[Thiol-dependent activation of guanylate cyclase by organic nitrates]

Organic nitrates produce their pharmacological effect by an intracellular stimulation of the enzyme guanylate cyclase (E. C. 4.6.1.2). We could show that the stimulatory effect of organic nitrates on the activity of guanylate cyclase is strongly dependent on the number of nitrate residues per molecule. The EC50 values found for the tetra-, tri- di-, and mononitrates differed from each other by the factor 4. In contrast to investigations carried out with the perfused isolated Langendorff heart there was no correlation between the lipophilicity of these substances and the EC50 in our guanylate cyclase preparation, as penetration of cell membranes is not required. Other authors have found that organic nitrates are able to activate the enzyme guanylate cyclase only in the presence of cysteine. There is general agreement in the literature that organic nitrates have to be cleaved before they become biologically active. During the transformation which takes place in the presence of cysteine or by means of enzymatic catalysis the nitric oxide radical is liberated as the essential stimulatory agent. We found a strict correlation between the liberation of nitric oxide from different organic nitrates (GTN, IMDN, IIDN, ISDN, IS-2-N, IS-5-N) and the degree of enzyme activation. The Ec50 values of the organic nitrates were calculated from the concentration response curves which were obtained with a guanylate cyclase preparation from rat liver in the presence of cysteine. The degradation of the organic nitrates was measured under the same conditions by means of HPLC. The amount of nitric oxide set free was calculated by using the velocity constants k of organic nitrate degradation.(ABSTRACT TRUNCATED AT 250 WORDS)