Phosphoenolpyruvate carboxylase from cherimoya fruit:: properties, kinetics and effects of high CO2

Phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) regulatory properties were studied in non-photosynthetic (mesocarp) and photosynthetic (peel) tissues from cherimoya (Annona cherimola Mill.) fruit stored in air, in order to gain a better understanding of in vivo enzyme regulation. Analyses were also performed with fruit treated with 20% CO2-20% O-2 to define the role of PEPC as part of an adaptive mechanism to high external carbon dioxide levels. The results revealed that the special kinetic characteristics of the enzyme from mesocarp-high V-max and low sensibility to L-malate inhibition - are related to the active acid metabolism of these fruits and point to a high rate of reassimilation of respired CO2 into keto-acids. With respect to fruit stored in air, PEPC in crude extracts from CO2-treated cherimoyas gave a similar V-max (1.12 +/- 0.03 mu kat.mg(-1) protein), a lower apparent K-m (68 +/- 9 muM for PEP) and a higher I-50 of L-malate (5.95 +/- 0.3 mM). These kinetic values showed the increase in the affinity of this enzyme toward one of its substrate, PEP, by elevated external CO2 concentrations. The lower K-m value and lower sensitivity to L-malate are consistent with higher in vivo carboxylation reaction efficiency in CO2-treated cherimoyas, while pointing to an additional enzyme regulation system via CO2. (C) 2001 Elsevier Science Ltd. All rights reserved.