Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UMLS:C0019204 (hepatocellular carcinoma)
 71,386 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Nine independently derived clones of mutagenized rat hepatoma cells selected for resistance to 6-mercaptopurine (6-MP) or 6-thioguanine (6-ThioG) have been isolated. Each has severely reduced catalytic activity of hypoxanthine-guanine phosphoribosyltransferase (HPRT) and seven of them possess significantly increased activities of phosphoribosylpyrophosphate (PRPP) synthetase. The degrees of elevations of PRPP synthetase activities do not correlate with the degrees of deficiencies of HPRT activities. The cells from one of these clones, 1020/12, posses 40% of the normal HPRT catalytic activity and overproduce purines. We have extensively examined the cells from this clone. Immunotration studies of 1020/12 cells indicate that there is a mutation in the structural gene for HPRT. Although they possess increased specific catalytic activities of the enzyme. PRPP synthetase, the catalytic parameters, heat stability, and isoelectric pH of PRPP synthetase from 1020/12 cells are indistinguishable from those of the enzyme from wild-type cells. The cause of purine overproduction by 1020/12 cells appears to be the elevated PRPP synthetase activity, rather than a PRPP "sparing" effect stemming from reduced HPRT activity. Support for this idea is provided by the observation that the complete loss of HPRT activity in a clone derived from 1020/12 cells does not further enhance the levels of PRPP synthetase or purine overproduction. We propose that the elevated levels of PRPP synthetase activity in these HPRT deficient cells result from a mutational event in the structural gene for HPRT, and that this causes the disruption of a previously undescribed regulatory function of this gene on the expression of the PRPP synthetase gene.
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PMID:Increased PRPP synthetase activity in cultured rat hepatoma cells containing mutations in the hypoxanthine-guanine phosphoribosyltransferase gene. 17 76

This work tested the relationship of guanylate and adenylate biosynthesis during the display of the proliferative program of rat hepatoma 3924A cells. Since serine, the major source of one-carbon units, competed with the substrate [14C]formate for purine labeling, serine-free medium was used in the assays. The initial rates of purine de novo synthesis with [14C]formate or L-[3-14C]serine followed Michaelis-Menten kinetics yielding similar Vmax values with apparent Kms of 0.5 and 0.038 mM, respectively. During the transition of cancer cells from plateau phase into logarithmic proliferation the specific activity of 5-phosphoribosyl 1-pyrophosphate synthase (EC 2.7.6.1, ribose phosphate pyrophosphokinase) increased 2.2-fold, followed by a 14-fold elevation of the concentration of 5-phosphoribosyl 1-pyrophosphate with a subsequent 8-fold rise in de novo purine synthesis. The ratio of guanylate to adenylate synthesis from IMP in plateau phase cells was 0.24 to 1. After replating the resting cells there was a sharp increase in the relative labeling of guanylates with a concurrent marked decrease in that of the adenylates, reaching an 8-fold rise in the ratio of guanylate to adenylate synthesis from IMP at the maximum deviation in the late lag phase at 20 to 24 h after seeding. This striking redirection in the distribution of label from IMP utilization to the preferential synthesis of guanylates during the expression of the biochemical proliferative program of cancer cells supports the potential significance of this pathway as a target of chemotherapy.
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PMID:De novo guanylate synthesis in the commitment to replication in hepatoma 3924A cells. 312 Nov 75

The behavior of the activity of 5-phosphoribosyl 1-pyrophosphate (PRPP) synthetase (ribosephosphate pyrophosphokinase, EC 2.7.6.1) was elucidated in normal rat liver, in 11 hepatomas of different growth rates, and in rapidly growing differentiating and regenerating liver. Tissue extracts were prepared by centrifugation of 10% homogenates at 100,000 X g for 30 min, and enzyme activity was measured in the protein fractions obtained by 40 and 47% ammonium sulfate saturation of the supernatant fluids from livers and hepatomas, respectively. In the tissue extracts, there was no interfering enzyme activity that utilized PRPP under the standard assay conditions. The affinity of PRPP synthetase for its substrates, ribose 5-phosphate and adenosine triphosphate (ATP), and to Mg2+ was similar in liver and hepatoma extracts. The Km for ribose 5-phosphate was 0.3 mM; for ATP, it was 0.1 mM in the presence of excess Mg2+. The Km for Mg2+ ATP was 1.2 mM in the presence of excess ATP. There was no difference in the affinity of the enzyme for its activators, Mg2+ and inorganic phosphate, in liver and hepatoma preparations; the Km for Mg2+ was 0.6 mM in the presence of excess ATP; the Km for inorganic phosphate was 14.0 mM. The requirement of hepatoma extracts for full phosphate saturation was higher than that of liver extracts (85 versus 65 mM). A standard assay was worked out for the liver and hepatoma systems; in liver, the enzyme activity was linear for 30 min incubation, and in hepatoma it was linear for 15 min incubation. PRPP synthetase activity was proportionate with amounts of protein added over a range of 0.4 to 3.0 mg in both liver and hepatoma extracts. In the liver of normal adult Wistar rats, PRPP synthetase activity was 108 +/- 10 nmol/hr/mg protein. In rat tissues of high cell renewal activity, thymus, testis, spleen, and small intestine, synthetase specific activity was 3.7-, 3.6-, 1.2-, and 1.3-fold higher than that of normal liver. The synthetase specific activity in hepatomas of slow growth rate increased 1.2- to 1.5-fold, and in intermediate and rapidly growing hepatomas it was elevated 1.9- to 4.1-fold higher than that of normal liver.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:Increased 5-phospho-alpha-D-ribose-1-diphosphate synthetase (ribosephosphate pyrophosphokinase, EC 2.7.6.1) activity in rat hepatomas. 609 67

