Modified erythrocyte uroporphyrinogen I synthase assay, and its clinical interpretation

Abstract

Assay of erythrocyte uroporphyrinogen I synthase is an accepted diagnostic test for acute intermittent porphyria, particularly in those individuals who are asymptomatic or in whom the disease is not biochemically manifested by excretion of excess porphyrin precursor. The assay described is based upon a coupled enzyme procedure in which added δ-aminolevulinic acid and its dehydratase present in erthrocytes are used to generate porphobilinogen as substrate for uroporphyrinogen synthase. Zinc and dithiothreitol are added with preincubation to give maximum activity and reproducibility. These agents also prevent inhibition by lead. Healthy young women had a mean activity of 40 nmol of porphyrin formed per milliliter of erthrocytes per hour, men an activity of 38 nmol/ml/h. Preparation of control specimens is described. This assay gave within-day CVs ranging from 1.9 to 2.8%. Precautions in interpretation of results are discussed.