In order to make further progress in elucidating the mechanism of NOS catalysis it will be essential to throw light on the interaction between the enzyme and its substrate. An understanding of the catalytic site will also assist the development of therapeutically important NOS inhibitors. In particular. it will be useful to uncover any differences that exist between the substrate binding sites of the three NOS isozymes which might be exploited for the development of isoform selective NOS inhibitors. A comparison of NOS to other Arg-binding proteins has shown no significant sequence homology...