L-Cysteine is a competitive inhibitor of pyruvate kinase from the intertidal sipunculan, Phascolosoma arcuatum

Abstract

L-Cysteine is a competitve inhibitor of the pyruvate kinase (PK) isozymes from the body wall and introvert of the sipunculan Phascolosoma arcuatum exposed to normoxia or anoxia. Firstly, the 1/V versus 1/[phosphoenolpyruvate] plot shows that the Vmax values of PK isozymes from these body parts were unaffected by L-cysteine with exception of the body wall from the anoxic worm. Secondly, the Dixon plot shows that the percentage inhibition of PK activity by L-cysteine decreased with increasing concentrations of phosphoenolpyruvate. Kinetic properties of PK isozymes from the body wall and introvert of P. arcuatum could be altered by anoxic exposure. In anoxia, the modified PK isozymes from these body parts had lower affinity to phosphoenolpyruvate and L-cysteine. Despite the increase in Ki(L-cysteine) value upon anoxic exposure, the [I]0.1, [I]0.5 and [I]0.9 values of L-cysteine for these PK isozymes were lower than those obtained for the normoxic worms. L-Cysteine was a more effective inhibitor than alanine for PK isozymes from the body wall and introvert. However, the effect of L-cysteine on PK was tissue specific and it had no effect on the isozyme obtained from the internal organs.