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 retardation, results from a mutation in phenylalanine hydroxylase or an Enzyme required for biosynthesis or regeneration of its cofactor, tetrahydrobiopterin. The hyperphenylalaninemia variant primapterinuria is characterized by the excretion of 7-biopterin (primapterin). This disorder is thought to be 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 with primapterinuria are associated with either a single amino acid change or a premature stop codon. Only C81R has been successfully expressed in soluble form, and was found to have 40% of normal activity. Tissues which could be obtained by minimally invasive procedures were analyzed for dehydratase activity. None was detected in normal human white cells or fibroblasts. However, activity was found in intestine of rat, dog, pig, and particularly humans where it was only eight times lower than in liver. Distribution along the length and across the wall of small intestine was relatively uniform. Moreover, the dehydratases from human liver and intestinal mucosa have identical kinetic properties. A biopsy of duodenal mucosa from a patient with homozygous E96K dehydratase had activity of 55 nmol. min(-1)g(-1) mucosa compared 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 account for the biochemical symptoms of primapterinuria in this patient. However, accumulation of a 4a-hydroxy-tetrahydrobiopterin degradation product (a side-chain cyclic adduct), which has been observed in vitro and appears to be a dehydratase inhibitor, may further exacerbate the problem.