Implications for isoform-selective inhibitor design derived from the binding mode of bulky isothioureas to the heme domain of endothelial nitric-oxide synthase

Nitric oxide produced by nitric-oxide synthase (NOS) is not only involved i n a wide range of physiological functions but also in a variety of patholog ical conditions, Isoform-selective NOS inhibitors are highly desirable to r egulate the NO production of one isoform beneficial to normal physiological functions from. the uncontrolled NO production of another isoform that acc ompanies certain pathological states. Crystal structures of the heme domain of the three NOS isoforms have revealed a very high degree of similarity i n the immediate vicinity of the heme active site illustrating the challenge of isoform-selective inhibitor design, Isothioureas are potent NOS inhibit ors, and the structures of the endothelial NOS heme domain complexed with i sothioureas bearing small S-alkyl substituents have been determined (Li, H, , Raman, C,S,, Martasek, P,, Kral, V., Masters, B.S.S., and Poulos, T.L. (2 000) J. Inorg. Biochem, 81, 133-139), In the present communication, the bin ding mode of larger bisisothioureas complexed to the endothelial NOS heme d omain has been determined, These structures afford a structural rationale f or the known inhibitory activities, In addition, these structures provide c lues on how to exploit the longer inhibitor substituents that extend out of the active site pocket for isoform-selective inhibitor design.