Effect of Light on Phosphatidate Phosphohydrolase Activity of Retina Rod Outer Segments. The Role of Transducin

PASQUARÉ S.J.; SALVADOR G.A.; ROQUE, M.E.; GIUSTO, N.M.

ARCHIVES OF BIOCHEMISTRY AND BIOPHYSICS

ELSEVIER SCIENCE INC

Año: 2000 vol. 379 p. 299 - 306

0003-9861

The aim of the present paper is to evaluate the modulation of phosphatidate phosphohydrolase (PAPase) and diacylglyceride lipase (DGL) activities in bovine rod outer segment (ROS) under dark and light conditions and to evaluate the role of transducin (T) in this phenomenon.In dark-adapted ROS membranes exposed to light, PAPase activity is inhibited by 20% with respect to the activity found under dark conditions. To determine whether the retinal G protein, T, participates in the regulation of PAPase activity in these membranes, the effects of GTPgS and GDPbS on enzyme activity were examined. Under dark conditions in the presence of GTPgS, which stabilizes T in its active form (Ta 1 Tbg), enzyme activity was inhibited and approached control values under light conditions. GDPbS, on the other hand, which stabilizes the inactive state of T (Tabg), stimulated PAPase activity by 36% with respect to control light conditions. ADP-ribosylation by cholera and pertussis toxin was also studied. In ADP-rybosilatedROS membranes with pertussis toxin under dark conditions, PAPase activity was 36% higher than the activity found under control light conditions. ADP-ribosylation by CTx, on the other hand, inhibited PAPase activity by 22%, with respect to dark control conditions, mimickinglight effect. The effects of GTPgS and GDPbS and conditions of ADP-ribosylation by PTx and CTx on DGL activity were similar to those of PAPase activities. Based on NEM sensitivity we have also demonstrated that the PAPase present in ROS is the PAP 2 isoform. Our findingstherefore suggest that light inhibition of PAP 2 in ROS is a transducin-mediated mechanism.