Carnitine-acylcarnitine translocase catalyzes an equilibrating unidirectional transport as well

Besides carnitine-acylcarnitine translocase-catalyzed exchange diffusions, a relatively much slower unidirectional transport of carnitines also proceeds in mitochondria. The latter proceeds in the direction of the concentration gradient of carnitines and leads to the equilibration of carnitine concentration across the inner membrane. The same translocase that catalyzes exchange diffusion appears to catalyze unidirectional transport inasmuch as the two processes showed similar substrate specificity, sensitivity to inhibitors, high temperature coefficients, apparent affinity for carnitine, and a lack of energy requirement. The unidirectionally imported carnitine readily exchanged against medium carnitine, indicating that the two transport processes share the same pool of mitochondrial carnitine. Unidirectional transport appears to be functional in vivo; its operation seems not only to allow mitochondria to acquire carnitine but to adjust the rates of their faster exchange-diffusion reactions with changes in total tissue carnitine concentrations. The carnitine-acylcarnitine translocase was found to be about 15 times as active in mitochondria of rat liver as in those of heart.