Studies on the Phenylalanine Hydroxylase System in Viva AN IN VW0 ASSAY BASED ON THE LIBERATION OF DEUTERIUM OR TRITIIJM INTO THE BODY WATER FROM RING-LABELED I/PHENYLALANINE

The rate of release of deuterons into the body water from 2,3,4,5,6-pentadeutero-L-phenylalanine has been shown to be a valid measure of the activity of the phenylalanine hydroxylase system in vivo. At a dose of 0.5 g/kg, the rate of release of deuterons is linear for 60 to 90 min. Male rats, which had previously been shown to have 22 to 25% more phenylalanine hydroxylase activity in liver extracts than female rats, produced deuterons from deuterated phenylalanine at a rate 20 to 30% greater than female rats. p-Chlorophenylalanine, which irreversibly inhibits phenylalanine hydroxylase in uivo, caused a similar degree of inhibition of the rate of deuteron formation as was found when phenylalanine hydroxylase was measured in extracts from the same group of animals. Methotrexate, which inhibits the phenylalanine hydroxylase system by preventing regeneration of the tetrahydropteridine cofactor, caused parallel inhibition of the in uivo assay as well as when the conversion of phenylalanine to tyrosine was measured in liver slices. Randomly ring-tritiated phenylalanine can be used interchangeably with ring-deuterated phenylalanine if greater sensitivity is needed in the in uiuo assay for phenylalanine hydroxylase. However, a dose of 20 to 30 wCi/kg is required. The in uivo deuterium release assay described in this paper should be useful in studying the physiological control of the phenylalanine hydroxylating system. It also may be of value in differentiating between individuals who are heterozygotes for phenylketonuria and those who are homozygotes for hyperphenylalaninemia.