Red cell uroporphyrinogen I synthetase in acute intermittent porphyria

Uroporphyrinogen I synthetase (URO-S) activity in red cells was measured in 49 patients with acute intermittent porphyria (AIP), in their relatives, in 16 patients with variegate porphyria, and in patients with various forms of anaemia. URO-S activity was clearly lower in patients with AIP (mean 30.5 U, SD 9.7) than in controls (mean 49.5 U, SD 6.4) and in patients with variegate porphyria. There was an overlapping of the values of controls and those of AIP patients, seven patients having fully normal values. Out of 63 relatives eight prepubertal children and two adults with normal urine analysis had URO-S activity below normal. Two newborn infants out of the five studied who had a prophyric parent had lowered URO-S activity in cord blood. URO-S activity was usually elevated in anaemias and correlated to the reticulocyte count. It is concluded that measurement of URO-S activity in red cells is a valuable supplementary method in searching for latent cases of AIP. It is the only method that can disclose the disease before puberty and even neonatally. The major limitation is the occurrence of normal values in some patients with AIP.