Primapterinuria: A New Variant of Atypical Phenylketonuria

Primapterin (7-iso-biopterin) is a new pterin metabolite found recently in the urine of a child with mild hyperphenylalaninaemia (Dhondt et al.,1988). Besides primapterin two other new 7-substituted pterins, namely 6-oxo-primapterin and anapterin (7-iso-neopterin), were also found in the patient’s urine, however in much lower concentrations (Curtius et al., 1988). In the meantime a second patient has been diagnosed with primapterinuria and transient hyperphenylalaninaemia (Blaskovics, personal communication). In both cases a tetrahydrobiopterin (BH4) loading test normalized plasma phenylalanine levels after 4 to 6 h. In both patients the cerebrospinal fluid (CSF) neurotransmitter metabolites, 5-hydroxyindoleacetic acid and homovanillic acid were in the normal range and the activities of all enzymes of the BH4 biosynthesis and regeneration (GTP cyclohydrolase I, 6-pyruvoyltetrahydropterin synthase, sepiapterin reductase and dihydropteridine reductase) were found to be normal. The neopterin to biopterin ratio of the patients’ urine was significantly increased. Following oral tetrahydrobiopterin administration (2 mg kg−1d−1) neopterin normalized and biopterin as well as primapterin increased about eight-fold in one of the patients. This finding suggests that primapterin is formed from biopterin by an isomerization reaction. However, the exact metabolic origin of primapterin and anapterin is still obscure.