Temperature dependence of activity and inhibition by lead with 5-aminolevulinic acid dehydratase

5-Aminolevulinic acid dehydratase (EC 4.2.1.24) was purified 230-fold from human erythrocytes. The effects of temperature on various properties of the isolated enzyme were studied. The maximal activity is observed at 60 degrees C. The pH optimum is shifted from pH 7.3 at 5 degrees C to pH 6.6 at 55 degrees C. The Michaelis constant increases 5-fold from 5 to 55 degrees C. Inhibition by lead is more pronounced at elevated temperatures. This effect is in contradiction to the restoration of enzyme activity by heating hemolysate after inhibition by lead in vivo, which has been described in previous reports.