Metabolism of Z-ascorbic Acid and L-tyrosine in Guinea Pig Liver by Keith G. Rienits

The experiments of Sealock et al. (l-3) and of Levine et al. (4) have given reasons for believing that one of the functions of ascorbic acid in the animal body is to facilitate the metabolism of L-tyrosine and L-phenylalanine. Sealock and Silberstein (2) showed that the administration of tyrosine orally to scorbutic guinea pigs resulted in the excretion of metabolites containing the benzene ring, accounting for up to 80 per cent of the original tyrosine. Abnormal metabolites were not excreted by animals receiving Z-ascorbic acid. Levine et al. (4) obtained similar results with premature infants. Lan and Sealock (5) examined the livers of guinea pigs, both control and scorbutic, for ability to oxidize tyrosine as measured by increased Q,-,, in slices. The livers from scorbutic animals showed diminished ability to oxidize the tyrosine when compared with livers from guinea pigs receiving adequate ascorbic acid. The oxidizing ability of scorbutic liver slices could be restored by the addition of ascorbic acid. Darby et al. (6) could find no difference in rate of metabolism of L-tyrosine or conjugation of phenol (as measured by disappearance of the hydroxyphenyl groups) in liver slices from scorbutic and non-scorbutic guinea pigs. They concluded that the main defect in the metabolism of aromatic compounds by scorbutic guinea pigs is the inability of the scorbutic liver to oxidize the side chain of tyrosine rather than to oxidize the ring or conjugate the phenolic group. However, Painter and Zilva (7) pointed out that excessive doses of tyrosine were necessary for guinea pigs to excrete metabolites containing the benzene ring, and that this abnormal excretion could only be prevented by having the guinea pig tissues saturated with Z-ascorbic acid. They concluded that there was not necessarily a connection between the normal functions of Z-ascorbic acid and normal metabolism of L-tyrosine. The experiments reported below support the view that tyrosine metabolism in the liver is dependent on the presence of Z-ascorbic acid.