Colon tumor: enzymology of the neoplastic program

The biochemical strategy of colon tumor was investigated by comparing the enzymic programs of glycolysis, pentose phosphate production and purine and pyrimidine biosynthesis and degradation in liver, normal colon mucosa and transplantable colon adenocarcinoma in the mouse. In normal colon mucosa the carbohydrate and pentose phosphate enzymes were 2- to 9-fold higher in specific activity than those in liver. Among the enzymes of CTP synthesis, CTP synthetase was the rate-limiting one in both liver and colon. In colon tumor CTP synthetase, OMP decarboxylase, uracil phosphoribosyltransferase and thymidine kinase activities increased to 927, 863, 597 and 514% of activities of normal colon. In contrast, the activity of the catabolic enzymes, dihydrothymine dehydrogenase and uridine phosphorylase, decreased to 51 and 25%. The ratios of activities of uridine kinase/uridine phosphorylase and thymidine kinase/dihydrothymine dehydrogenase were elevated 6- and 10-fold. The activity of the key purine synthetic enzyme, glutamine PRPP amidotransferase, increased 7-fold and the opposing rate-limiting enzyme of purine catabolism, xanthine oxidase, decreased to 7%. The ratio of amidotransferase/xanthine oxidase was elevated to 8, 150%. Activities of glucose-6-phosphate dehydrogenase and transaldolase did not increase, but that of pyruvate kinase was elevated to 154%. Similar enzymic programs were observed in a transplantable adenocarcinoma of the colon in the rat. The alterations in gene expression in colon tumor manifested in an integrated pattern of enzymic imbalance indicate the display of a program, a segment of which is shared with rat and human liver and kidney tumors. These alterations in gene expression should confer selective advantages to colon tumor cells. The striking increases in the activities of CTP synthetase, OMP decarboxylase, glutamine PRPP amidotransferase and thymidine kinase mark out these enzymes as potentially sensitive targets for combination chemotherapy by specific inhibitors of these enzyme activities.