Plant Physiol, June 2000, Vol. 123, pp. 625-636
The Photoreduction of H2O2 by
Synechococcus sp. PCC 7942 and UTEX
6251
Anthony G.
Miller,*
Kristopher J.
Hunter,2
Stephen J.B.
O'Leary,3 and
Leslie J.
Hart
Department of Biology, St. Francis Xavier University, Antigonish,
Nova Scotia, Canada B2G 2W5
It has been claimed that the sole
H2O2-scavenging system in the cyanobacterium
Synechococcus sp. PCC 7942 is a cytosolic
catalase-peroxidase. We have measured in vivo activity of a
light-dependent peroxidase in Synechococcus sp. PCC 7942 and UTEX 625. The addition of small amounts of
H2O2 (2.5 µM) to illuminated
cells caused photochemical quenching (qP) of chlorophyll fluorescence
that was relieved as the H2O2 was consumed. The
qP was maximal at about 50 µM
H2O2 with a Michaelis constant of about 7 µM. The H2O2-dependent qP strongly indicates that photoreduction can be involved in
H2O2 decomposition. Catalase-peroxidase
activity was found to be almost completely inhibited by 10 µM NH2OH with no inhibition of the H2O2-dependent qP, which actually increased,
presumably due to the light-dependent reaction now being the only route
for H2O2-decomposition. When
18O-labeled H2O2 was presented to
cells in the light there was an evolution of
16O2, indicative of
H216O oxidation by PS 2 and formation of
photoreductant. In the dark 18O2 was evolved
from added H218O2 as expected for
decomposition by the catalase-peroxidase. This evolution was completely
blocked by NH2OH, whereas the light-dependent evolution of
16O2 during
H218O2 decomposition was unaffected.
1
This work was supported by grants from the
National Sciences and Engineering Research Council of Canada (to
A.G.M.).
2
Present address: Waterloo Biotelemetry
Institute, Department of Biology, University of Waterloo, 200 University Avenue West, Waterloo, ON, Canada N2L 3G1.
3
Present address: Department of Biology,
University of Victoria, P.O. Box 3020 Station CSC, Victoria, BC, Canada
V8W 3N5.
*
Corresponding author; e-mail amiller{at}stfx.ca; fax 902-867-2389.
© 2000 American Society of Plant Physiologists
