Abstract
The kinetics of the exchange reactions between internal l‐malate and external 2–oxoglutarate or malonate have been measured at 4°C in the presence of external l‐malate (external‐product inhibitor), using preparations of rat‐heart mitochondria under conditions where the oxoglutarate translocator is operating exclusively. Measurements of initial rates were made at three concentrations of the internal substrate (l‐malate), three concentrations of the external substrate (2‐oxoglutarate or malonate) and three concentrations of the external reaction product (l‐malate).The Michaelis constant for the internal malate is unaffected by the presence of the external product whereas the apparent‐maximum rate for each concentration of external substrate decreases when the concentration of external product increases. On the other hand the Michaelis constant for the external substrate, oxoglutarate or malonate, increases when the concentration of external malate increases.The external product inhibits the exchange in a competitive way when external substrate is varied and in a non‐competitive way when internal substrate is varied.The inhibition constant determined for the external malate is equal to the previously determined dissociation constant of the corresponding external‐malate · translocator complex.These results are in perfect agreement with the previously proposed mechanism (rapid‐equilibrium random bi‐bi) and a mixed dead‐end and product inhibition.
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