Soluble guanylate cyclase: effect of YC-1 on ligation kinetics with carbon monoxide

Recently it has been reported that in the presence of YC-1, a benzyl indazole derivative, carbon monoxide activates soluble guanylate cyclase, GC, to about the same extent as its best known activator, nitric oxide. Kinetic studies utilizing flash photolysis of GC complexed with CO in the presence and absence of YC-1 show, in contrast to another recent report of a mixing experiment, 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 kinetics in the presence of 200 microM YC-1: This major fraction rebinds CO approximately 1000-fold more rapidly than in the absence of YC-1. Another portion, considerably less than half, exhibits kinetics that are almost exactly the same as in the absence of YC-1. It is now clear that both YC-1 and CO have 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 activation, a hypothesis that would have been difficult to maintain if the earlier report, that YC-1 has no effect on CO binding, were correct. Possible reasons for the discrepancy between the two measurements are suggested.