Biochemical strategy of the cancer cell: Malignant transformationlinked enzymatic imbalance

Abstract The biochemical strategy of the cancer cell was examined in terms of malignant transformation-linked enzymatic imbalance. Previous work established that the activities of two of the key enzymes of the oxidative and non-oxidative synthetic pathways of pentose phosphate production were increased in all hepatomas. Thus, the activities of glucose-6-phosphate dehydrogenase and transaldolase were increased in all liver tumors irrespective of growth rate and degree of differentiation. The present work reports that amidophosphoribosyltransferase (glutamine PRPP amidotransferase, EC 2.4.2.14) and UDP kinase (ATP:UDP phosphotransferase, EC 2.7.4.6) were markedly increased in all hepatomas. In studying the behavior of amidophosphoribosyltransferase the kinetic parameters in normal liver and rapidly growing hepatomas were compared. For both normal liver and hepatoma 3924-A apparent K m 's for glutamine and magnesium were 1.7 and 0.4 m m , respectively. In contrast, the liver enzyme exhibited sigmoid properties toward the substrate, PRPP, but the enzyme in hepatoma 3924-A did not. In liver and hepatoma the apparent K m 's for PRPP were 0.9 and 0.4 m m , respectively. In the standard assay that was developed, proportionality was achieved for enzyme activity with amount of enzyme added and with time elapsed in normal and neoplastic livers. In all hepatomas amidophosphoribosyltransferase activity was increased 1.7- to 3.1-fold over the normal liver values. The hepatoma enzyme was less sensitive to inhibition by AMP than the liver enzyme. Studies in kidney tumors of different growth rates indicated that amidotransferase activity was markedly increased over the values of normal rat kidney cortex. The enzyme activity was also increased in human primary hepatomas. Since the markedly increased amidotransferase activity and the hyperbolic affinity for PRPP were present only in tumors and not in rapidly growing normal differentiating or regenerating liver, the altered activity of this key purine-synthesizing enzyme appears to be specific to the neoplastic transformation. The increased amidotransferase activity, the enhanced affinity of the hepatoma enzyme to the substrate, PRPP, and the decreased sensitivity to inhibition by AMP confer selective biological advantages to the cancer cell. In examining the behavior of UDP kinase it was observed that the kinetic parameters were similar for normal and neoplastic liver. The apparent K m values for UDP, ATP and magnesium were 0.5, 3.0 and 3.0 m m , respectively. Through kinetic studies a standard assay was developed where good proportionality was achieved for enzyme activity with amount of enzyme added and time elapsed in both normal and neoplastic liver. The UDP kinase activities were approximately 2- to 4-fold of those observed in the liver of control normal rats. Studies in two different primary hepatomas from man also indicated that UDP kinase activity was increased 2- to 3-fold above the activities observed in normal human liver. The increased UDP kinase activity occurred only in liver tumors, and the enzyme activity did not rise in rapidly growing normal, differentiating or regenerating liver; therefore, the increased activity of this UTP-synthesizing enzyme appears to be characteristic of the neoplastic transformation in the liver. As the hepatomas contain markedly more UDP kinase activity than the normal liver the cancer/cells possess an increased capacity for UTP biosynthesis. This alteration in UDP kinase activity indicates a reprogramming of gene expression that should confer selective biological advantages to the neoplastic cells. Since the marked increases in activities of amidophosphoribosyltransferase, UDP kinase, glucose-6-phosphate dehydrogenase and transaldolase occur in all hepatomas, including even the slowest-growing and most-differentiated tumors, these alterations indicate reprogramming of gene expression that is connected with the malignant transformation per se .