We have found a new ability of sepiapterin reductase, which has been known to show a strict substrate specificity for the 6-lactyl sidechain of sepiapterin to produce 6-dihydroxypropyl sidechain of dihydrobiopterin in the biosynthesis of tetrahydrobiopterin, to reduce many dicarbonyl compounds with NADPH as effectively utilized substrates. By analysis of diacetyl reduction by purified sepiapterin reductase, it was observed that both of the carbonyl groups of the compound are finally sequentially reduced by the enzyme with NADPH to hydroxyl groups. And we expect that this enzyme may reduce "Compound X", which is an intermediate of tetrahydrobiopterin synthesis and would be a dicarbonyl derivative of pteridine (Tanaka et. al., 1980), to dihydrobiopterin via sepiapterin.