Soluble guanylate cyclase: Effect of YC-1 on ligation kinetics with carbonmonoxide

Recently it has been reported that in the presence of YC-1, a benzyl indazo le derivative, carbon monoxide activates soluble guanylate cyclase, GC,to a bout the same extent as its best known activator, nitric oxide. Kinetic stu dies utilizing hash photolysis of GC complexed with CO in the presence and absence of YC-1 show, in contrast to another recent report of a mixing expe riment, that YC-1 has a profound effect on bimolecular association kinetics and a smaller, but significant, effect on ligand affinity. Most prominent is the appearance of a major, new phase in the bimolecular recombination ki netics in the presence of 200 mu M YC-1: This major fraction rebinds CO sim ilar to 1000-fold more rapidly than in the absence of YC-1. Another portion , considerably less than half, exhibits kinetics that are almost exactly th e same as in the absence of YC-1. It is now clear that both YG-1 and CO hav e a strong synergistic effect on enzyme activity and also a dramatic effect on ligand binding behavior. It is, therefore, a reasonable inference that ligand binding at the heme iron atom is intimately connected with enzyme ac tivation, a hypothesis that would have been difficult to maintain if the ea rlier report, that YC-1 has no effect on CO binding, were correct. Possible reasons for the discrepancy between the two measurements are suggested. (C ) 1999 Academic Press.