FIGURE 5.

A: arginase and NOS compete for their common substrate l-arginine for the formation of their products. B: elevation of arginase activity by reducing the availability of l-arginine to NOS can reduce NO formation and uncouple NOS, causing it to produce more superoxide (O₂^.−), which rapidly reacts with NO to produce another potent oxidant, ONOO⁻. Both oxidants reduce levels of BH₄, which is also required for normal NOS coupling. Elevated l-ornithine levels allow greater production of polyamines and proline.