Modulation of L-Arginine metabolism and bioavailability by free plasma heme

Severe falciparum malaria is associated with high levels of free heme [Fe(III)-protoporphyrin IX, FP], hypoargininemia and low nitric oxide (NO). Hypoargininemia can be partially explained by the increased level of plasma arginase due to the disease-induced hemolysis, but its precise cause is unknown. The aim of this work was to investigate whether FP can affect the membrane transport of arginine (ARG) and/or the activity of plasma and red blood cell (RBC) arginase, therefore limiting ARG bioavailability for NO production. Design and Methods. ARG transport was evaluated in human RBC by measurement of influx of radiolabeled ARG and resolved into the saturable transport components y+ and y+L by selective inhibition of the two systems with 2 mM N-ethylmaleimide or 1 mM leucine. respectively. Arginase activity was evaluated after pre-treatment of RBC or cell-free extract with FP in different experimental conditions and expressed as mol urea produced/gr Hb. Results. FP impairs the influx of ARG into RBC in a dose dependent manner with a more pronounced effect on the transport system y+L. FP-treated RBC show an enhancement of the arginase activity. Experiments performed in different experimental conditions have shown that arginase activation is not simply related to oxidative modifications induced by the porphyrin Conclusions: impairment of ARG influx and activation of arginase could limit ARG availability for NO production by RBC NOS. Additionally, release of a more active arginase by damaged RBC during intravascular hemolysis could worsen hypoargininemia, reducing ARG availability for NO production also in endothelial cells.