Convenient isolation and kinetic mechanism of glutathionylspermidine synthetase from Crithidia fasciculata

Trypanothione, the essential metabolite in the oxidant defense system of trypanosomatids, is synthesized by two distinct proteins, glutathionylspermidine synthetase and trypanothione synthetase. Glutathionylspermidine synthetase was purified to homogeneity from the trypanosomatid Crithidia fasciculata by aqueous two-phase systems and chromatography. The enzyme showed a specific activity of 38 mu mol of glutathionylspermidine formed per min per mg of protein. Its molecular mass was 78 kDa in SDS polyacrylamide gel electrophoresis, and it appeared predominantly monomeric in native polyacrylamide gel electrophoresis and gel filtration. The isoelectric point was at pH 4.6, and the pH optimum was near 7.6, Partial amino acid sequencing revealed homology with, but low similarity to, the glutathionylspermidine synthetase/amidase of Escherichia coli, and amidase activity was not detected in glutathionylspermidine synthetase of C. fasciculata. The kinetics of trypanosomatid glutathionylspermidine synthetase revealed a rapid equilibrium random mechanism with limiting K-m values for Mg2+-ATP, GSH, and spermidine of 0.25 +/- 0.02, 2.51 +/- 0.33, and 0.47 +/- 0.09 mM, respectively, and a k(cat) of 415 +/- 78 min(-1). Partial reactions at restricted cosubstrate supply were not detected by P-31 NMR, supporting the necessity of a quarternary complex formation for catalysis. ADP inhibited competitively with respect to ATP (K-i = 0.08 mM) and trypanothione exerted a feedback inhibition competitive with GSH (K-i = 0.48 mM).