Capacity of Enzymes of the Euphorbiacea Aleurites montana Involved in CO2- Fixation, Compared to Plants Having C3-, C4- and Crassulacean Acid Metabolism

Capacities of phosphoenolpyruvate carboxylase (PEP-Co), ribulose bisphosphate carboxylase (Rubisco), NADP malic enzyme (ME) and of malate dehydrogenase (MDH) were measured in the Euphorbiacea Aleurites montana, grown under 700 ppm CO for four weeks prior to enzyme extraction. For comparison Bryophyllum daigremontiana (CAM), Saccharum officinarum (C ) and Capsicum frutescens (C ) were treated in the same way. PEP-Co capacity of Aleurites was in the range of 12-, that of Capsicum approx. 26 nmol × min × mg protein , without significant influence of the light period or CO -treatment. In contrast, the activity of the enzyme from Saccharum was. depending on the duration of light, 160- respectively 96 times higher than that of the tung-oil tree. In Bryophyllum a rather low activity in the morning was increased during the day to approx. 230 nmol × min × mg protein in plants grown in the greenhouse and to approx. 115 nmol × min × mg protein in those from the growth chamber. Malate was hardly detectable in extracts of Aleurites, whereas it was high in Bryophyllum, depending on the light period. The ratio of average PEP-Co to Rub-Co capacity was high for the CAM-plant (20:1), somewhat lower for sugar cane (10:1), but almost at equality for Aleurites (0.9:1) and chilli (0.8:1). For the NADP malic enzyme, low capacity (20 to 28 nmol x min × mg protein ) was found for Aleurites and for Capsicum, whereas it was 10 to 17 times higher in Saccharum. In Bryophyllum, the activity was up to 80 nmol × min × mg protein , dependent on light period. MDH capacity was extremely high in all plants investigated. Highest rates (10-20 μmol × min × mg protein ), were obtained for Bryophyllum, followed by sugar cane and Capsicum with 5 -8 μmol × min-1 x mg protein-1. Again, the lowest capacity was found in extracts of Aleurites with approx. 1.3 to 1.6 μmol × min × m protein . Thus, in Aleurites montana no indication for C or Crassulacean acid metabolism was obtained. Therefore, the earlier observed very efficient uptake of CO cannot be explained by a high expression of the PEP-Co protein, known to occur in CAM- and C plants.