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6-Pyruvoyl-tetrahydropterin synthase, which catalyzes the first step in the conversion of 7,s-dihydroneopterin triphosphate to tetrahydrobiopterin, was purified approximately 140,000-fold to apparent homogeneity from human liver. The molecular mass of the enzyme is estimated to be 83 kDa. 7,X-Dihydroneopterin triphosphate was a substrate of the enzyme in the presence of Mg2+, and the pH optimum of the reaction was 7.5 in Tris HCI buffer. The K, value for 7,fLdihydroneopterin triphosphate was 10 pM. The product of this enzymatic reaction was the presumed intermediate 6-pyruvoyl-tetrahydropterin. This latter compound was converted to tetrahydrobiopterin in the presence of NADPH and partially purified sepiapterin reductase from human liver. The conditions and the effect of N-acetylserotonin on this reaction, and on the formation of the intermediates 6-(1 '-hydroxy-2'-oxopropyl)-tetrahydropterin and 6-( 1'0x0-2'-hydroxypropy1)-tetrahydropterin have been studied.

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