Evidence for the existence of an ornithine/citrulline antiporter in rat liver mitochondria

In the urea cycle in rat liver, the enzymes carbamoyl phosphate synthase and ornithine transcarbamylase are situated in the mitochondria while the other enzymes of the cycle are cytoplasmic. Thus during the operation of the urea cycle, it is necessary for ornithine to enter the mitochondria and for citrulline to leave. These two amino acids are presumably transported by specific carrier systems, but the systems involved have not been adequately characterised.