Affordable Access

Publisher Website

Light-induced de-epoxidation of violaxanthin in lettuce chloroplasts. III. Reaction kinetics and effect of light intensity on de-epoxidase activity and substrate availability

Authors
Journal
Biochimica et Biophysica Acta (BBA) - Bioenergetics
0005-2728
Publisher
Elsevier
Publication Date
Volume
357
Issue
1
Identifiers
DOI: 10.1016/0005-2728(74)90119-4
Disciplines
  • Biology

Abstract

Abstract The 505 nm change in isolated chloroplasts of Lactuca sativa var. Manoa showed a linear correlation with violaxanthin decrease. The corresponding difference extinction coefficient was established as 27.3 mM −1 · cm −1. The 505 nm change followed first-order kinetics. The first-order rate constant, a measure of de-epoxidase activity, and the final extent of the 505 nm change, a measure of the extent of de-epoxidation, varied with light intensity. In 670 nm light the activity remained about half-maximal at intensities from 1 to 4 kerg · cm −2 · s −1 before reaching saturation at 9 kergs · cm −2 · s −1. In contrast, the extent showed a simpler response and reached saturation already at 4 kergs · cm −2 · s −1. These results suggest that different photosynthetic factors determine de-epoxidase activity and de-epoxidation extent. De-epoxidase activity is known to reflect steady-state thylakoid pH whereas the de-epoxidation extent was found to reflect light-dependent variable substrate availability. The available fraction varied in size from 40% of the total violaxanthin at 1 kerg · cm −2 · s −1 to 67% at saturation. The possibility that variable availability reflects conformational changes of the membrane near Photosystem II which result in variable exposure of the substrate to enzyme is discussed.

There are no comments yet on this publication. Be the first to share your thoughts.