Cyclic Nucleotide Metabolism in Human Photoreceptors

¹ Division of Ophthalmology and the Department of Pharmacology, University of Colorado Medical Center, 4200 E. Ninth Ave., Denver, Colo. 80220

Human photoreceptor outer segments were prepared by sucrose density centrifugation of homogenized retinas. This preparation was found to contain high levels of the enzymes catalyzing synthesis and degradation of cyclic GMP and, to a lesser extent, cyclic AMP. Light and calcium ion influenced cyclic GMP synthesis. Cyclic nucleotide-dependent protein kinase, the presumed site of action of cyclic nucleotides, was also present at high leveb. This enzyme was stimulated to the same extent by cyclic GMP and cyclic AMP, but was more sensitive to cyclic AMP. In all the parameters measured, the enzymes of the human preparation resembled those found in bovine rod outer segments.

Key Words: outer segments • cyclic AMP • cyclic GMP • phosphodiesterase • cyclase • protein kinase • adaptation

Submitted on April 8, 1974