Arginine Metabolite Profiling in Sickle Cell Disease: Abnormal Levels and Correlations with Pulmonary Hypertension, Desaturation, Hemolysis and Organ Dysfunction.

Pulmonary arterial hypertension (PAH) in patients with sickle cell disease (SCD) is linked to intravascular hemolysis, renal dysfunction, systolic hypertension, cholestasis, and early mortality. Although the pathophysiology of PAH in SCD is multifactorial, one important and fundamental factor is impaired nitric oxide bioavailability. Severe intravascular hemolysis releases hemoglobin and arginase into blood plasma, leading to consumption of nitric oxide and its plasma precursor L-arginine, the obligate substrate for the nitric oxide synthases (NOS). In order to explore other potential alterations in the arginine pathway that might affect arginine bioavailability and nitric oxide production, we used high-performance liquid chromatography-tandem mass spectrometry to determine the plasma concentrations for several key metabolites that may affect NOS activity or arginine transport: asymmetric dimethylarginine (ADMA), symmetric dimethylarginine (SDMA), N-monomethyl-L-arginine (MMA), and N-ω-hydroxy-L-arginine (NOHA). Plasma levels of ADMA, SDMA and MMA are significantly higher in all forms of SCD than in healthy African American control subjects (Table 1). NOHA, the intermediate species in nitric oxide synthesis from L-arginine, is significantly lower in sickle-β-thalassemia (Sβ-thal) patients and homozygous SCD (SS). L-arginine levels are significantly lower in all forms of SCD, as previously reported.

PAH as assessed by echocardiography screening was correlated to SDMA (r=0.30, p<0.0001) and NOHA (r=0.23, p=0.002). Similar correlations were observed to NT-proBNP, another marker of PAH. Low oxygen saturations were linked to high levels of all four arginine metabolites. ADMA levels were elevated with severe hemolysis, and unexpectedly lower with renal dysfunction. Levels of SDMA and NOHA were significantly related to renal dysfunction (p<0.01), with an additional link of NOHA to systemic hypertension (p<0.001). In addition, Cox proportional hazard analysis showed a relationship of the arginine/SDMA ratio to early mortality (p<0.001).

In summary, levels of the endogenous NOS inhibitor ADMA are highly elevated in SCD and linked to hemolysis, and may contribute to hemolysis-associated endothelial dysfunction. The levels of SDMA, a competitive inhibitor of arginine transport and intracellular bioavailability, are also elevated and linked to PAH, desaturation, renal dysfunction and early mortality risk. The low levels of arginine and NOHA in SCD are consistent with low substrate availability for NOS, and may also limit NO production. The role of arginine metabolites in dysregulation of the arginine-nitric oxide axis and pulmonary hypertension in SCD merits further investigation.