Enzymatic Isomerization of Sepiapterin to 6-1'-Hydroxy-2' -oxopropyl 7,8-dihydropterin

Abstract

6-1'-Hydroxy-2' -oxopropyl 7,8-dihydropterin, which has been believed to be too labile to actually exist, was produced from sepiapterin (6-lactoyl 7,8-dihydropterin). It was formed by the isomerase activity of sepiapterin reductase, which was recen tly observed in the isomerization of 6-lactoyl 5,6,7,8-tetrahydropterin (15), an intermediate of tetrahydrobiopterin biosynthesis. This product from sepiapterin was detectable electrochemically at 500 mV and fluorometrically on HPLC analysis. The product was converted into 7,8-dihydrobioptcrin (6-1 ' ,2'-dihydroxypropyl-7,8-dihydropterin) if it was further incubated with sepiapterin reductase and NADPH. The structure of the product was identified by 'H-NMR analysis in D2O. The pH optimum of the isomerization of sepiapterin was 8.6 and the UV-absorption spectrum of the product was quite similar to that of 7,8-dihydrobiopterin. The finding of isomerization of sepiapterin, besides that of 6-lactoyl tetrahydropterin. to the C2' -keto isomer strongly supports the real existence of isomerase activity in the sepiapterin reductase molecule.