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To the Editor.—By "transient phenylalaninemia," I meant a temporary increased concentration of phenylalanine in the blood. To call such a state "transient tyrosinemia" would be as misleading as using the word "diabetes mellitus" instead of "hyperglycemia." It is true that tyrosinemia, through certain mechanisms, may lead to phenylalaninemia. Nevertheless, in that particular part of the review article, I was concerned with increased concentration of phenylalanine in the blood, ie, phenylalaninemia.
Many investigators, including Helen K.Berry, MD, who was referred to in my article, have suggested that "delayed maturation of phenylalanine hydroxylase and paraphydroxyphenylpyruvic oxidase systems" play a role in transient phenylalaninemia. Obviously, many factors are involved. Enzymologists have not, as yet, come up with any final answers. The complexity of the situation has been referred to in a recent book entitled Amino Acid Metabolism and Genetic Variation edited by William L. Nyhan, MD, PhD, and published by McGraw-Hill.
Regarding Dr. Dyken's observation on