Extracellular human PMN arginase specifically depletes arginine under physiologic conditions. (A) RPMI 1640 cell culture medium containing 150 μM l-arginine (Arg(+) medium) was incubated for 12 hours at 37°C either alone or with an aliquot of a sonicate of human PMNs (PMN-S) of a defined arginase activity (300 mU/mL). Alternatively, human recombinant arginase I (final activity also 300 mU/mL) was used instead of PMN-S. When indicated, arginase activity was blocked with nor-NOHA (1 mM) during the incubation. The concentrations of arginine and ornithine were determined by ion exchange chromatography at the end of the incubation period. Representative data from 1 of 3 independent experiments are shown. (B) Experimental set-up was as in panel A. To allow for better comparison between experiments, the concentration of each amino acid in Arg(+) medium was used as control and set as 100%. The relative concentration of each amino acid (in percentage of control) after incubation with PMN-S (with or without nor-NOHA inhibition) is shown. Data are demonstrated as mean ± SD from 3 independent experiments.