Hyperphenylalaninaemia due to impaired dihydrobiopterin biosynthesis: leukocyte function and effect of tetrahydrobiopterin therapy.

We have described the clinical and biochemical status of two patients with tetrahydrobiopterin (BH4) deficiency due to impaired dihydrobiopterin biosynthesis. BH4 administration appeared to improve the mental and psychological status more than did neurotransmitter replacement therapy alone. This enhancement of activities of daily life was seen with a dose of BH4 as low as 1.25 mg kg-1 day-1. Granulocyte adherence capacity was below normal and recovered after BH4 therapy in both patients. B-cell differentiation capacity was altered either before or after therapy.