Inhibition of erythrocyte aminolevulinate dehydratase by a 56.2-kD peptidefrom uremic plasma

Among the abnormalities in erythrocyte porphyrin metabolism already describ ed in patients with chronic renal failure on hemodialysis, a decrease in bl ood aminolevulinate dehydratase activity has been reported, suggesting the presence in uremic plasma of an inhibitor of the enzyme. The aim of this wo rk has been to isolate and characterize such an inhibitor. Blood samples fr om 105 patients with chronic uremia were collected; plasma was applied to S ephadex G-100 columns and the fraction with the highest inhibiting capacity was identified and purified by subsequent SDS-polyacrylamide gel electroph oresis, followed by electroelution and electroblotting. It was demonstrated that the factor present in plasma of uremic patients inhibited blood amino levulinate dehydratase in a concentration-dependent manner; its inhibitory properties were abolished after heat, trypsin and TCA treatment indicating its peptidic nature. The purified inhibitor has an apparent molecular mass of 56.2 kD, it inhibits blood aminolevulinate dehydratase in a competitive way and the K-i value is 12 x 10(-6) M. The amino acid composition of the i nhibitor has been determined and it has been found that its N-terminal amin o acid is blocked. The isolated peptide may play a role in heme biosynthesi s disturbances and in the pathogenesis of uremic anemia.