Capacity of enzymes of the euphorbiacea Aleurites montana involved in CO2-fixation, compared to plants having C-3-, C-4- and Crassulacean acid metabolism

Capacities of phosphoenolpyruvate carboxylase (PEP-Co), ribulose bisphospha te carboxylase (Rubisco), NADP(+) malic enzyme (ME) and of malate dehydroge nase (MDH) were measured in the Euphorbiacea Aleurites montana, grown under 700 ppm CO2 for four weeks prior to enzyme extraction. For comparison Bryo phyllum diagremontiana (CAM), Saccharum officinarum (C-4) and Capsicum frut escens (C-3) were treated in the same way. PEP-Co capacity of Aleurites was in the range of 12-, that of Capsicum approx. 26 nmol x min(-1) x mg prote in(-1), without significant influence of the light period or CO2-treatment. In contrast, the activity of the enzyme from Saccharum was, depending on t he duration of light, 160- respectively 96 times higher than that of the tu ng-oil tree. In Bryophyllum a rather low activity in the morning was increa sed during the day to approx. 230 nmol x min(-1) x mg protein in plants gro wn in the greenhouse and to approx. 115 nmol x min(-1) x mg protein(-1) in those from the growth chamber. Malate was hardly detectable in extracts of Aleurites, whereas it was high in Bryophyllum, depending on the light perio d. The ratio of average PEP-Co to Rub-Co capacity was high for the CAM-plan t (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 cap acity (20 to 28 nmol x min(-1) x mg protein(-1)) was found for Aleurites an d for Capsicum, whereas it was 10 to 17 times higher in Saccharum. In Bryop hyllum, the activity was up to 80 nmol x min(-1) x mg protein(-1), dependen t on light period. MDH capacity was extremely high in all plants investigat ed. Highest rates (10-20 mu mol x min(-1) x mg protein(-1)), were obtained for Bryophyllum, followed by sugar cane and Capsicum with 5-8 mu mol x min( -1) x mg protein. Again, the lowest capacity was found in extracts of Aleur ites with approx. 1.3 to 1.6 pmol x min(-1) x m protein(-1). Thus, in Aleur ites montana no indication for C-4- Or Crassulacean acid metabolism was obt ained. Therefore, the earlier observed very efficient uptake of CO2 cannot be explained by a high expression of the PEP-Co protein, known to occur in CAM- and C-4-plants.