Fig. 5.

Scheme of the amino acid and apolipoprotein systems. When a tracer is introduced, that amino acid is in an unsteady state, but the apolipoprotein remains in a steady state. The dashed line separates the unsteady amino acid system from the steady apolipoprotein system. The number of pools is for illustrative purposes. The unaltered synthesis paths are shown by double arrows. Total masses are denoted by M, and fluxes are denoted by R. Tracer masses are denoted by m; labeled fluxes, which equal the corresponding total fluxes multiplied by the source pool enrichments, are given in terms of total fluxes and tracer-to-total mass ratios or enrichments. For instance, the total flux from protein pool 1 to pool 2 is R₂₁, and the corresponding tracer flux is R₂₁m₁(t)/M₁, or R₂₁ multiplied by the tracer enrichment in pool 1. Tracer quantities are shown in italics, below or to the right of the corresponding total quantities.