Journal of Experimental Botany, Vol. 51, No. 352, pp. 1903-1909,
November 1, 2000
© 2000 Oxford University Press
Original Papers |
Phosphoenolpyruvate carboxylase in cucumber (Cucumis sativus L.) roots under iron deficiency: activity and kinetic characterization
Dipartimento di Produzione Vegetale, University of Milan, Via Celoria 2, I-20133 Milano, Italy
Phosphoenolpyruvate carboxylase (PEPCase) activity was investigated in cucumber roots grown under iron starvation. The enzyme extracted from plants grown in the presence and in the absence of Fe was characterized both kinetically and biochemically. Extractable PEPCase activity was increased by 4-fold in the absence of Fe. This increase began about 5 d after Fe starvation. Western blot analysis revealed the presence of two polypeptides with apparent molecular masses of 103 and 108 kDa. At the beginning both the polypeptides were equally present in the control and in the Fe-deficient roots. After 10 d of Fe starvation the increase was clearly evident and concerned, in particular, the polypeptide of 103 kDa whose enhancement was around 3-fold with respect to the control. Re-supply of iron to Fe-starved roots decreased both the activity and the concentration of the enzyme to the control values. Determination of kinetic parameters revealed that the Km values for the substrates were the same, while the Vmax was increased by four times for the enzyme extracted from Fe-deficient roots. Also the responses to pH changes and to the allosteric modulators malate and glucose-6-phosphate were different. The kinetic data, the increase in PEPCase specific activity and in the PEPCase polypeptides concentration seem to indicate that under Fe deficiency the enzyme regulation might be, in part, exerted at the transcriptional level.
Key words: Cucumis sativus, Fe-deficiency, phosphoenol-pyruvate carboxylase.
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