MECHANISM OF YC-1-INDUCED ACTIVATION OF SOLUBLE GUANYLYL CYCLASE

The signaling molecule nitric oxide (NO) mediates many of its effects by the stimulation of soluble guanylyl cyclase (sGC). The activation p rocess is initiated by high-affinity binding of NO to the enzyme's pro sthetic heme group. Despite its poor sGC-activating properties, carbon monoxide (GO) has also been suggested as a physiological activator of sGC. Recently, we have shown that the substance YC-1, a benzyl indazo le derivative, stimulates sGC by 10-fold (independently of NO) potenti ates the stimulatory effect of NO, and turns CO into a potent activato r of sGC. In the present study, we show that activation of sGC by prot oporphyrin IX, a ligand-independent activator, was potentiated by YC-1 , yet a shift of the concentration-response curve as seen with NO and CO was not observed. YC-1 slowed down the dissociation rates for NO an d CO from the activated enzyme as monitored by cGMP accumulation after addition of the NO and CO scavenger oxyhemoglobin. A direct interacti on of YC-1 with the heme group can be ruled out because YC-1 did not c hange the Soret absorption of basal or stimulated sGC and, in addition , still bound to the heme-depleted enzyme. Together, our results indic ate that YC-1 increases the maximal catalytic rate and sensitizes the enzyme toward its gaseous activators by binding to an allosteric site on sGC molecules, thereby reducing the ligand dissociation rate from t he heme group.