The cytotoxicity of 6-thioguanine and 6-mercaptopurine to cultured lymphoblasts and fibroblasts was strongly antagonized by pretreatment of the cells with 100 microM adenosine. Administration of adenosine 2 hours after the antipurine agent did not cause antagonism. In two rat hepatoma cell lines, adenosine pretreatment did not protect cells from the antipurines. Treatment of lymphoblasts or fibroblasts with 100 microM adenosine gave increases up to 150% in cellular ATP and ADP and decreases greater than 80% in UTP and UDP. In the hepatoma lines, adenine nucleotides did not increase by greater than 45%, and uridine nucleotides did not decrease by greater than 40% following adenosine treatment. The selective protection of the normal cells from 6-thioguanine and 6-mercaptopurine was probably the consequence of phosphoribosylpyrophosphate (PRPP) depletion, since adenosine pretreatment decreased PRPP pools by greater than 90% in the normal cells but by only 30% in the malignant hepatoma cells. In the absence of PRPP the antipurines would not be metabolically activated. The selectivity of the adenosine and antipurine combinations was probably attributable to the low activity of adenosine kinase and high activities of adenosine deaminase and PRPP synthetase characteristic of malignant hepatomas.
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PMID:Biochemical approaches to enhancement of antitumor drug selectivity: selective protection of cells from 6-thioguanine and 6-mercaptopurine by adenosine. 616 56

The aminopyrimidopyrimidine nucleoside 4-amino-8-(beta-D-ribofuranosylamino)pyrimido[5,4-d]pyrimidine (APP), which was previously shown to possess experimental antitumor and antiviral activity, was metabolized within WI-L2 human lymphoblastoid cells to a derivative identified as the beta-D-ribonucleotide (APP-MP). In a subline of WI-L2 cells deficient in adenosine kinase, this metabolite was not formed and APP was not cytotoxic, suggesting that APP is converted by adenosine kinase to its 5'-monophosphate. Because no evidence of di- or triphosphates was seen, the monophosphate appeared to be the active species. Treatment of WI-L2 or L1210 cells with APP (10 microM) for 30 min caused extensive depletion of both purine and pyrimidine ribonucleotides. Purine and pyrimidine deoxyribonucleotides were also depleted. Cells were not protected from the cytotoxicity of APP by hypoxanthine plus uridine, but uridine plus adenosine plus 2-deoxycoformycin gave considerable protection. This result was consistent with APP-MP acting as an inhibitor of 5-phosphoribosyl-1-pyrophosphate (PRPP) synthetase, a hypothesis that was confirmed by preparing PRPP synthetase from Novikoff hepatoma cells; APP-MP was a noncompetitive inhibitor, with a Ki of 0.43 mM. APP-MP was found to accumulate in APP-treated cells to a concentration of almost 3 mM. The relevance of PRPP synthetase inhibition to the cytotoxic mechanism of APP is indicated by the fact that depletion of the PRPP pool was seen as early as 15 min after treatment, before any change was apparent in cellular levels of ATP or UTP. DNA synthesis was markedly suppressed within 30 min of APP treatment of WI-L2 cells, and a lesser degree of inhibition of RNA synthesis was apparent after 45 min.
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PMID:Inhibition of 5-phosphoribosyl-1-pyrophosphate synthetase by the monophosphate metabolite of 4-amino-8-(beta-D-ribofuranosylamino)pyrimido[5,4-d]pyrimidine: a novel mechanism for antitumor activity. 768 45