Localization of nonspecific lipid transfer protein (nsLTP = sterol carrier protein 2) and acyl-CoA oxidase in peroxisomes of pigment epithelial cells of rat retina.

We investigated the localization of nonspecific lipid transfer protein (nsLTP) in rat retina, especially in the pigment epithelial (RPE) cells, by the avidin-biotin-peroxidase complex method on cryosections for light microscopy and by the cryoimmunogold method for electron microscopy. Light microscopic observation revealed that the RPE, inner segment layer, nerve fiber layer, and Müller cells contain nsLTP. In the RPE cells gold particles were exclusively concentrated in the small peroxisomes (microperoxisomes; 0.1-0.3 micron in diameter), which were identified by double staining using anti-nsLTP and anti-catalase antibodies. In the peroxisomes gold particles were distributed homogeneously in the matrices and no preferential binding to the limiting membrane was observed. Acyl-CoA oxidase was also localized in the matrices of the peroxisomes. We suggest that the peroxisomes in RPE cells play important roles in the metabolism of lipids of the outer segment disk membranes, especially in the beta-oxidation of polyunsaturated long-chain and very long-chain fatty acids, such as docosahexaenoic acid which is composed of approximately one third of fatty acids in the disk membranes.