Purine mutants of mammalian cell lines. II. Identification of a phosphoribosylpyrophosphate amidotransferase-deficient mutant of Chinese hamster lung cells

In Silico Structure Modeling and Molecular Docking Analysis of Phosphoribosyl Pyrophosphate Amidotransferase (PPAT) with Antifolate Inhibitors

BACKGROUND

Cancer remains one of the most serious disease worldwide. Robust metabolism is the hallmark of cancer. PPAT (phosphoribosyl pyrophosphate amidotransferase) catalyzes the first committed step of de novo purine biosynthesis. Hence PPAT, the key regulatory spot in De novo purine nucleotide biosynthesis, is an attractive and credible drug target for leukemia and other cancer therapeutics.

OBJECTIVE

In the present study, detailed computational analysis has been performed for PPAT protein, the key enzyme in de novo purine biosynthesis which is inhibited by many folate derivatives, hence we aimed to investigate and gauge the inhibitory effect of antifolate derivatives; lomexterol (LTX) methotrexate (LTX), and pipretixin (PTX) with human PPAT to effectively capture and inhibit De novo purine biosynthesis pathway.

METHODS

The sequence to structure computational approaches followed by molecular docking experiments was performed to gain insight into the inhibitory mode, binding orientation and binding affinities of selected antifolate derivatives against important structural features of PPAT.

RESULTS

Results indicated a strong affinity of antifolate inhibitors for the conserved active site of PPAT molecule encompassing a number of hydrophobic, hydrogen bonding, Vander Waals and electrostatic interactions.

CONCLUSION

Conclusively, the strong physical interaction of selected antifolate inhibitors with human PPAT suggests the selective inhibition of De novo purine biosynthesis pathway by antifolate derivatives towards cancer therapeutics.

Purine biosynthesis in mutant mammalian cells

De novo purine biosynthesis has been studied in lymphocyte cell lines established from Lesch-Nyhan patients deficient in hypoxanthine-guanine phosphoribosyltransferase (HGPRT), in in vitro differentiating erythroleukaemic cell lines cloned from cells charactistic of virus-induced murine leukaemia, and in mutant hamster cells deficient in amidophosphoribosyltransferase. The relationship between cellular phosphoribosylpyrophosphate (PP-ribose-P) metabolism and the activity of the enzymes which catalyse the early steps of de novo purine biosynthesis has been explored. It was found that hamster cells deficient in amidophosphoribosyltransferase did not accumulate PP-ribose-P as do HGPRT-deficient cells. In these model systems, an accelerated rate of de novo purine biosynthesis tended to be associated with an increase in cellular PP-ribose-P cotent, but decreases in this rate results from the reduction in the activity of amidophosphoribosyltransferase. Regulation of ammonia-dependent de novo purine biosynthesis was similar to that of glutamine-dependent purine biosynthesis.

A novel synergistic effect of alanosine and guanine on adenine nucleotide synthesis in mammalian cells. Alanosine as a useful probe for investigating purine nucleotide metabolism

A novel synergistic effect of the antitumor agent alanosine (2-amino-3-(hydroxynitrosoamino) propionic acid), which specifically inhibits the enzyme adenylosuccinate synthetase (ASS) and guanine on the growth of Chinese hamster ovary (CHO) cells and human diploid fibroblasts (HDF) has been observed. In the presence of subinhibitory concentrations of alanosine, both CHO cells and the HDF show excessive sensitivity to exogenous guanine--a phenotype which closely resembles that seen with some of the mutants containing reduced enzymatic activity of ASS. The growth inhibitory effects of alanosine, or alanosine and guanine, on CHO cells are completely reverted by the addition of adenine to the culture medium, and the synergistic effect of guanine is not observed in mutants which lack the enzyme hypoxanthine-guanine phosphoribosyl transferase. These resuls suggest that guanine nucleotides exert a regulatory effect on the activity of the enzyme adenylosuccinate synthetase. The ability to confer the guanine-sensitive phenotype and its modulation by subinhibitory concentrations of alanosine in different cell types indicates that alanosine provides a useful probe for investigating the regulation of purine nucleotide metabolism in mammalian cells.

Inhibition of DNA chain elongation in a purine-auxotrophic mutant of Chinese hamster

DNA synthesis has been examined in a purine-auxotrophic mutant cell line of Chinese hamster (V79 pur 1) under conditions of purine deprivation. At 6 h after the removal of purines from the growth medium, there is a decrease in semiconservative DNA replication. Alkaline velocity centrifugation of the DNA synthesized during a 1-min pulse under conditions of purine deprivation shows that approximately 50% of the newly replicated DNA is the size of Okazaki pieces. These are not incorporated into bulk DNA during a 1-h chase. If the purine supply is restored to the growth medium, these short DNA pieces are jointed to full-sized DNA within 1 h. DNA fiber autoradiolgraphy reveals a retardation in the rate of DNA replication fork movement but no apparent inhibition of initiation of synthesis on replication units within clusters actively engaged in replication. Our results indicate that purine deprivation specifically inhibits elongation of nascent dna chains.

