Abstract. Hepatic conversion of porphobilinogen to porphyrins was less than 50% of control levels in human subjects with the genetic disease, intermittent acute porphyria. This relative block in heme biosynthesis may be relevant to a concomitant 6-to 10-fold elevation in 5-aminolevulinic acid synthetase activity, since this first and rate-controlling enzyme in the biosynthetic pathway is subject to negative feedback regulation by the end product, heme. A micro-radiochemical assay of 6-aminolevulinic acid synthetase, and some of its applications, are described.Intermittent acute porphyria (IAP) is a rare genetic disease characterized by an acute neurological syndrome often precipitated by therapeutic dosages of drugs of diverse structure. There is an increased urinary excretion of porphyrin precursors. In the two other types of genetically transmitted hepatic porphyrias, variegate porphyria and hereditary coproporphyria, the neurological syndrome is identical but each type has a distinct pattern of porphyrin and precursor excretion.' Another form of hepatic porphyria, porphyria cutanea tarda, often appears to be acquired. Patients with this disorder do not display neurological abnormalities. They primarily excrete excessive quantities of uroporphyrin but normal amounts of porphyrin precursors. Because increased activity of hepatic -aminolevulinic acid synthetase (ALA-S) has been observed in IAP,2,3 and IAP is transmitted as an autosomal dominant, a defect in genetic regulation, possibly caused by an operator constitutive mutation, has been con-
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