L-alanyl-L-tyrosine as a tyrosine source during intravenous nutrition of the rat.

Tyrosine is considered an essential amino acid for some premature infants, but its limited solubility prevents addition to intravenous solutions in adequate amounts. Tyrosine peptides with solubilities greater than tyrosine were synthesized to evaluate potential for intravenous use. Intraperitoneal injection of L-alanyl-L-[U-14C]tyrosine (0.5 mmoles/kg) into adult rats resulted in a rapid labeling of tissue pools and production of 14C-CO2. When the peptide was infused as part of an intravenous nutrition solution (0.5 mmoles/kg/24 hours) into adult rats, plasma and tissue tyrosine pools were rapidly labeled. The radioactivity distribution after 24 hours of infusion was: 41% CO2, 13.4% muscle, 7.7% urine, 7.1% liver, 5.5% intestine, 1.4% kidney, with the remainder in other carcass organs. No unhydrolyzed peptide was found in tissue homogenates. Between 9% and 17% of the radioactivity present in tissue was free tyrosine, with the remainder in tissue protein. Tyrosine accounted for more than 96% of this protein bound radioactivity. The data indicate good utilization of alanyl-tyrosine as a tyrosine source when administered intravenously.