Characterization and kinetics of isoenzymes of pyruvate kinase from developing castor bean endosperm.

Isozymes of pyruvate kinase (PK) have been isolated from developing castor bean endosperm. One isozyme, PK(c), is localized in the cytosol, and the other, PK(p), is in the plastid. Both isozymes need monovalent and divalent cations for activity, requirements which can be filled by K(+) and Mg(2+). Both isozymes are inhibited by citrate, pyruvate, and ATP. PK(c) has a much broader pH profile than PK(p) and is also more stable. Both have the same K(m) (0.05 millimolar) for PEP, but PK(p) has a 10-fold higher K(m) (0.3 millimolar) for ADP than PK(c) (0.03 millimolar). PK(c) also has a higher affinity for alternate nucleotide substrates than PK(p). The two isozymes have different kinetic mechanisms. Both have an ordered sequential mechanism and bind phosphoenolpyruvate before ADP. However, the plastid isozyme releases ATP first, whereas pyruvate is the first product released from the cytosolic enzyme. The properties of the two isozymes are similar to those of their counterparts in green tissue.