Peg-Arg I depletes L-Arginine in vitro and in vivo and impairs malignant T-cell proliferation. (A) RPMI medium was treated with peg-Arg I (0.5-4 IU/mL) or PBS and the levels of L-Arginine measured by HPLC after 15 and 30 minutes. (B) Mice were injected with PBS (Control) or peg-Arg I (0.1-1 mg/mouse) and the levels of L-Arginine in the serum were measured every day by HPLC. Five mice were used for each condition. (C) Malignant T-cell lines (1 × 10⁵) were cultured for 72 hours in the presence or the absence of peg-Arg I (2 IU/mL), or the absence of L-Arginine (No-L-Arg), and proliferation determined by MTT. (D) Malignant T-cell lines (1 × 10⁶) were cultured for 24 hours in medium with or without peg-Arg I, after which the cell-cycle progression was determined by propidium iodide. Results are expressed as the percentage of cells in the G₀-G₁ phase of the cell cycle. Experiments were repeated at least 3 times obtaining similar results.