Each frame on this page can be independently resized. This page uses JavaScript to simulate the labeling kinetics of the pools of choline (Cho), phosphoryl-choline (P-Cho), phosphatidyl-choline (Ptd-Cho) and glycine betaine (GlyBet) in leaf disks from immature leaves of non-salinized, transgenic tobacco plants expressing choline monooxygenase in the chloroplast, supplied with [¹⁴C]-choline. The JavaScript language is supported by Netscape Navigator and Microsoft Internet Explorer 3.0 or above. If you are using a JavaScript-compatible browser, try varying the pool sizes and Km and Vm values (upper Table of the lower right frame) to determine how this influences the simulated labeling patterns in comparison to observed data.

Definitions of each variable are given in the upper right frame. The assumptions of the model are shown in the upper left frame, which illustrates the metabolic scheme and the typical graphical output from the Visual Basic version of this same model using the default values provided.

The Km and Vm values allow the fluxes to behave with Michaelis-Menten kinetics. Thus, rate D3 (the rate of synthesis of P-Cho from the cytosolic pool of choline) = (Vm2*A2)/(Km2+A2). As the cytosolic pool of choline expands due to uptake of exogenously supplied [¹⁴C]-choline, the rate of synthesis of P-choline (D3) increases hyperbolically.

Starting values should be in numerical format, and should not exceed 6 characters in length. Errors resulting in script termination will occur when values are entered as text rather than in numerical format. Upon encountering such errors it may be necessary to "Refresh" the page. Observed radioactivity and sampling time data should be entered in the format indicated by the default values provided; the last time value should be the greatest time value in the data series; all time values should be greater than 0. "Alerts" will be provided when these conditions are not satisfied.

After entering pool sizes, Km and Vm values, click the Calculate button. Simulated values will then be displayed in the text boxes below the "Calculate" button. The Calculate button can be reused repetitively after modifying input values. The objective is to find a set of Km, Vm and pool size values which minimizes the mean absolute deviations between observed and simulated radioactivity.

With certain browsers (e.g. Internet Explorer 3.0), the simulated values of radioactivity, and pool sizes displayed in the text boxes (lower right frame) are floating-point (e.g. 0.019995 may be displayed as 1.9995E-02). Thus, it may be necessary to use your mouse to highlight certain text boxes to view the full floating-point notations for values suspected to be less than 0.1.