Definitions and Assumptions:

  • P1 = Specific activity of exogenously supplied [35S]-methionine (nCi.nmol-1)
  • P2 = Initial pool size of exogenously supplied [35S]-methionine (nmol.gfw-1)
  • P3 = Estimated pool size of exogenously supplied [35S]-methionine sulfoxide (nmol.gfw-1) [not taken up!]
  • Note that P2 + P3 = 5.0 nmol.gfw-1
  • TS = Total time of the simulation (min)
  • k = A variable that determines the rate of uptake of the exogenously supplied methionine, A8 = P2.k; as the pool of exogenous methionine (P2) declines, the rate of uptake of methionine declines proportionately.
  • F1 = Specific activity of endogenous unlabeled homocysteine (nCi.nmol-1)
  • A1 = Initial specific activity of endogenous free methionine (nCi.nmol-1)
  • A2 = Initial pool size of endogenous free methionine (nmol.gfw-1)
  • A3 = Rate of synthesis of methionine from unlabeled homocysteine (nmol.min-1.gfw-1)
  • A7 = Rate of utilization of methionine in "other" reactions (nmol.min-1.gfw-1)
  • B1 = Initial specific activity of protein bound methionine (nCi.nmol-1)
  • B2 = Initial pool size of protein bound methionine (nmol.gfw-1)
  • B3 = Rate of utilization of methionine in protein synthesis (nmol.min-1.gfw-1) [note that in this specific model, a better fit to the observed protein methionine labeling data was obtained if the rate of protein methionine synthesis was envisaged to transiently increase in response to the rapid influx of methionine from the medium. Thus, the rate of protein synthesis = B3 + A8, where A8 is the methionine uptake rate = P2.k (see above)].
  • B4 = Rate of release of protein methionine back to the free methionine pool (nmol.min-1.gfw-1)
  • C1 = Initial specific activity of MTHB (4-methylthio-2-hydroxybutyrate) (nCi.nmol-1)
  • C2 = Initial pool size of metabolic pool of MTHB (nmol.gfw-1)
  • C3 = Rate of synthesis of metabolic pool of MTHB from methionine (nmol.min-1.gfw-1)
  • C4 = Rate of exchange between metabolic and storage pools of MTHB (nmol.min-1.gfw-1)
  • C5 = Initial specific activity of storage of pool MTHB (nCi.nmol-1)
  • C6 = Initial pool size of storage pool of MTHB (nmol.gfw-1)
  • D1 = Initial specific activity of metabolic of pool DMSHB (4-dimethylsulfonio-2-hydroxybutyrate) (nCi.nmol-1)
  • D2 = Initial pool size of metabolic pool of DMSHB (nmol.gfw-1)
  • D3 = Rate of synthesis of metabolic pool of DMSHB from MTHB (nmol.min-1.gfw-1)
  • D4 = Rate of exchange between metabolic and storage pools of DMSHB (nmol.min-1.gfw-1)
  • D5 = Initial specific activity of storage of pool DMSHB (nCi.nmol-1)
  • D6 = Initial pool size of storage pool of DMSHB (nmol.gfw-1)
  • E1 = Initial specific activity of end-product, DMSP (3-dimethylsulfoniopropionate) (nCi.nmol-1)
  • E2 = Initial pool size of DMSP (nmol.gfw-1)
  • E3 = Rate of synthesis of DMSP from DMSHB (nmol.min-1.gfw-1)
  • gfw = g fresh weight