Combined use of nonmyelosuppressive nitrosourea analogues with hydroxyurea in the induction of F-cell production in a human erythroleukemic cell line

In vitro evidence of the inhibitory capacity of chloroquine on arginase activity in sickle erythrocytes

Increased levels of erythrocyte arginase activity are believed to play a key role in the pathogenesis of sickle cell disease (SCD). Because increased arginase activity has been implicated in the exacerbation of pulmonary hypertension in SCD, this enzyme is considered an important therapeutic target for identifying new drugs to treat and manage SCD and other inflammatory disorders. Although chloroquine (CQ) is prescribed as an anti-malarial and anti-rheumatoid drug, the mechanism of its anti-inflammatory activity is largely unknown. The present study found that CQ inhibited arginase in a dose-dependent manner, and also displayed a linear competitive inhibition on sickle erythrocyte arginase. The apparent K(M) values in the presence of inhibitor were considerably reduced at both physiological and slightly acidic pHs. Slope replots of the double reciprocal plots at pH 6.8 and 7.4 also indicated simple competitive inhibitory mechanism of CQ, and Ki values for CQ were within micromolar levels. To our knowledge, this is the first example of an anti-malarial and anti-inflammatory agent displaying competitive inhibition kinetics on arginase. The outcome of this study may provide a template for the rational design of specific agents either alone or in combination with CQ for the inhibition of arginase activity.

Modulation of erythrocyte arginase activity in sickle cell disease patients during hydroxyurea therapy

An elevated erythrocyte arginase activity with a corresponding decrease in nitric oxide (NO) level has been implicated in the pathophysiology of sickle cell disease (SCD). Recent studies have shown that hydroxyurea (HU) increases the production of NO, which increases the soluble guanylate cyclase activity and fetal haemoglobin (HbF) synthesis. To study the effects of HU on the arginase and nitric oxide synthase (NOS) activities in SCD patients, we compared levels of arginase activity and NO metabolites in red blood cells and plasma, respectively, from 23 patients with SCD (HbSS) receiving HU therapy, with those of 12 SCD patients not receiving HU treatment. Patients on HU therapy showed significantly lower arginase activity than that of HbSS patients not on HU therapy (1.36+/-0.2 U/10(8) cells vs. 3.31+/-0.29 U/10(8) cells). NOS activity was higher in patients on HU therapy than in untreated patients (0.72+/-0.4 nmol/ml/min vs. 0.35+/-0.15 nmol/ml/min, P<0.05). Among the HU-treated patients, the decreased level of arginase activity correlated (r=0.71) with HbF level as well as the mean corpuscular haemoglobin content. These data suggest that one of the beneficial effects of HU in vivo may involve the regulation of arginase activity and a concomitant induction of NOS activity, which may lead to an increased production of NO. The outcome of this study may lead to the development of improved NO-based treatments for SCD.