Carnitine acylcarnitine translocase of rat liver mitochondria, assayed by following the uptake of radioactive carnitine, resembled the translocase of heart mitochondria described earlier (Pande, SV, and Parvin, R.(1976) J. Biol. Chem. 251, 6683-6691) in that it showed first order kinetics for the initial uptake and a marked sensitivity to inhibition by mersalyl, N-ethylmaleimide, and (+)-decanoylcarnitine.(-)-Carnitine and its esters were transported more actively than were the corresponding(+)-carnitines. Fasting and alloxan diabetes enhanced the rates of carnitine acylcarnitine translocase-catalyzed transport of carnitine in liver mitochondria. Measurements of K, for carnitine transport and intramitochondrial carnitine concentrations showed that carnitine acylcarnitine translocase normally remains subsaturated with respect to the level of matrix carnitine and that the enhancement of transport on fasting and alloxan diabetes results from an elevation of the intramitochondrial carnitine. These results indicate that the ability of liver to transport fatty acids into mitochondria is increased under ketogenic conditions. The intramitochondrial carnitine content was found positively related to liver carnitine in a variety of conditions.

Fasting for upto 48 h had little effect on serum total carnitine but decreased the urinary excretion of carnitine and deoxycarnitine and appeared to enhance carnitine retention in body. The ratio of esterified to free carnitine, in serum as well as urine, rose on fasting. It seems that under conditions of active hepatic fatty acid oxidation, like acetoacetate and/3-hydroxybutyrate, short chain acylcarnitines are produced in and exported out of liver to serve as fuel for extrahepatic tissues.