Considerable
thymidine kinase and
pyrroline-5-carboxylate reductase activities were found in the plasma of rats bearing a transplanted
lymphoma; neither activity was detected in plasma of hosts carrying hepatic, renal, mammary, or submaxillary gland
tumors. All host livers exhibited signs of biochemical immaturity as indicated by the appropriate increases or decreases in the concentrations of the nine
enzymes measured. The extent and time schedule of the changes in host liver varied with the
enzyme and with the
tumor that caused them. The hepatic concentrations of
ornithine aminotransferase,
arginase,
pyrroline-5-carboxylate reductase, and
glucokinase (all diminished), and of peptidyl
proline hydroxylase and
hexokinase (increased) were sensitive indicators of
tumor growth in general. The concentration of
ornithine aminotransferase decreased before the
tumors became palpable. At more advanced stages, the high hepatic
thymidine kinase activity distinguished the presence of
hepatoma and
lymphoma from those of all other equally fast-growing
tumors. However, only in
lymphoma-bearing rats did a fivefold elevation of hepatic
thymidine kinase occur as early as 4 days after implantation. Additional observations on the
lymphoma itself, on blood cells, and on the involuting thymus of normal rats indicate that the striking systemic effects of this
tumor cannot be explained by a release of
enzymes from the thymus or by the increased number of
lymphoma cells present in blood or liver.