2 FORMS OF PHOSPHOENOLPYRUVATE CARBOXYLASE IN CHLORELLA-KESSLERI

FPLC of crude cell extracts of Chlorella kessleri reveals two protein fractions with phosphoenolpyruvate carboxylase activity. Their mole ma sses, 955 kDa and 543 kDa, suggest that they are dimer and monomer of the same enzyme. Further data, however, indicate that they are more li kely two different iso-forms of phosphoenolpyruvate carboxylase: Attem pts to interconvert both proteins in vitro, such as through treatment with NaCl, metabolites and thiol- or histidine-group effecting reagent s, were unsuccessful. The substrate affinity of the large protein was slightly higher than that of the small protein (K(m) = 1.13 and 0.76 m m, respectively); and the sensitivity to enhanced temperature was more pronounced in the large than in the smaller protein (half lifes = 23 min and 55-60 min, respectively). Some properties of both fractions, h owever, proved identical: 1. pH optima at pH 8.5-9, 2. Hill coefficien ts approx. 1, 3. no significant regulatory effect of glutamine, glutam ate, aspartate and malate, 4. increase in K(m) and in Hill coefficient by citrate, and 5. identical behaviour in ion exchange chromatography . Functions and localization of the assumed two iso-forms of phosphoen olpyruvate carboxylase remain to be clarified. No specific effects of red or blue light during autotrophic growth on either protein with PEP Co activity could be found.