A Microassay for Uroporphyrinogen I Synthase, One of Three Abnormal Enzyme Activities in Acute Intermittent Porphyria, and its Application to the Study of the Genetics of this Disease

A new spectrofluorometric assay is described for quantitating uroporphyrinogen I synthase (EC 4.3.1.8) activity in volumes of human blood as small as 2 μl. By this sensitive assay the inheritance of the enzyme's activity has been studied and the genetic defect for acute intermittent porphyria has been confirmed to be autosomal dominant in nature. There is a 3-fold range of uroporphyrinogen I synthase activity in erythrocytes in the normal population, with a meanV max ± SD of 35.7 ± 8.4 nmol of uroporphyrinogen I formed per ml of erythrocytes per hr, at 37°. One-half this level of enzyme activity (18.0 ± 5.0) is found in erythrocytes from patients with clinically manifest acute intermittent porphyria; and in erythrocytes from those of their relatives, including prepubertal children, who have the latent gene defect for the disease. TheKm of erythrocyte enzyme of normal people is 12.3 ± 3.9 μM, whereas theKm of the erythrocyte enzyme of patients with acute intermittent porphyria is 6.2 ± 3.9 μM, as determined on whole blood lysates. Three enzymic changes have now been identified in patients with acute intermittent porphyria; a high level of δ-aminolevulinate synthase activity; a low level of uroporphyrinogen I synthase activity; and a deficiency of steroid Δ4 -5α reductase activity.