Fig. 4

Illustration of the natural abundance correction. The left panels show peak areas (arbitrary units) of ATP and GSH (glutathione), of the same experiment as in Fig. 3, before the natural abundance correction was performed. The interpretation of the results is difficult, particularly when no unlabelled reference is reported. The right panel shows the same compounds after the correction, here the amount of ¹³C incorporation can be deduced much easier. For the legend, see Fig. 3, top