Subcellular fractionation of resting human PMNs demonstrates that arginase I is present in azurophil granules. (A) Equal amounts of postnuclear extract (E), soluble (S), and membrane (M) proteins from resting human PMNs were run on SDS-PAGE and analyzed by immunoblotting for arginase I. Human liver protein extract (L) was used as a positive control. (B) Postnuclear fractions from resting neutrophils were subjected to subcellular fractionation and analyzed for specific markers for the distinct subcellular organelles, which are plotted normalized to the fraction with maximal activity. The following markers were assayed: cytosol, lactate dehydrogenase (⋄; CYT); plasma membrane, HLA (▴; PM); tertiary granules, gelatinase (▵; TG); specific granules, Lf (•; SG); azurophil granules, MPO (□; AG). (C) Proteins from subcellular fractions 1 to 8 of resting human neutrophils were assayed for arginase I protein by immunoblotting and in parallel for enzymatic activity. All data shown are representative of 4 separate experiments.