PLANT PHYSIOLOGY , Vol 101, Issue 1 65-71, Copyright © 1993 by American Society of Plant Biologists
The pH Dependence of Violaxanthin Deepoxidation in Isolated Pea Chloroplasts
E. E. Pfundel and R. A. Dilley
Purdue University, Department of Biological Sciences, West Lafayette, Indiana 47907
The absorbance change at 505 nm was used to monitor the kinetics of
violaxanthin deepoxidation in isolated pea (Pisum sativum) chloroplasts
under dark conditions at various pH values. In long-term measurements (65
min) a fast and a slow exponential component of the 505-nm absorbance
change could be resolved. The fast rate constant was up to 10 times higher
than the slow rate constant. The asymptote value of the fast kinetic
component was twice that of the slow component. The pH dependency of the
parameters of the fast kinetic component was analyzed from pH 5.2 to pH
7.0. It was found that the asymptote value dropped slightly with increasing
pH. The rate constant was zero at pH values greater than 6.3 and showed
maximum values at pH values less than 5.8. Hill plot analysis revealed a
strong positive cooperativity for the pH dependency of the fast rate
constant (Hill coefficient nH = 5.3). The results are discussed with
respect to published activity curves of violaxanthin deepoxidation.