BH4 [3] to control the regulation of the synthesis of monoamines in nerve tissue, metabolism of phenylalanine in the liver, or formation of nitrogen oxide in macrophages. Thus understanding of the mechanisms of reaction and regulation

INTRODUCTION As well as folate (a conjugated pteridine) in plants and microorganisms, tetrahydrobiopterin (BH4, an unconjugated pteridine) in animals is also biosynthesized from GTP. So far three essential enzymes have been identified as being active in BH4 synthesis[1,2], while only the initial enzyme of the pathway is common with that of the folate pathway (Fig. 1). Since the cell level of BH4 in humans is kept relatively low, rapid modulation of the activity of BH4requiring enzymes could occur in vivo by small changes in the available pool of BH4 [3] to control the regulation of the synthesis of monoamines in nerve tissue, metabolism of phenylalanine in the liver, or formation of nitrogen oxide in macrophages. Thus understanding of the mechanisms of reaction and regulation of BH4 synthesis is quite important. GTP cyclohydrolase I is the initial and