Vertebrate photoreceptors are among the most differentiated eukaryotic cells. These cells rapidly hyperpolarize when exposed to light, and the speed and sensitivity of this response has been optimized by the evolution of a unique cellular structure and enzymatic cascade. The photoresponse is made up of unique electrical and metabolic changes that are initiated by rhodopsin photolysis.

Perhaps the best understood metabolic photoresponse, a rapid light-induced hydrolysis of cGMP appears to be an essential intermediate in phototransduction. Other chapters in this volume cover the molecular properties of rhodopsin (17), the electrical properties of photoreceptor plasma membranes (75), and the coupling between the light-induced metabolic changes and the plasma membrane (79). This review details several biochemical processes that are unique to photoreceptor cells. Only the enzymatic hardware that couples rhodopsin photolysis to cGMP hydrolysis will be described in detail here.