According to our present understanding organic nitrates like glycerine trinitrate mediate their pharmacological effect by an intracellular stimulation of the enzyme guanylate cyclase (E.C. 188.8.131.52) [1, 10]. The exact molecular mechanism underlying the process of enzyme activation is still a matter of controversial discussion. But there is general agreement in literature about the fact that organic nitrate compounds are able to activate the enzyme guanylate cyclase only in the presence or by the interaction of the amino acid cysteine [3, 5]. The stimulatory activity of nitric oxide-containing compounds may be due, at least in part, to the formation of active, unstable intermediate S-nitrosothiols, i.e. S-nitrosocysteine in case of the organic nitrates . According to Craven and DeRubertis , the active intermediates of guanylate cyclase stimulation are represented by nitric oxide-heme complexes. There is, however, substantial evidence that the 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, nitric oxide radicals are reductively split off the molecule from which (via the intermediate formation of salpetric acid) the nitric oxide is liberated as the essential stimulatory agent. In this study we examined the transformation of glycerine trinitrate and other organic nitrates under the influence of different thiols and a purified soluble rat liver guanylate cyclase preparation. At the same time the stimulation of guanylate cyclase in the presence of the thiols mentioned was quantitatively estimated. Only in case of cysteine did we find a strict correlation between the liberation of nitric oxide from different organic nitrates and the degree of enzyme activation. Several other thiols were also able to liberate nitric oxide, but surprisingly enough, there was no equivalent stimulation of guanylate cyclase. We believe that these findings thus provide new information about the subcellular linkage of biochemical reactions involved in the mediation of the pharmacological activity of organic nitrates, because there is now considerable evidence that there must be further biochemical steps which link the liberation of nitric oxide on the one hand with the activation of guanylate cyclase on the other.
- Guanylate cyclase
- Nitric oxide