Hyperphenylalaninemia and 7-pterin excretion associated with mutations in 4a-hydroxy-tetrahydrobiopterin dehydratase/DCoH: Analysis of enzyme activity in intestinal biopsies

Hyperphenylalaninemia, which can cause neurological disorders and mental re tardation, results from a mutation in phenylalanine hydroxylase or an enzym e required for biosynthesis or regeneration of its cofactor, tetrahydrobiop terin. The hyperphenylalaninemia variant primapterinuria is characterized b y the excretion of 7-biopterin (primapterin). This disorder is thought to b e due to a deficiency of 4a-hydroxy-tetrahydrobiopterin dehydratase (pterin -4a-carbinolamine dehydratase), but a lack of tissue activity has not been directly demonstrated. The five mutations so far recognized in patients wit h primapterinuria are associated with either a single amino acid change or a premature stop codon. Only C81R has been successfully expressed in solubl e form, and was found to have 40% of normal activity. Tissues which could b e obtained by minimally invasive procedures were analyzed for dehydratase a ctivity. None was detected in normal human white cells or fibroblasts. Howe ver, activity was found in intestine of rat, dog, pig, and particularly hum ans where it was only eight times lower than in liver. Distribution along t he length and across the wall of small intestine was relatively uniform. Mo reover, the dehydratases from human liver and intestinal mucosa have identi cal kinetic properties. A biopsy of duodenal mucosa from a patient with hom ozygous E96K dehydratase had activity of 55 nmol.min(-1)g(-1) mucosa compar ed to 329 +/- 32 nmol min(-1)g(-1) tissue in controls (n = 12). The sixfold lower tissue activity of the E96K mutant alone may not be sufficient to ac count for the biochemical symptoms of primapterinuria in this patient. Howe ver, accumulation of a 4a-hydroxy-tetrahydrobiopterin degradation product ( ai side-chain cyclic adduct), which has been observed in vitro and appears to be a dehydratase inhibitor, may further exacerbate the problem, (C) 2000 Academic Press.