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Volume 271, Number 32, Issue of August 9, 1996 pp. 19209-19218
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.

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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.''

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