|
Effects of lead exposure in vitro to δ-aminolevulinate dehydratase (δ-ALAD-1 EC 4.2.1.24) on optimal pH, sensitivity to heat denaturation, stability to storage and freeze-thawing, sulfhydryl nature and Michaelis-Menten constant (Km) and maximal velocity (Vmax) values were studied with the crude enzyme in the supernatant of liver homogenate obtained from ICR strain of mice. Optimal pH for the enzyme activity in the normal and the leaded groups was identical with a value of 6.5. Activity of the enzyme in the normal group steadily decreased as the temperature was raised, whereas in the leaded group, enzyme activity was enhanced at temperature range of 40℃~-50℃ with the highest activity at 45℃, and then decreased. The enzyme could be stored frozen at -30℃ in a deep freezer longer than 3 weeks without significant loss of the activity, but when it was stroed at 4℃ in a refrigerator the activity dropped steadily from the second day of the storage. The enzyme, however, was quite resistant to freeze-thawing. The enzyme was activated maximally by sulfhydryl reagent, β-mercaptoethanol, at a concentration of 100 mM and no activity was demonstrated without addition of β-mercaptoethanol, after DEAE cellulose column chromatography or with addition of iodoacetamide. The enzyme activity was inhibited markedly with addition of lead in vitro with the values of Michaelis-Menten constant (Km) 2.79×10^-4M and maximal velocity (Vmax) 2.33 unit/mg protein.
|