Discovery of a new tetrahydrobiopterin-synthesizing enzyme in the lemon mutant of the silkworm Bombyx mori

A new tetrahydrobiopterin-synthesizing enzyme, which is different from sepiapterin reductase (EC 1.1.1.153), was discovered in the integument of the lemon mutant of the silkworm Bombyx mori. This enzyme converted 6-pyruvoyltetrahydropterin to tetrahydrobiopterin, an essential cofactor in the hydroxylation of aromatic amino acids, in the presence of NADPH. The reaction proceeded via 6-lactoyltetrahydropterin and 1'-hydroxy-2'-oxopropyltetrahydropterin as intermediates. The molecular mass of this enzyme was estimated to be 40 kDa. N-Acetylserotonin, a potent inhibitor of sepiapterin reductase, slightly inhibited the enzymatic reaction. In the presence of 0.5 mM N-acetylserotonin, the formation of tetrahydrobiopterin by sepiapterin reductase purified from the normal strain silkworm was completely inhibited. However, the formation of tetrahydrobiopterin by the enzyme purified from the lemon mutant was inhibited by only about 50%. These results suggest an alternative biosynthetic pathway to tetrahydrobiopterin.