Purification and identification of photoreceptor guanylate cyclase

Photoreceptor guanylate cyclase was solubilized and purified from bovine rod outer segments with 50-150-fold increase in specific activity using the nonionic detergent n-dodecyl-beta-D-maltoside. Guanylate cyclase activities correlated with the enrichment of a protein with an apparent Mr = 112,000. The purified enzyme showed specific activities of 100-700 nmol of cGMP produced/min/mg protein and exhibited positive cooperativity with respect to MnGTP (Hill coefficient n = 1.6 +/- 0.1). The apparent Km was 274 +/- 67 microM, and the turnover number was determined to be 0.2-1.3 cGMP produced/s. The molar ratio of the 112-kDa protein to rhodopsin corresponds to 1:104. This indicates that the amount of guanylate cyclase in rod photoreceptors is nearly equimolar to the amount of the phosphodiesterase.