Temporal expression of different pathways of 1-arginine metabolism in healing wounds

Arginine can be metabolized by inflammatory cells through at least two pathways. One is an oxidative l-arginine deiminase (OAD) that results in the formation of citrulline and reactive nitrogen intermediates. The other is arginase, which determines the production of ornithine and urea. The temporal expression of these pathways in an experimental wound model (s.c. implanted polyvinyl alcohol sponges in the rat) was investigated by examining the concentrations of amino acids and of nitrite in fluids obtained from the sponges 6 h to 15 day after implantation. These analyses revealed two distinct periods during which the arginine concentration in the fluids was markedly below plasma levels. During the early period (less than 3 days after sponge implantation) wound fluid contained more citrulline and nitrite than at any other time, suggesting OAD activity. In contrast, ornithine accumulated in the fluids during the late decrease in arginine concentration that extended beyond day 3, during which time the wound fluid also contained a high arginase activity. This time-dependent expression of different pathways of arginine metabolism in wounds was confirmed in sponge cultures containing [guanido-14C]-l-arginine. Cells contained in sponges harvested less than 48 h after implantation metabolized labeled arginine mainly to labeled citrulline, whereas labeled urea was produced during culture of sponges harvested after this time. The low arginine content of wound fluid did not appear to be rate limiting for the expression of OAD in late sponges because no OAD activity was evidenced when 4 mM arginine was added to the cultures. These results indicate that the OAD pathway is expressed in this model predominantly during the early, polymorphonuclear leukocyte-predominant, phase of repair. At this time, the reactive nitrogen intermediates resulting from the metabolism of arginine may mediate some of the events characteristic of early inflammation, including microbiostasis, vasodilation, and inhibition/reversal of platelet aggregation. In turn, the late suppression of this pathway and the catabolism of arginine through arginase may promote macrophage function within wounds.