Neobiosynthesis of Phosphatidylinositol and of Other Glycerolipids in the Entire Cattle Retina

Abstract

The vertebrate retina comprises two different portions, a neural part and a specialized layer of photoreceptor cells. Because the former is an integral part of the central nervous system it contains a large surface of excitable membranes. Also, a very high proportion of the photoreceptor cell mass is made up of tightly folded membranes. Each portion of the retina requires polar lipids for its membranes. However, the lipid composition and metabolism of the two portions differ strikingly. This is in part an assumption since whole photoreceptor cells have not yet been isolated and direct biochemical investigations are thus not possible. However, isolated rod outer segments are currently being intensively studied. They have a high phospholipid content and are very rich in polyenoic acyl chains (Anderson, 1970; Mason et al., 1973; Daemen, 1973). Although retina homogenates and subcellular fractions display several enzymatic activities for phospholipid synthesis and breakdown the rod outer segments seem to be devoid of these pathways (Swartz & Mitchell, 1970, 1973, 1974). The temperature — and light-stimulated continuous photoreceptor membrane renewal points to an active and specialized membrane biosynthetic process in the inner segment. Moreover, Bibb & Young (1974) concluded from autoradiographic analysis that phospholipids formed from (2-³H)-glycerol are concentrated initially in the inner segments, then aggregated at the base of rod outer segments and later appear in the outer segments.