Isoprenoid biosynthesis in the retina. Quantitation of the sterol and dolichol biosynthetic pathways

The isoprenoid pathway provides several important products for retina function. In this study the sterol and dolichol pathways were investigated in retinas from Rana pipiens in order to assess the contribution of de novo synthesis. Levels of 5.9±2.0 (n=13) nmol/retina for squalene, 134±27 (n=16) nmol/retina for cholesterol, and 0.14±0.04 (n=11) nmol/retina for dolichyl phosphate (Dol-P) were determined by high performance liquid chromatography analysis. When whole retinas were incubated with ³H₂O, radioactivity was incorporated into compounds which chromatographed on reversed-phase and silica high performance liquid chromatography at the elution positions of squalene, cholesterol, lathosterol, and methyl sterols. From these results, the upper limit for the absolute rate of the sterol pathway was estimated to be 3.4 pmol/h. When retinas were incubated with [³H]acetate, the major labeled product was squalene. The relatively low level of incorporation into cholesterol was apparently due to a substantial pool of squalene which accumulated de novo incorporated [³H]acetate. Dol-P was also labeled with [³H]acetate, and by comparing the ratio of ³H incorporation into Dol-P/squalene with the absolute rate of the sterol pathway, the absolute rate of Dol-P synthesis was determined to be 0.022 pmol/h. Our calculations indicate that the retina does not synthesize sufficient quantities of cholesterol de novo to account for that which is utilized in the biogenesis of rod outer segment membranes.