The enzymic capacities of the de novo and the salvage pathways for
purine nucleotide synthesis were compared in rat in normal, differentiating, and regenerating liver, and in three
hepatomas of widely different growth rates. The activities of the key de novo and salvage
enzymes were also determined in mouse lung and
Lewis lung carcinoma, in human kidney and liver, and in
renal cell carcinoma and
hepatocellular carcinomas. A precise and reproducible assay was worked out for measuring the activities of
adenine phosphoribosyltransferase (EC 2.4.2.7) and
hypoxanthine-guanine phosphoribosyltransferase (
HGPRT; EC 2.4.2.8) in crude liver and
hepatoma systems. Kinetic studies on the salvage
enzymes were carried out in the crude 100,000 X g supernatant fluid from normal liver and rapidly growing
hepatoma 3924A. In both
tissue extracts, Michaelis-Menten kinetics was observed for
adenine phosphoribosyltransferase and
HGPRT. The reciprocal plots for 5-phosphoribosyl-1-pyrophosphate (PRPP) of liver and
hepatoma enzymes gave apparent KmS of 2 microM for
adenine phosphoribosyltransferase and 4 microM for
HGPRT, showing two orders of magnitude higher affinities for PRPP than that of the rate-limiting
enzyme of de novo
purine synthesis,
amidophosphoribosyltransferase (EC 2.4.2.14) (Km = 400 to 900 microM). The apparent Km values for
adenine of liver and
hepatoma adenine phosphoribosyltransferase were 0.6 to 0.9 microM, respectively. For both liver and
hepatoma HGPRT, the reciprocal plots for
hypoxanthine and
guanine yielded the same Km of 3 microM. The specific activities of
purine phosphoribosyltransferases were markedly higher than that of
amidophosphoribosyltransferase in rat thymus, spleen, testis, bone marrow, colon, liver, kidney cortex, lung, heart, brain, and skeletal muscle, but were lower in the small intestine. In
hepatomas and regenerating and differentiating liver, the activities of the salvage
enzymes were 2.1- to 32-fold higher than that of
amidophosphoribosyltransferase. The
purine phosphoribosyltransferase activities were also higher than that of
amidophosphoribosyltransferase in
Lewis lung carcinoma (8.2- to 32-fold), human
renal cell carcinoma (3.5- to 22-fold), and
hepatocellular carcinoma (3.4- to 30-fold). The high activities and the high affinity to PRPP of the
purine phosphoribosyltransferases might explain the lack of linkage of the behavior of these enzymic activities with proliferation in normal, regenerating, differentiating, or neoplastic tissues. In contrast, the specific activity of the
amidophosphoribosyltransferase, which is lower than that of the salvage
enzymes, is linked with transformation as it is increased in all examined
tumors.4