Recognition of alpha-amino acids bearing various C=NOH functions by nitric oxide synthase and arginase involves very different structural determinants.
Several alpha-amino acids bearing a C=NOH function separated from the Calpha carbon by two to five atoms have been synthesized and tested as substrates or inhibitors of recombinant nitric oxide synthases (NOS) I and II and as inhibitors of rat liver arginase (RLA). These include four N-hydroxyguanidines, N(omega)-hydroxy-L-arginine (NOHA) and its analogues homo-NOHA, nor-NOHA, and dinor-NOHA, two amidoximes bearing the -NH-C(CH(3))=NOH group, and two amidoximes bearing the -CH(2)-C(NH(2))=NOH group. Their behavior toward NOS and RLA was compared to that of the corresponding compounds bearing a C=NH function instead of the C=NOH function. The results obtained clearly show that efficient recognition of these alpha-amino acids by NOS and RLA involves very different structural determinants. NOS favors molecules bearing a -NH-C(R)=NH motif separated from Calpha by three or four CH(2) groups, such as arginine itself, with the necessary presence of delta-NH and omega-NH groups and a more variable R substituent. The corresponding molecules with a C=NOH function exhibit a much lower affinity for NOS. On the contrary, RLA best recognizes molecules bearing a C=NOH function separated from Calpha by three or four atoms, the highest affinity being observed in the case of three atoms. The presence of two omega-nitrogen atoms is important for efficient recognition, as in the two best RLA inhibitors, N(omega)-hydroxynorarginine and N(omega)-hydroxynorindospicine, which exhibit IC(50) values at the micromolar level. However, contrary to what was observed in the case of NOS, the presence of a delta-NH group is not important. These different structural requirements of NOS and RLA may be directly linked to the position of crucial residues that have been identified from crystallographic data in the active sites of both enzymes. Thus, binding of arginine analogues to NOS particularly relies on strong interactions of their delta-NH and omega-NH(2) groups with glutamate 371 (of NOS II), whereas binding of C=NOH molecules to RLA is mainly based on interactions of their terminal OH group with the binuclear Mn(II).Mn(II) cluster of the enzyme and on possible additional bonds between their omega-NH(2) group with histidine 141, glutamate 277, and one Mn(II) ion. The different modes of interaction displayed by both enzymes depend on their different catalytic functions and give interesting opportunities to design useful molecules to selectively regulate NOS and arginase.