Abnormal regulation of de novo purine synthesis and purine salvage in a cultured mouse T-cell lymphoma mutant partially deficient in adenylosuccinate synthetase

The isolation and characterization of a mutant murine T-cell lymphoma (S49) with altered purine metabolism is described. This mutant, AU-100, was isolated from a mutagenized population of S49 cells by virtue of its resistance to 0.1 mM 6-azauridine in semisolid agarose. The AU-100 cells are resistant to adenosine mediated cytotoxicity but are extraordinarily sensitive to killing by guanosine. High performance liquid chromatography of AU-100 cell extracts has demonstrated that intracellular levels of GTP, IMP, and GMP are all elevated about 3-fold over those levels found in wild type cells. The AU-100 cells also contain an elevated intracellular level of pyrophosphoribosylphosphate (PPriboseP), which as in wild type cells is diminished by incubation of AU-100 cells with adenosine. However AU-100 cells synthesize purines de novo at a rate less than 35% of that found in wild type cells. In other growth rate experiments, the AU-100 cell line was shown to be resistant to 6-thioguanine and 6-mercaptopurine. Levels of hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) measured in AU-100 cell extracts, however, are 50-66% greater than those levels of HGPRTase found in wild type cell extracts. Nevertheless this mutant S49 cell line cannot efficiently incorporate labeled hypoxanthine into nucleotides since the salvage enzyme HGPRTase is inhibited in vivo. The AU-100 cell line was found to be 80% deficient in adenylosuccinate synthetase, but these cells are not auxotrophic for adenosine or other purines. The significant alterations in the control of purine de novo and salvage metabolism caused by the defect in adenylosuccinate synthetase are mediated by the resulting increased levels of guanosine nucleotides.

Biochemical genetics of Chinese hamster cell mutants with deviant purine metabolism: isolation and characterization of a mutant deficient in the activity of phosphoribosylaminoimidazole synthetase

A new purine-requiring mutant of Chinese hamster ovary cells (CHO-Kl) is described. This mutant, Ade-G, grows on aminoimidazole carboxamide, hypoxanthine, or adenine. It complements all eight of our other previously described Ade- mutants. Biochemical analysis of de novo purine synthesis in whole cells suggests that Ade-G is capable of the first four reactions of de novo purine biosynthesis and that it synthesizes and accumulates phosphoribosylformylglycinamidine (FGAM). Direct enzyme assay in cell-free extracts confirms that Ade-G is defective in phosphoribosylaminoimidazole synthetase activity and does not convert FGAM to phosphoribosylaminoimidazole (AIR), the next intermediate in the de novo biosynthetic pathway.

Regulation of de novo purine biosynthesis in normal and 8-azaguanine-resistant Chinese hamster cells

Azaguanine-resistant mutants of Chinese hamster ovary cells were isolated following mutagenesis with ICR-17OG. Of the eight mutant isolates examined, only one, Ag-5 had detectable hypoxanthine(guanine)phosphoribosyltransferase activity. Under normal conditions of growth, de novo purine biosynthesis in the mutants was not significantly different from wild type. However, when the cultures were starved for glutamine over a 2 h period before measuring 5'phosphoribosyl formylglycinamide (a relative measure of de novo purine biosynthesis), elevated levels of 5'-phosphoribosyl formylglycinamide accumulated in some of the mutants, and decreased levels in wild type and Ag-5. The level of purine biosynthesis could be related to the levels of glutamine in the pregrowth medium. The rate of purine biosynthesis correlated with 5-phosphoribosyl pyrophosphate levels, which were enhanced in the mutant (Ag-C) following the starvation period. No alterations were found in levels of 5-phosphoribosyl pyrophosphate synthetase or glutamine synthetase. The extent of feedback inhibition was normal in both mutant and wild type cells. These data suggest that the hypoxanthine (guanine) phosphoribosyltransferase locus is a regulatory gene.

Biochemical genetics of Chinese hamster cell mutants with deviant purine metabolism: characterization of Chinese hamster cell mutants defective in phosphoribosylpyrophosphate amidotransferase and phosphoribosylglycinamide synthetase and an examination of alternatives to the first step of purine biosynthesis

Activities of the first three enzymes in the de novo purine biosynthetic pathway have been measured in cell-free extracts of the Chinese hamster ovary cell (CHO-K1) and two purine-requiring auxotrophs of this cell. Ade-A has been found to be defective in phosphoribosylpryophosphate (PRPP) amidotransferase while Ade-C has been found to be defective in glycinamide ribonucleotide (GAR) synthetase. Neither enzyme deficiency is due to the presence of an excess of diffusible inhibitor, and mixed extracts of Ade-A and Ade-C are capable of performing both enzymatic steps in a coupled assay. Assays of GAR formyltransferase show that it is present in Ade-A and Ade-C, indicating that these cell types are defective in only one enzyme each of the early purine biosynthetic enzymes. Using the Ade-A mutant, analysis of alternatives to PRPP plus glutamine as substrates for the first step in the purine biosynthetic pathway showed that a common genetic unit must direct the synthesis for both PRPP plus glutamine and PRPP plus ammonia activities. Although ribose-5-phosphate plus ammonia can be used in cell-free extracts to perform the first step in purine biosynthesis, it is shown that this activity is apparently not used by intact CHO-K1 cells.

