Characterization of guanylate cyclase of rod outer segments of the bovine retina.

Guanylate cyclase (GTP pyrophosphate-lyse (cyclizing), EC 4.6.1.2.) of bovine retinal rod outer segments is almost completely particulate, i.e. associated with rod outer segment membranes. In contrast to particulate guanylate cyclase in other tissues, treatment of rod outer segments with Triton X-100 does not solublize the enzyme but inhibits it. Enzyme activity is dependent on the presence of divalent cation, especially Mn2+ with only poor activation by Mg2+ (10-fold lower) and no activation seen with other cation. Ezpression of maximal activity required Nm2+ and GTP in equimolar concentrations with an apparent Km of 8 . 10(-4) M and V of 10 nmol/min per mg protein. Excess of Mn2+ over that required for the formation of the Mn . GTP complex was inhibitory. Ca2+, Ba2+ and Co2+ inhibited enzyme activity when assayed with the Mn . GTP substrate complex. In the presence of a fixed concentration of 1mM Mn2+, the enzyme exhibited strong negative cooperative interactions with GTP, characterized by an intermediary plateau region in the substrate vs. enzyme activity curve, a curve of downward concavity in the double reciprocal plot and a Hill coefficient of 0.5. Nucleotides such as ITP, ATP and UTP at higher concentrations (1 mM) stimulates activity by 40%. NaN3 has no effect on the guanylate cyclase. It is thus possible that the guanylate cyclase may be regulated in vivo by both the metal : GTP substrate ratio and the free divalent cation concentration as well as by the ATP concentration and thus play an important but yet undefined role in the visual process.