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

L-Cysteine is a competitve inhibitor of the pyruvate kinase (PK) isozymes f rom the body wall and introvert of the sipunculan Phascolosoma arcuatum exp osed to normoxia or anoxia. Firstly, the 1/V versus 1/[phosphoenolpyruvate] plot shows that the V-max values of PK isozymes from these body parts were unaffected by L-cysteine with exception of the body wall from the anoxic w orm. Secondly, the Dixon plot shows that the percentage inhibition of PK ac tivity by L-cysteine decreased with increasing concentrations of phosphoeno lpyruvate. Kinetic properties of PK isozymes from the body walt and introve rt of P. arcuatum could be altered by anoxic exposure. In anoxia, the modif ied PK isozymes from these body parts had lower affinity to phosphoenolpyru vate and L-cysteine. Despite the increase in K-i(L-cysteine) value upon ano xic 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 t he body wall and introvert. However, the effect of L-cysteine on PK was tis sue specific and it had no effect on the isozyme obtained from the internal organs.