Tyrosine is an essential amino acid in the queuine-deficient germfree mouse

Tyrosine has been shown, by others, not to be an essential amino acid in conventional mice. It is synthesized de novo from the amino acid phenylalanine in a reaction catalyzed by phenylalanine hydroxylase using O₂ and a pterin cofactor. This cofactor is also utilized in the reactions of tyrosine and tryptophan hydroxylases. We have found, however, that the queuine-deficient germfree mouse develops neurological abnormalities and cannot survive in the absence of dietary tyrosine. In the presence of queuine, a modified guanine residue found in the tRNAs of histidine, aspartic acid, asparagine, and tyrosine, these germfree mice are healthy and do not develop the bizarre neurological symptoms seen in queuine deficient mice fed a diet without tyrosine. The biological role of queuine is unknown but this work suggests that queuine may be an important dietary component necessary for normal metabolism of tyrosine and possibly other amino acids. Further studies are needed to test for the biochemical basis of this phenomenon.