Phenylalanine-pyruvate aminotransferase in immature and adult mammalian tissues induction in fetal rat liver

Normal human fetal liver contains little phenylalanine-pyruvate aminotransferase: between the 11th and 22nd week of gestation its activity (per g) is 8.8% of that in adult liver. In rat liver this enzyme begins to rise a few hours before birth. Precocious increases in the phenylalanine-pyruvate aminotransferase activity of fetal rat liver (but not kidney or brain) were evoked by premature delivery and also by the administration of thyroxine or glucagon in utero. These results, Discussed in relation to related observations on other enzymes, suggest that thyroxine secreted by the fetus, and also another factor relaesed at the beginning of labour, may be the natural stimuli for the developmental formation of phenylalanine-pyruvate aminotransferase. The regulation of hepatic phenylalanine-pyruvate aminotransferase and phenylalanine hydroxylase (L-phenylalanine, tetrahydropteridine:oxygen oxidoreductase (4-hydroxylating), EC 1.14.16.1) during fetal development is different: in both man and rat, phenylalanine hydroxylase begins to rise earlier and is unaffected by the treatments which enhanced the formation of phenylalanine-pyruvate aminotransferase. In suckling rats (but not in fetuses and adults), an injection of cortisol increased the levels of both enzymes. Hepatocarcinomas of the adult rat were devoid phenylalanine hydroxylase as well as phenylalanine-pyruvate aminotransferase. However, suppression in vivo by substrate analogues (α-methylphenylalanine and p-chlorophenylalanine) was unique for phenylalanine hydroxylase.