Volume 271, Number 32,
Issue of August 9, 1996
pp. 19209-19218
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.
Presence of a Light-independent Phospholipase A2 in
Bovine Retina but Not in Rod Outer Segments
(Received for publication, September 8, 1995, and in revised form, April 23, 1996)
Michèle
Jacob
,
Philip K.
Weech
§
and
Christian
Salesse
From the
Centre de Recherche en Photobiophysique,
Université du Québec à Trois-Rivières,
Trois-Rivières, Québec, Canada G9A 5H7 and the
§ Merck Frosst Centre for Therapeutic Research,
Pointe-Claire-Dorval, Québec, Canada M9R 4P8
Rod outer segments (ROS) are responsible for the
visual transduction process. Rhodopsin, which constitutes 85-90% of
ROS proteins, absorbs light photons, changes its conformation, and then
binds to a heterotrimeric G-protein called transducin. As a
consequence, transducin dissociates into T
and T
subunits. The
presence in ROS of a phospholipase A2 (PLA2)
stimulated by light and guanosine 5
-O-(3-thio)triphosphate
was first demonstrated in 1987 (Jelsema, C. L. (1987) J. Biol. Chem. 262, 163-168). This led that author to conclude that
ROS PLA2 could be involved in the phototransduction
process, and raised the possibility of receptor-mediated activation of
PLA2 via G-proteins in cell types other than rods. However,
the biochemical characteristics and the role of this PLA2
have not been fully elucidated. We have tried to reproduce some of the
results previously reported in order to further characterize this
enzyme. We have found that, in our hands, there is neither
light-dependent nor GTP-dependent
PLA2 activity in intact purified ROS. We also failed to
detect PLA1 activity in those ROS preparations.
Nevertheless, we detected significant amounts of PLA2
activity in two subretinal fractions adjacent to ROS: RPE (enriched
with retinal pigment epithelial cells) and P200 (presumably containing
neuronal cells, Müller cells, and rod inner segments). The enzyme
present both in RPE and P200 is light- and GTP-independent,
Ca2+- and Mg2+-independent, and seems to be
optimally active in the alkaline pH range. Our results suggest that
there is, if any, vanishingly little PLA2 or
PLA1 activity in intact purified ROS and that the activity
levels previously reported in the literature could have been due to a
contamination by either RPE or P200. This is supported by our
observation that some contaminated ROS preparations were
``PLA2 active.''