Purification of guanylyl cyclase from rod outer segments

The particulate form of guanylyl cyclase from bovine rod outer segments has been solubilized and purified to near homogeneity by a combination of liquid chromatography and native gel electrophoresis. The procedure enriches enzyme activity 6700-fold from rod outer segment extracts to a final specific activity of 17.5 mumol/min per mg (when assayed with Mn-GTP as substrate). Purified preparations of guanylyl cyclase contain a single glycoprotein with an apparent molecular mass of 60,000 Da and a native isoelectric point of 7.6. Although crude or partially purified enzyme activity is modulated by sub-micromolar concentrations of Ca2+, the fully purified enzyme is insensitive to this cation. However, the purified enzyme remains sensitive to nitrovasodilators, being stimulated over 10-fold by sodium nitroprusside. These data suggest that retinal rods contain a unique isoform of guanylyl cyclase.