Agmatine: at the crossroads of the arginine pathways.

In acute inflammatory responses, such as wound healing and glomerulonephritis, arginine is the precursor for production of the cytostatic molecule nitric oxide (NO) and the pro-proliferative polyamines. NO is an early phase response whereas increased generation of polyamines is requisite for the later, repair phase response. The temporal switch of arginine as a substrate for the inducible nitric oxide synthase (iNOS)/NO axis to arginase/ornithine decarboxylase (ODC)/polyamine axis is subject to regulation by inflammatory cytokines as well as interregulation by the arginine metabolites themselves. Herein we describe the capacity of another arginine pathway, the metabolism of arginine to agmatine by arginine decarboxylase (ADC), to aid in this interregulation. Agmatine is an antiproliferative molecule due to its suppressive effects on intracellular polyamine levels, whereas the aldehyde metabolite of agmatine is a potent inhibitor of iNOS. We propose that the catabolism of agmatine to its aldehyde metabolite may act as a gating mechanism at the transition from the iNOS/NO axis to the arginase/ODC/polyamine axis. Thus, agmatine has the potential to serve in the coordination of the early and repair phase pathways of arginine in inflammation.