cDNA clones for human 6-pyruvoyl-tetrahydropterin synthase, the second enzyme in the biosynthetic pathway of tetrahydrobiopterin, were isolated from a human Molt-4 cell cDNA library by cross-hybridization with a rat cDNA. One cDNA clone contained the entire coding sequence of 435 base pairs. The cDNA was expressed in Escherichia coli using the expression vector pMAL as a fusion protein with maltose-binding protein. After affinity purification through its maltose-binding protein domain, the fusion protein was digested by factor Xa at a specific cleavage site inserted between the domains. The main product was a protein species with a native molecular mass of 90 kDa and a subunit molecular mass of 17 kDa, and the molecular masses and its kinetic properties were similar to those of the human enzyme purified from the liver.