1. The activities of the enzymes of haem biosynthesis were studied in 23 patients with acute intermittent porphyria. The mitochondrial enzymes δ-aminolaevulinate synthase, coproporphyrinogen oxidase and ferrochelatase were measured in leucocytes and the cytosolic enzymes δ-aminolaevulinate dehydratase, porphobilinogen deaminase and uroporphyrinogen decarboxylase in erythrocytes.

2. Leucocyte δ-aminolaevulinate synthase activity was elevated (P < 0·001), with marked diminution of porphobilinogen deaminase activity (P < 0·001) and reduction in the activities of δ-aminolaevulinate dehydratase (P < 0·01) and uroporphyrinogen decarboxylase (P < 0·005).

3. A therapeutic regimen based on intravenous laevulose infusion was studied. In four patients in acute attack and one in remission laevulose treatment was associated with a fall in δ-aminolaevulinate synthase activity, a rise in porphobilinogen deaminase and uroporphyrinogen decarboxylase activities, and a fall in urinary porphyrin precursor excretion (P < 0·001). These studies provide a basis for the evaluation of the use of sugars in acute intermittent porphyria.

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