On the levels of phenylalanine, tyrosine and tetrahydrobiopterin in the blood of tumor-bearing organisms.

The levels of free phenylalanine and free tyrosine (and m-tyrosine) as well as of protein-bound tyrosine were determined by means of automated aminoacid analysis and by reaction with 1-nitrosonaphthol(2). Their absolute values as well as their percentages relative to total aminoacids of the blood were markedly increased in all tumor patients tested, as well as in experimental tumor rats. Although tetrahydrobiopterin could scarcely be detected by fluorometric methods in the blood of control persons, it is accumulated blood of all tumor patients tested. The major part was found in the erythrocyte fraction. The results are discussed with respect to decreased phenylalanine hydroxylation rate and to depressed tyrosine catabolism recently found by means of in vivo experiments in tumor-bearing rats.