[1] Exhalation of isotopic CO2

The collection of 14CO2 derived from appropriately labeled test compounds has several methodological advantages. The procedure is noninvasive, and therefore does not disturb the organism being tested. The technical realization is quite easy, requiring no sophisticated equipment other than a liquid scintillation counter. The method is extremely versatile because it can be applied to a wide range of test compounds, at pharmacological or tracer doses, in many species and in isolated organs or cells. The most frequently used 14CO2 breath tests have been demethylation reactions. For this purpose, the test compounds must be labeled at a methyl group that is demethylated within the organisms. In most instances, demethylation results in CO2 only after several further oxidation steps with formaldehyde, formic acid, and carbonic acids as intermediates. Thus, the label has to pass through the pools of several physiological intermediary metaholites resulting in the changes of specific activity and in losses to other routes of metabolism. Under these circumstances, the rate of 14CO2 formation reflects the rate of metabolism of the administered test compound only if the modifications occurring at the level of the intermediary metabolites are predictable—that is, remain the same for all the investigated experimental conditions. It is also necessary that the initial demethylation reaction should be the rate-limiting step and that all further oxidations are much more rapid.