Biochemical genetic analysis of pyrimidine biosynthesis in mammalian cells: I. Isolation of a mutant defective in the early steps of de novo pyrimidine synthesis

The isolation and characterization of a new mutant of Chinese hamster ovary cells is described. This mutant, Urd-A, shows an absolute requirement for exogenously added pyrimidines for growth. Complementation analysis indicates that the lesion in this mutant is recessive. Revertants can be isolated at frequencies suggesting that it is a single gene alteration. Biochemical analysis of cell-free extracts of CHO-K1 (Urd+) and Urd-A revealed that Urd-A possesses no more than 10% of wild-type levels of carbamyl phosphate synthetase (EC 2.7.2.9) activity, no more than 1% of wild-type levels of aspartate transcarbamylase (EC 1.2.3.2) activity, and undetectable levels of dihydroorotase (EC 3.5.2.3) activity. Thus, this mutant appears simultaneously to possess marked or complete deficiencies in the activities of the first three enzymes of pyrimidine biosynthesis. Activities of the other enzymes of the pathway appear normal. The use of this mutant for biochemical-genetic studies of pyrimidine biosynthesis is discussed.

Purine mutants of mammalian cell lines: III. Control of purine biosynthesis in adenine phosphoribosyl transferase mutants of CHO cells

Spontaneous and mutagen-induced 2,6-diaminopurine-resistant mutants of Chinese hamster ovary (CHO-K1) cells were isolated. Such mutants fell into two classes: spontaneous and ethylmethane-sulfonate-induced mutants had approximately 5% wild-type adenine phosphoribosyl transferase (APRT) activity, whereas ICR-170G-induced mutants had barely detectable APRT activity. Since it has been reported that human hypoxanthine-guanine phosphoribosyl transferase (HGPRT) (Lesch-Nyhan syndrome) and APRT mutants over-produce purines, we examined the control and rate of purine biosynthesis in the Chinese hamster mutants. End product inhibition by adenine could not be demonstrated in such mutants, indicating that the active feedback inhibitor is a nucleotide rather than the free purine base, HGPRT activity was normal in all mutants examined except in one isolate. Purine biosynthesis as measured by the accumulation of the purine biosynthetic intermediate phosphoribosyl formylglycineamide was not elevated in the mutants as might have been predicted from work with Lesch-Nyhan cells. The data also suggest that our strain of CHO-K1 is physically or functionally haploid for the APRT locus.

A purine auxotroph deficient in phosphoribosylpyrophosphate amidotransferase and phosphoribosylpyrophosphate aminotransferase activities with normal activity of ribose-5-phosphate aminotransferase

Three enzyme reactions have been reported to catalyze the synthesis of phosphoribosylamine in eukaryotic cells. These activities are glutamine phosphoribosylpyrophosphate (P-Rib-P-P) amidotransferase [amidophosphoribosyl-transferase; 5-phosphoribosylamine: pyrophosphate phosphoribosyltransferase (glutamate-amidating) EC 2.4.2.14], ammonia P-Rib-P-P aminotransferase, and ammonia ribose-5-phosphate aminotransferase. A purine auxotroph derived from a cell line of Chinese hamster fibroblasts was shown to be deficient in catalytic activities of glutamine P-Rib-P-P amidotransferase and ammonia P-Rib-P-P aminotransferase. Extracts from this cell line had normal ammonia ribose-5-phosphate aminotransferase activity. The defect in purine biosynthesis in the mutant cell line was localized to the synthesis of phosphoribosylamine. These results indicate that glutamine P-Rib-P-P amidotransferase or ammonia P-Rib-P-P aminotransferase or both are important for phosphoribosylamine synthesis, but that ammonia ribose-5-phosphate aminotransferase activity probably does not play a significant role in this eukaryotic cell line. The simultaneous disappearance of both P-Rib-P-P-dependent activities suggests these two enzyme activities are closely related structurally or genetically.

Biochemical genetics of Chinese hamster cell mutants with deviant purine metabolism III. Isolation and characterization of a mutant unable to convert IMP to AMP

The isolation and characterization of a new mutant of Chinese ovary cells (CHO-K1) is described. This mutant, Ade-H, has the following properties: (1) it forms a new genetic complementation group; (2) it specifically requires adenine for growth and will not grow on aminoimidazole carboxamide (AIC) or hypoxanthine; (3) it accumulates IMP; (4) it cannot synthesize adenine nucleotides; (5) its phenotype can be mimicked by treatment of CHO-K1 (the wild type parental strain) with hadacidin, an inhibitor of adenylosuccinate synthetase (E.C.6.3.4.4). Thus, the site of the defect in this mutant is presumed to involve the step in adenylate biosynthesis catalyzed by this enzyme. The usefulness of Ade-H for the study of regulation of purine biosynthesis in mammalian cells is discussed.