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. Author manuscript; available in PMC: 2008 Mar 17.
Published in final edited form as: J Nutr. 2007 Jun;137(6 Suppl 1):1549S–1575S. doi: 10.1093/jn/137.6.1549S

FIGURE 1.

FIGURE 1

Model to measure conversion of phenylalanine to tyrosine using a phenylalanine tracer. The top half shows the reaction converting phenylalanine to tyrosine via phenylalanine hydroxylase. The bottom half shows the model equivalent. Circles denote the free phenylalanine (Phe) and tyrosine (Tyr) pools. Arrows indicate rates of appearance and disappearance from the free pools due to whole body protein synthesis and breakdown. The wide arrow indicates administration of a phenylalanine tracer (usually by continuous infusion) into the free phenylalanine pool. Tracer abundance (enrichment) is measured in free phenylalanine and in tyrosine (indicated by the ball and stick figure to denote sampling from the free pools). The ratio of the tyrosine tracer enrichment divided by the phenylalanine tracer enrichment defines the fraction of free tyrosine derived from phenylalanine.