Presence of a Light-independent Phospholipase A2 in Bovine Retina but Not in Rod Outer Segments

Abstract

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 Talpha and Tbetagamma 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."