Assessing Metabolic Flux in Plants with Radiorespirometry.

Carbohydrates are the dominant respiratory substrate in many plant cells. However, the route of carbohydrate oxidation varies depending on the relative cellular demands for energy, reductant, and precursors for biosynthesis. During these processes individual substrate carbon atoms are differentially released as carbon dioxide by specific reactions in the network, and this can be measured by monitoring the release of 14CO2 from a range of positionally labeled forms of [14C]glucose. Although the relative amounts of carbon dioxide produced from different carbon positions do not allow precise determination of fluxes, they are indicative of the route of carbohydrate utilization. Such information can be used to determine whether a comprehensive metabolic flux analysis is merited, and also to facilitate independent verification of flux maps generated by other techniques. This chapter describes an approach to determine and interpret the pattern of oxidation of carbohydrates by monitoring 14CO2 release during metabolism of exogenously supplied [1-14C]-, [2-14C]-, [3,4-14C]-, and [6-14C]glucose. The method is exemplified by studies on Arabidopsis cell suspension cultures, but the protocol can be easily adapted for the investigation of other plant materials.