione reductase system.