The source of aromatic ketoacids in tyrosinaemia and phenylketonuria.

The studies reported here support the observation that elevated excretion of ~-hydrox~henylp~~uvic acid could occur in the presence of deficient hepatic tyrosine aminotransferase activity. The ketoacid need not have come either from the liver or from the kidney. It appears possible that the urinary ketoacid, both in tyrosine aminotransferase deficiency and in phenylketonuria, originates not in the liver but in other tissues which possess transaminase but which lack hydroxylase activity. What emerges from these studies is the view that the consequence of a single enzyme deficiency in one tissue may be modified by the ~~istribution of isozymes or related enzymes in that tissue as well as in other tissues.