HISTOCHEMISTRY OF GUANYLATE-CYCLASE ACTIVITY

Guanylate cyclase liberates pyrophosphate from guanosine triphosphate (GTP). In studies published previously, this phosphate is trapped by l ead ions even though it is known that free lead ions inactivate a cons iderable proportion of this enzymatic activity. To overcome the damagi ng effects of fixation, this study used fresh cryostat sections stabil ized with a sufficient concentration of a collagen-derived polypeptide to ensure no measurable loss of guanylate cyclase activity. To avoid the damaging influence of free lead ions, we used a hidden metal captu re reagent, i.e., a complex of lead ammonium citrate/acetate that does not react with GTP but which rapidly forms a precipitate with the pyr ophosphate liberated by the enzyme. The lead precipitate is then conve rted into the colored sulfide which is measured in individual cells by microdensitometry. This system was used to measure guanylate cyclase activity in individual cells in unfixed sections of rat liver.