Presence of a light-independent phospholipase A(2) in bovine retina but not in rod outer segments

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 alpha and T beta gamma subunits, The presence in ROS of a phospholipase A, (PLA,) 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 PLA(2) could be involved in the phototransduction process, and raised the possibility of receptor-mediated activation of PLA(2) via G-proteins in cell types other than rods, However, the biochemical characteristics and the role of this PLA(2) 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 PLA(2) activity in intact purified ROS, We also failed to detect PLA(2) activity in those ROS preparations, Nevertheless, we detected significant amounts of PLA(2) activity in two subretinal fractions adjacent to ROS: RPE (enriched with retinal pigment epithelial cells) and P200 (presumably containing neuronal cells, Muller 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 PLA(2) or PLA(1) 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 ''PLA(2) active.''