Synergistic activation of soluble guanylate cyclase by YC-1 and carbon monoxide: implications for the role of cleavage of the iron-histidine bond during activation by nitric oxide

Background: Nitric oxide (. NO) is used in biology as both an intercellular signaling agent and a cytotoxic agent. In signaling, submicromolar quantities of . NO stimulate the soluble isoform of guanylate cyclase (sGC) in the receptor cell. . NO increases the V-max of this heterodimeric hemoprotein up to 400-fold by interacting with the heme moiety of sGC to form a 5-coordinate complex. Carbon monoxide (CO) binds to the heme to form a 6-coordinate complex, but only activates the enzyme 5-fold. YC-1 is a recently discovered compound that relaxes vascular smooth muscle by stimulating sGC, Results: In the presence of YC-1, CO activates sGC to the same specific activity as attained with . NO. YC-1 did not affect the NO-stimulated activity. The on-rate (k(on)) and off-rate (k(off)) of CO for binding to sGC in the presence of YC-1 were determined by stopped-flow spectrophotometry. Neither the k(on) nor the k(off) varied from values previously obtained in the absence of YC-1, indicating that YC-1 has no effect on the affinity of CO for the heme, In the presence of YC-1, the visible spectrum of the sGC-CO complex has a Soret peak at 423 nm, indicating the complex is 6-coordinate. Conclusions: YC-1 has no effect on the affinity of CO for the heme of sGC. In the presence of YC-1, maximal activation of sGC by CO is achieved by formation of a 6-coordinate complex between CO and the heme indicating that cleavage of the Fe-His bond is not required for maximal activation of sGC.