Characterization of protein inhibitors of guanylate cyclase activation from rat heart and bovine lung.

Sodium azide activated soluble rat liver guanylate cyclase but not crude preparations from rat heart or bovine lung. Mixing supernatant fractions from liver preparations with those from heart or lung resulted in an inhibition of sodium azide activation. The inhibitory activity was nondialyzable, heat-labile, and resistant to trypsin treatment. Chromatography of heart or lung supernatant preparations on Sephadex G-100 separated guanylate cyclase from inhibitory materials and the partially purified guanylate cyclase could be activated with azide. The inhibitory materials co-chromatographed with hemoglobin or myoglobin. These heme proteins inhibited activation of guanylate cyclase by azide as well as NaNO,, sodium nitroprusside, and nitric oxide. Hemoglobin and myoglobin, but not methemoglobin, also inhibited azide activation of partially purified guanylate cyclase when catalase, cytochrome b,, or cytochrome c reductase was substituted for the azide activator protein requirement. Hemoglobin readily reversed the activation of guanylate cyclase induced by several agents. Hemoglobin also decreased basal guanylate cyclase activity assayed with Mn’+, but not M g z+, as sole cation cofactor, suggesting that some of the basal enzyme activity in liver preparations is partially activated enzyme. Thus, the tissue specificity of azide activation of guanylate cyclase is attributable to the presence of metalloprotein activators that can convert azide to the active agent nitric oxide and the presence of heme (Fez+) protein inhibitors. The mechanism of inhibition is probably due to scavenging of nitric oxide and other radicals.