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. Author manuscript; available in PMC: 2009 Aug 1.
Published in final edited form as: Metabolism. 2008 Aug;57(8):1078–1087. doi: 10.1016/j.metabol.2008.03.011

Figure 1.

Figure 1

An extremely simple model for apoB turnover in VLDL and IDL with a labeled amino acid precursor as tracer. The square P denotes the precursor system, consisting of one or more pools. Pool 1 is for VLDL apoB and pool 2 for IDL apoB. The k symbols denote rate constants for the pathways, defined as fluxes divided by the source pool mass, with the first subscript indicating the destination and the second the source of the pathway. Thus, k02 is IDL apoB FCR, the rate constant into 0 from 2, equal to the mass flux to the outside from 2 (IDL) divided by Q2, the mass of the source pool 2 (IDL). The extension to this definition is that, when describing synthesis or entry from the outside, the second subscript is zero or P, and the division is by the destination pool mass. Thus, the FSR of VLDL apoB is k1P, the flux into 1 (VLDL) from P (the precursor) divided by Q1, the mass of the destination pool. A: The model has a direct removal pathway from VLDL (k01), making the relationship between the two masses an identifying constraint. B: The model has no direct removal pathway from VLDL (k01=0), making the relationship between the two masses a nonidentifying constraint.