| Abstract
| - Several α-amino acids bearing a CNOH function separated from the Cα carbon by two tofive 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,Nω-hydroxy-l-arginine (NOHA) and its analogues homo-NOHA, nor-NOHA, and dinor-NOHA, twoamidoximes bearing the −NH−C(CH3)NOH group, and two amidoximes bearing the −CH2−C(NH2)NOH group. Their behavior toward NOS and RLA was compared to that of the corresponding compoundsbearing a CNH function instead of the CNOH function. The results obtained clearly show that efficientrecognition of these α-amino acids by NOS and RLA involves very different structural determinants.NOS favors molecules bearing a −NH−C(R)NH motif separated from Cα by three or four CH2 groups,such as arginine itself, with the necessary presence of δ-NH and ω-NH groups and a more variable Rsubstituent. The corresponding molecules with a CNOH function exhibit a much lower affinity forNOS. On the contrary, RLA best recognizes molecules bearing a CNOH function separated from Cαby three or four atoms, the highest affinity being observed in the case of three atoms. The presence oftwo ω-nitrogen atoms is important for efficient recognition, as in the two best RLA inhibitors,Nω-hydroxynorarginine and Nω-hydroxynorindospicine, which exhibit IC50 values at the micromolar level.However, contrary to what was observed in the case of NOS, the presence of a δ-NH group is not important.These different structural requirements of NOS and RLA may be directly linked to the position of crucialresidues 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 δ-NH and ω-NH2groups with glutamate 371 (of NOS II), whereas binding of CNOH molecules to RLA is mainly basedon interactions of their terminal OH group with the binuclear Mn(II)···Mn(II) cluster of the enzyme andon possible additional bonds between their ω-NH2 group with histidine 141, glutamate 277, and oneMn(II) ion. The different modes of interaction displayed by both enzymes depend on their different catalyticfunctions and give interesting opportunities to design useful molecules to selectively regulate NOS andarginase.
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