Guanylate cyclase in neuroblastoma N1E 115 cells: presence of endogenous activator

Guanylate cyclase in cultured neuroblastoma N1E 115 cells was readily solubilized. MgCl2 as well as MnCl2 served as a metal cofactor of the guanylate cyclase. The maximal guanylate cyclase activity obtained with MgC12 was 80% of that with MnCl2. When the supernatant of cell homogenate was adjusted to pH 5.2, all of enzyme activity was precipitated. The guanylate cyclase activity recovered in the pH 5.2 precipitate was reduced to about 10% of the original supernatant. Combination of the pH 5.2 supernatant and precipitate fractions, however, restored guanylate cyclase activity, indicating that the pH 5.2 supernatant contains an endogenous activator for guanylate cyclase. The activating factor in the pH 5.2 supernatant remained in the aqueous phase after proteins were removed by perchloric acid. The factor was filterable through Diaflo ultrafilter membranes UM 2 and UM 10 indicating that the factor is a small molecule. The activation by the endogenous activator was prevented by N-methylhydroxylamine and lysolecithin.