Phosphoenolpyruvate carboxylase isolated from Panicům maximum Jacq. leaf presented a hysteretic behaviour that resulted in a kinetical lag in the reaction progress curve. This lag depended on the concentration of etuiyme, total phosphoenolpyruvate (PEP) and total Mg2+. Through an analysis of these dependences it is suggested that the hysteresis is due to an association-dissociation process influenced by union, to one form of the enzyme, of more than one of the three possible ligands; free Mg2+, free PEP or PEP-Mg2+ complex. The partially purified enzyme showed, during steady statě, a Michaelis-Menten kinetics for PEP and Mg2+ (total concentrations) and pH optima between 7.8 and 8.2.
Carbonic anhydrase (CA) activity and net photosynthetic rate (/’N) in the leaves, and plant dry mass (DM) in twelve cultivars of mustard (Brassica juncea L.) were measured 50, 70 and 90 d after sowing. At different sampling stages CA was strongly associated with PN and DM.