dihydrobiopterin

**Biochemistry** - BH4 is synthesized from GTP via GTP cyclohydrolase I (rate-limiting step), then 6-pyruvoyltetrahydropterin synthase (PTPS), then sepiapterin reductase. - During hydroxylation reactions, BH4 is oxidized to quinonoid dihydrobiopterin (qBH2), which is the substrate for DHPR. DHPR reduces qBH2 back to BH4 using NADH. Quinonoid BH2 can spontaneously rearrange to the stable 7,8-BH2, which can be reduced to BH4 by DHFR (salvage pathway). This pathway is NADPH-dependent. - BH4 is a cofactor for: - Phenylalanine hydroxylase (Phe → Tyr) - Tyrosine hydroxylase (Tyr → L-DOPA, the rate-limiting step in catecholamine synthesis) - Tryptophan hydroxylase (Trp → 5-HTP, which leads to serotonin and melatonin) - Nitric oxide synthase (Arg → NO + citrulline) - Alkylglycerol monooxygenase

**Clinical significance** - *BH4 deficiency* accounts for about 2% of hyperphenylalaninemia cases. Causes include GTP cyclohydrolase I deficiency, PTPS deficiency (the most common), and DHPR deficiency (which impairs BH2 → BH4 recycling). - Presentation: elevated phenylalanine, plus neurotransmitter deficiency (low dopamine, norepinephrine, and serotonin). This causes progressive neurologic deterioration despite dietary Phe restriction. Features include hypotonia, seizures, developmental delay, movement disorders, and microcephaly. - Diagnosis: newborn screen shows high Phe. Confirm with a urinary or blood pterin profile and a DHPR enzyme assay. CSF neurotransmitter metabolites (low HVA and 5-HIAA) are also useful. - Treatment: - Sapropterin (synthetic BH4) - Neurotransmitter precursors (L-DOPA/carbidopa and 5-HTP) - Dietary Phe restriction as needed - Folinic acid in DHPR deficiency (DHPR deficiency causes secondary cerebral folate deficiency) - Sapropterin is also used in BH4-responsive PKU, which is caused by PAH mutations with residual activity. BH4 acts as a chaperone and cofactor in these cases.

**Other associations** - *Dihydrofolate reductase (DHFR) inhibitors* such as methotrexate and trimethoprim can impair BH2 → BH4 salvage. This is mainly of theoretical or experimental importance. - *eNOS uncoupling:* oxidative stress oxidizes BH4 to BH2, and BH2 competes with BH4 for eNOS binding. eNOS then produces superoxide instead of NO, causing endothelial dysfunction. The BH4:BH2 ratio matters more than the absolute BH4 level. This is relevant in hypertension, diabetes, and atherosclerosis.

**High-yield pearls** - Hyperphenylalaninemia that persists despite a low-Phe diet, with neurologic decline, suggests BH4 deficiency ("malignant PKU"). - Classic PKU (PAH deficiency) is autosomal recessive and causes a musty/mousy odor, fair skin, eczema, intellectual disability, and tyrosine becoming essential. - BH4 deficiency is also autosomal recessive. - Phenylalanine hydroxylase needs BH4 and O2; DHPR needs NADH.