Amino acid oxidation in the tumor-bearing rat.

It has been proposed that the presence of a tumor leads to uncontrolled oxidation of the branched chain amino acids in the peripheral tissue of the host animal. This was investigated in Buffalo strain rats bearing the tumor, Morris hepatoma 5123tc. The rate of 14CO2 production from [1-14C] leucine was observed to be significantly increased in the tumor-bearing rats in vivo. This increase may be attributed to an observed increase in the activity of the rate-limiting enzyme of leucine oxidation (2-oxoisocaproate dehydrogenase) not only in host skeletal muscle but also in the tumor. In contrast, the oxidative capacities of host tissues (liver and muscle) towards tyrosine were, however, decreased by the presence of a tumor, an observation consistent with tumor induction of fetal-like changes in host tissue metabolism.