Theoretical interpretation of isotope labelling experiments in cells in which the label is chemically incorporated: the example of orthophosphate.

Studies of transport across the plasma membrane in intact cells frequently involve measuring the incorporation of a labelled extracellular species into the cells. Unfortunately, if the labelled species is metabolized in the cell, the kinetics of labelling are made more complicated. Using the example of the incorporation of 32P-labelled orthophosphate into cells, we describe a mathematical model which allows for this complication, and show how this may alter the interpretation of experiments. The analysis is widely applicable to cellular labelling studies with any species that undergoes chemical exchange with a large cellular pool.