Clinical Chemistry, Vol 22, 1991-1994, Copyright © 1976 by American Association for Clinical Chemistry

A spot test for uroporphyrinogen I synthase, the enzyme that is deficient in intermittent acute porphyria

HM Schumaker, PV Tishler and DJ Knighton

We describe a spot test for detecting deficiency of uroporphyrinogen I synthase (EC 4.3.1.8), which is characteristic of intermittent acute porphyria. The specimens used for enzyme assay are 6.5-mm filter paper discs saturated with dried blood (less than 15 mul) that was collected by direct application from a fingerstick or from venipuncture, with or without anticoagulant. The enzyme in such specimens is stable for at least nine days at -20 or c degrees C or for two days at room temperature. The discs are incubated with porphobilinogen (0.11 mmol/liter) in tris(hydroxymethyl)aminomethane HCl buffer, pH 8.2, in the dark at 37 degrees C for 3.5 h. Trichloroacetic acid is added and, after centrifugation, the supernate is examined visually with a long- wavelength ultraviolet lamp. Samples from normal and porphyric subjects are readily differentiated, both by color and intensity of the resulting porphyrin fluorescence. Anemia is a potential source of falsely positive tests, but one may accurately determine the concentration of hemoglobin in the whole blood on the filter paper discs. Moreover, the fluorescence of normal but anemic samples clearly differs qualitatively from that of porphyric specimens. Another source of falsely positive tests, variation in enzyme activity creating an overlap zone of normal and porphyric results, has not been a confounding problem. The method thus seems to offer promise for screening populations for this disorder.