Phenylalanine Tolerance Test for Heterozygous Carriers of Phenylketonuria

Abstract

HSIA et al.¹ found that plasma phenylalanine-levels were substantially higher after the ingestion of a standard dose, based on body-weight, of this substance by parents of phenylketonuric patients than the levels found in a comparable (age, sex, etc.) normal control group. The same observation was made by Berry, Sutherland and Guest². Hsia et al. interpreted their results as indicating that a deficiency of the enzyme responsible for phenylalanine metabolism exists in the heterozygous carriers. However, if the data of Hsia et al.¹ and Berry, Sutherland and Guest² on phenylalanine concentration as a function of time are plotted³, it will be seen that the slope of the disappearance curve is essentially the same in the test group and in the control group. The data of Berry, Sutherland and Guest² actually suggest that phenylalanine is removed more rapidly by the test group than it is by normal subjects.