Tetrahydro-sepiapterin is an intermediate in tetrahydrobiopterin biosynthesis.

7,8-Dihydrobiopterin is not an intermediate in the de novo biosynthesis of tetrahydrobiopterin, the cofactor required for aromatic amino acid hydroxylations. However, N-acetyl-serotonin inhibition of sepiapterin reductase, an enzyme whose previously only known function was the reduction of sepiapterin to 7,8-dihydrobiopterin, completely inhibited biosynthesis of tetrahydrobiopterin by bovine adrenal medulla extracts. We have now shown that sepiapterin reductase catalyzes the reduction of tetrahydro-sepiapterin to tetrahydrobiopterin and that this reaction is N-acetyl-serotonin-sensitive. A new pathway for tetrahydrobiopterin biosynthesis is proposed which takes these observations into account and which involves tetrahydro intermediates.