Measurements of the kinetics of hyperpolarized (13)C label exchange between [1-(13)C]
pyruvate and
lactate in
suspensions of intact and lysed murine
lymphoma cells, and in cells in which
lactate dehydrogenase expression had been modulated by inhibition of the PI3K pathway, were used to determine quantitatively the role of
enzyme activity and membrane transport in controlling
isotope flux. Both steps were shown to share in the control of
isotope flux in these cells. The kinetics of label exchange were well described by a kinetic model that employed rate constants for the
lactate dehydrogenase reaction that had been determined previously from steady state kinetic studies. The
enzyme showed
pyruvate inhibition in steady state kinetic measurements, which the kinetic model predicted should also be observed in the
isotope exchange measurements. However, no such
pyruvate inhibition was observed in either intact cells or cell lysates and this could be explained by the much higher
enzyme concentrations present in the
isotope exchange experiments. The kinetic analysis presented here shows how
lactate dehydrogenase activity can be determined from the
isotope exchange measurements. The kinetic model should be useful for modeling the exchange reaction in vivo, particularly as this technique progresses to the clinic.