Cytochemical localization of Mn2+-dependent pyrimidine 5'-nucleotidase activity in isolated rod outer segments

A cytochemical method was developed for localization in isolated rod outer segments of manganese-dependent pyrimidine 5'-nucleotidase (MDPNase), an enzyme activity with possible relevance to shedding that we recently reported in photoreceptors and retinal pigment epithelial (RPE) cells in the intact rat retina. The purpose of this study was to eliminate the possibility that the previously observed cytochemical staining of the rods was due to diffusion of reaction product from the RPE cell lysosomes, which were also heavily stained. Rod outer segments (ROS) were isolated on continuous sucrose gradients from retinal homogenates prepared from rats raised in cyclic light (12 hr light:12 hr dark) and killed during the first 2 hr after light onset. ROS-containing bands were removed from the gradients and the isolated rods were fixed in 0.25% glutaraldehyde and pelleted. Chopped sections of the pellets were incubated in cytochemical medium for MDPNase activity and processed for light- and electron-microscopic localization of the enzyme activity. Two patterns of cytochemical staining were seen in ROS isolated from retinas obtained at this time of day. A few of the pellets contained clusters of ROS that were heavily coated along their surfaces and seemingly interconnected by thick strands of highly reactive extracellular material that displayed a punctate pattern of cytochemical staining. This material may have originated from the apical processes of the RPE cells, which were heavily stained in tissue fixed in situ around the time of light onset. The second staining pattern, visible only by electron microscopy, was more commonly observed. In the majority of the isolated ROS profiles, discrete streaks of cytochemical reaction product were seen in association with the internal aspects of the discs, at sites that seemed to correspond to the rims, and to narrow zones within the disc interiors. This distribution of reactive sites closely resembled that observed over most of the length of the ROS in the intact retina fixed at the same time of day. Occasionally, ROS profiles were encountered in which additional reactive sites were localized to the interdisc spaces between the plasma membrane and the rims of the discs. The latter pattern resembled the distribution of reaction product seen during this period over the tips of the ROS fixed in situ. As in the intact retinas, the cytochemical staining of the isolated ROS was inhibited by fluoride ions and strongly stimulated by manganese ions.(ABSTRACT TRUNCATED AT 400 WORDS)