Guanylate cyclase from bovine lung. A kinetic analysis of the regulation of the purified soluble enzyme by protoporphyrin IX, heme, and nitrosyl-heme.

A kinetic characterization of the regulation of purified soluble guanylate cyclase from bovine lung by protoporphyrin M and hematin is reported.Purified guanylate cyclase was isolated with heme and had specific activities @mol of cGMP/min/mg of protein) of 0.1-0.2 and 0.3-0.6 in the presence of excess MgGTP and MnGTP, respectively, in the absence of added activators.Protoporphyrin IX, nitric oxide (NO), and NOheme increased the V,, up to 40-fold and decreased the K,,, for GTP (from 100 to 45-55 p ~) in the presence of excess Mg'.However, in the presence of excess Mn2+ the V,, was increased only slightly and the K , for GTP was unchanged.Protoporphyrin IX resembled NO and NO-heme also in lowering the K , and Ki for uncomplexed metal.This close similarity in the interactions of these activators with guanylate cyclase suggests that a common form of activated enzyme is generated.Hematin, in excess of 1.5 p ~, inhibited guanylate cyclase activity.Smaller concentrations of hematin competitively inhibited protoporphyrin IX (KI = 0.35 p ~) ,