Final answer:
The 13C label in tyrosine cannot trace back to phenylalanine due to the irreversible reaction catalyzed by phenylalanine hydroxylase, which converts phenylalanine to tyrosine in one direction.
Step-by-step explanation:
The 13C label in tyrosine does not appear as a 13C label in phenylalanine during regular metabolic activity in a rat liver because the reaction catalyzed by the enzyme phenylalanine hydroxylase is an irreversible reaction that converts phenylalanine to tyrosine, not the other way around. Therefore, radiolabels in tyrosine cannot be traced back to phenylalanine because the metabolic pathway proceeds in one direction—from phenylalanine to tyrosine.
In the context of metabolic disorders such as phenylketonuria (PKU), a defect in phenylalanine hydroxylase impairs the conversion of phenylalanine to tyrosine, leading to accumulation of phenylalanine and a deficiency in tyrosine. This scenario emphasizes the unidirectional nature of the pathway under normal metabolic conditions, affirming the absence of a reversible step that would allow tyrosine's 13C label to appear in phenylalanine.