Computer Simulation of Metabolism
Simulation of The Pathway of Synthesis of DMSP in Spartina alterniflora
Kocsis, M.G., Nolte, K.D., Rhodes, D., Shen, T.L., Gage, D.A. and Hanson, A.D.
This work was in part supported by a grant from the National Science Foundation (NSF)
In contrast to marine algae, the synthesis of 3-dimethylsulfoniopropionate (DMSP) in the higher plant Spartina alterniflora, proceeds via the intermediates S-methylmethionine (SMM) and dimethylsulfoniopropylamine (DA) (Kocsis et al, 1998).
This was verified by computer modeling, as shown in the figure, below.
[35S]-SMM (1.6 nmol.gfw-1; 5000 nCi.gfw-1; 3125 nCi.nmol-1) (SMM (ex)) was supplied to leaf disks of Spartina for 150 min, and the supply of
labeled SMM was then abruptly removed (i.e. chase start time = 150 min). Radioactivity in the endogenous pools of SMM, DMSP-amine (DA) and DMSP was determined as a function of time (Kocsis et al, 1998)
The labeling kinetics of DA and DMSP are quantitatively consistent with the pathway: SMM ---> DA ---> DMSP, provided that it is envisaged that both DA and SMM exist as metabolically active (met) pools, in slow equilibrium with metabolically
inactive or "storage" (stor) pools of these intermediates. In order to fully account for the complex labeling kinetics of SMM, it was necessary to envisage a small apoplastic pool of SMM (SMM(ap)) in rapid equilibrium with the medium, in addition to the
symplastic metabolically active (met) and storage (stor) pools of SMM. Simulated values for SMM in the figure shown above, represent the total radioactivity in the apoplastic pool of SMM and the two symplastic SMM pools (i.e. [ (B1*B2) + (C1*C2) +
(D1*D2)]). The rate of synthesis of DMSP estimated from this model is 2.7 nmol.min-1.gfw-1. Note that it is principally the endogenous flux from unlabeled methionine ---> SMM (at rate C4) that carries
the label from exogenously supplied SMM into the end-product, DMSP.
Pool sizes and fluxes were progressively adjusted in the model until the mean absolute deviations between observed (squares) and simulated (lines) values were minimized. Values for pool sizes and fluxes shown in the figure, above, represent "best-fit" values.
Download a version of the Visual Basic program illustrated above. To run this program you must have Visual Basic 5.0 (or greater) installed on your computer.
A version of the above program is available as a Java applet. This applet should function with both Netscape Navigator and Internet Explorer 3.0 or above. Simulated values are shown in graphical format. This
model currently does not provide numerical estimates of "goodness-of-fit".
Kocsis, M.G., Nolte, K.D., Rhodes, D., Shen, T.L., Gage, D.A. and Hanson, A.D. 1998. Dimethylsulfoniopropionate biosynthesis in Spartina alterniflora Loisel.: evidence that S-methylmethionine and dimethylsulfoniopropylamine are
intermediates. Plant Physiology 117: 273-281.