PTERIDINE BIOSYNTHESIS

National Institute Of Mental Health

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Abstract

Tetrahydrobiopterin is a required cofactor for the three aromatic amino acid hydroxylases that hydroxylate phenylalanine, tyrosine and tryptophan. Tissue levels of tetrahydrobiopterin are regulated mainly by the activity of the first enzyme on the de novo biosynthetic pathway, GTP cyclohydrolase I (GTPCH). Tetrahydrobiopterin is also required by the three isoforms of nitric oxide synthase, inducible, constitutive, and endothelial, and limits production of nitric oxide, endothelial derived relaxing factor, by macrophages, neurons, and endothelial cells, respectively. Availability of intracellular tetrahydrobiopterin likely plays a role in controlling nitric oxide-dependent vascular tone under normal and pathological conditions.Sphingolipid metabolites, ceramide, sphingosine, and sphingsosine-1-phosphate, play important signaling roles in many biological processes, including cell growth, death, and differentiation. We have now found that although ceramide plays a role in TNF-alpha induction of nitric oxide synthase and nitric oxide production in glial cells, TNF-alpha stimulates GTP cyclohydrolase activity and tetrahydrobiopterin synthesis by a ceramide independent mechanism. Furthermore, sphingosine kinase/sphingosine-1-phosphate signaling regulates GTPCH expression and tetrahydrobiopterin synthesis. This is the first indication that the pathways of NO and tetrahydrobiopterin synthesis may be regulated via divergent signaling pathways involving different sphingolipid metabolites. Overexpression of type 1 sphingosine kinase, the enzyme that catalyzes the conversion of sphingosine to sphingosine-1-phosphate, protected PC12 neuronal cells from apoptosis induced by trophic factor withdrawal by inhibiting activation of caspases and of the MAP kinases, JNK and p38.