Effects of structures of tetrahydropterin cofactors on rat brain tryptophan hydroxylase

Effects of structures of the side chain at position 6 of 12 tetrahydropterin cofactors including 4 stereoisomers of tetrahydrobiopterin and tetrahydroneopterin on the activity of rat brain tryptophan hydroxylase were examined. Tetrahydrobiopterins and tetrahydroeneopterins having a side chain of a L-erythro or or D-threo configuration showed lower Km values for both the pterin cofactor and tryptophan substrate and also higher V values than their enantiomers. L-erythro-Tetrahydrobiopterin had the highest cofactor activity among all the pterin cofactors examined. Since reduction of biopterin to tetrahydrobiopterin introduces another center of asymmetry at 6-position of the pterin ring, L-erythro-tetrahydrobiopterin obtained by chemical reduction is a mixture of two diastereoisomers. The two diastereoisomers of L-erythro--tetrahydrobiopterin, i.e. (6R)-L-erythro-tetrahydropterin and the (6S)-isomer were separated by high-performance liquid chromatography, and were examined for their cofactor activity. The two diastereoisomers gave similar Km values toward pterin itself and toward tryptophan, but the natural (6R)-isomer gave much higher V values than the (6S)-isomer; the (6S)-isomer was nearly inactive for rat brain tryptophan hydroxylase because of its very low V value. These results support the hypothesis that (6R)-L-erythro-tetrahydrobiopterin may be the natural cofactor of rat brain tryptophan hydroxylase.