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Adenylate cyclase is described in a number of photoreceptor membranes. Vertebrate rod outer segments contain light-regulated cyclase, and light regulation is abolished by digitonin. Disruption of microvilli in cone and rhabdomphotoreceptors is also associated with loss of light regulation and retention of full enzymic activity. The data suggest that inhibitory constraint provides regulation in cyclase systems and that disruption of membrane structure uncouples catalytic and regulatory elements.
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