Factors Influencing the pH-Activity Relationship of -Aminolevulinic Acid Dehydratase From Human Blood, and Their Relevance to Blood Lead Assay

Abstract

A systematic survey of factors affecting the pH-activity profile of δ-aminolevulinic acid dehydratase (ALA-D) in human blood hemolysate has been carried out, particularly in relation to the methodology for assessing lead poisoning. These factors include the variation in the buffer, hemolysate preparation, the addition of potassium ions, dithiothreitol, and the level of lead poisoning. It has been found that all of these factors give rise to serious changes in the pH of maximum enzyme activity, and that in fact, the alleged enzyme inactivation Induced by lead (at levels up to 51 µg %) is not enzyme inactivation, but a change in the shape of the pH-activity profile and a shift of pH of maximum activity to a more acidic value. Lead actually appears to act as a preservative. Because of the changes in the pH-activity profile, it is possible to vary the sensitivity of the assay method for lead at will by appropriate choice of the pH used for the assay. A tentative scheme to rationalize the results obtained is proposed.