The Arginine-to-Ornithine Ratio: Biomarker of Arginase Activity and Predictor of Mortality in Sickle Cell Disease.

Sickle cell disease (SCD) is characterized by a state of nitric oxide resistance and limited bioavailability of L-arginine, the substrate for nitric oxide synthesis. While nitric oxide resistance occurs secondary to inactivation of nitric oxide by plasma hemoglobin released during intravascular hemolysis and by reactive oxygen species, mechanisms that limit L-arginine are not known. We hypothesized that increased arginase activity in patients with SCD would shift arginine metabolism away from nitric oxide production and towards ornithine metabolism, contributing to endothelial dysfunction and the development of pulmonary hypertension. Furthermore, since arginine and ornithine compete for the same transport system for cellular uptake, a decrease in the Arginine-to-Ornithine ratio resulting from increased arginase activity could also impair arginine bioavailability for nitric oxide production. Our goal was to evaluate associations between plasma arginase, arginine metabolism and pulmonary hypertension and prospective mortality in SCD. Plasma and erythrocyte arginase activity and amino acid levels were determined for patients with SCD and compared to ethnically matched control subjects. A diagnosis of pulmonary hypertension by Doppler-echocardiogram and prospective mortality were determined over 30 months of sequential patient enrollment. Plasma arginase activity was significantly elevated in patients with SCD compared to controls (2.2±2, n=140 vs. 0.4±2 μmol/ml/hr, n=45, p=0.007), trending higher in subjects with pulmonary hypertension. Plasma arginase activity correlated with the Arginine-to-Ornithine ratio (r=−0.33, p=0.0004), and lower ratios were associated with greater severity of pulmonary hypertension (1.1±0.4 vs. 0.8±0.4 vs. 0.6±0.3, controls vs. SCD without pulmonary hypertension vs. SCD with pulmonary hypertension, respectively, p=0.01) and independently associated with mortality (0.7±0.4 vs. 0.5±0.2, alive vs. dead, p=0.003; Risk Ratio = 4.9 [CI: 1.4, 17.1], p=0.002, for a low Arginine-to-Ornithine ratio; 13 deaths total). The mortality risk ratio increased to 7.0 ([CI: 1.6, 31.6], p=0.01), when the Arginine-to-Ornithine ratio was adjusted for creatinine, likely reflecting the impact of renal disease. Plasma arginase activity correlated with markers of increased hemolytic rate, including LDH (r=0.44, p<0.001), AST (r=0.39, p<0.002), reticulocyte count (r=0.25, p<0.001), and Hct (r= −0.25, p<0.001), and was higher in erythrocytes of SCD patients compared to controls (37.7±2.9, n=16 vs 23.5±1.7 nmol/mg/min, n=45, p<0.0001), consistent with hemolytic release of erythrocyte arginase. These data support a novel mechanism of disease whereby hemolysis not only liberates vasoactive hemoglobin but also releases erythrocyte arginase, which contributes to impaired nitric oxide bioavailability, endothelial dysfunction, pulmonary hypertension and death. The Arginine-to-Ornithine ratio, a reflection of arginase activity, may represent a useful biomarker of disease severity and risk of death in patients with SCD.