Redistribution of Mn++-dependent pyrimidine 5'-nucleotidase (MDPNase) activity during shedding and phagocytosis

Ultrastructural cytochemistry was used to localize a previously undescribed retinal enzyme activity, ie, "manganese-dependent pyrimidine 5'-nucleotidase" (MDPNase) activity in retinas from rats raised in cyclic light and killed at various times in the lighting cycle. In retinas fixed during the last 3 hr of darkness, at the beginning of the shedding period, heavy cytochemical staining was observed over the extracellular surfaces of the apical processes of the retinal pigment epithelium (RPE) cells. The adjacent distal ends of the rod outer segments (ROS), ensheathed by the apical processes, were also stained along their surfaces. During the first 3 hr after light onset, at the time of maximal shedding and phagocytic activity, the tips of the ROS and ROS phagosomes were heavily labeled with reaction product distributed throughout the interdisc spaces. The RPE apical processes, previously heavily stained, were at this time weakly reactive relative to the ROS tips and phagosomes. After the shedding peak (4-8 hr after light onset), the tips of the ROS were no longer labeled with reaction product, and the apical processes were unreactive. The proximal portions of the ROS were weakly stained throughout the lighting cycle. The observed patterns of redistribution of MDPNase activity before, during, and after the shedding peak suggest that the presence of the enzyme in the ROS tips may be correlated with shedding. Changes in the staining of the RPE apical processes in relation to cyclic light further suggest that this enzyme may be transferred from the apical processes to the ROS tips prior to shedding.