Defective Uroporphyrinogen I Synthase in Acute Intermittent Porphyria

Acute intermittent porphyria (AIP) is linked to a genetic defect in uroporphyrinogen I synthase. In pursuit of the question whether this disturbance of heme synthesis is due to an abnormal quality or quantity of this enzyme, we purified uroporphyrinogen I synthase, separated into isoenzymes and tested the stability of them in five porphyric patients; three AIP and two porphyria cutanea tarda (PCT). Six isoenzymes were found in all these porphyric patients as well as in normals. No lack of the isoenzymes was found in AIP. In normals and in PCT, the isoenzymes with the highest percent activity were found to be the most labile ones, but this instability was even more pronounced in AIP patients. This suggests that the enzymatic defect in AIP is more likely a qualitative than a quantitative one, suggesting a structural gene defect.