Isolation of hypoxanthine phosphoribosyltransferase-defective mutants in chinese hamster V79 cells by tritium suicide

Tritium suicide was shown to be highly efficient method for isolating mutants defective in hypoxanthine incorporation in the Chinese hamster lung cell line V79. The tritium suicide procedure consisted of 3 kill cycles. Survivors of one kill cycle were used for the next kill cycle. The kill cycles involved incorporation of [³H]hypoxanthine for 5 or 10 min, followed by storage of ³H-labelled cells at ?70°C for 4–10 days. 12 clones that survived the 3rd kill cycle were tested for incorporation of [³H]hypoxanthine and all were found to be defective. At least 6 of the clones have defective hypoxanthine phosphoribosyltransferase (HPRT) activity. One mutant, H19, chosen for further characterization, had HPRT with a 13-fold elevation in apparent K_m for phosphoribosylpyrophosphate (PRPP). Thin-layer chromatography of cell extracts showed that this mutant was incapable of converting intracellular hypoxanthine to IMP or to other purine metabolites. In addition, H19 was resistant to 6-thioguanine.     

Characterization of HAT- and HAsT-resistant HPRT mutant clones of V79 Chinese hamster cells.

HPRT mutant clones of V79 Chinese hamster cells, isolated after 6-thioguanine (6TG) selection, normally exhibit sensitivity to growth in medium containing the folic acid inhibitor aminopterin or the glutamine analogue L-azaserine (e.g., HAT or HAsT medium). However, it has been shown that some HPRT- clones are resistant to both HAT and HAsT medium. The present study was undertaken to investigate whether any common structural gene alteration exists for such 6TGr-HATr-HAsTr clones. Four clones were studied, 1 of spontaneous origin, 2 induced by a low dose of MNU and 1 EMS-induced. In contrast to wild-type cells and a mutant clone carrying a complete deletion of the HPRT gene, these 4 investigated 6TGr-HATr-HAsTr clones all showed an enhanced incorporation of exogenous 3H-hypoxanthine in the presence of aminopterin and L-azaserine suggesting that these clones carry mutations in the structural part of the HPRT gene. Sequence analysis of PCR-amplified HPRT cDNA from these mutants showed that the spontaneous and the 2 MNU-induced mutant clones lacked exon 4, while the EMS-induced mutant had a GC to AT transition in exon 6. Southern blot analysis of genomic DNA after digestion with BglII, EcoRI and PstI showed no changes in fragment patterns as compared to the wild type. Further sequence analysis of PCR-amplified genomic DNA using exon 4-specific primers showed that all these 3 mutants had an AT to GC or GC to AT transition in exon 4, but had no alterations in the splice sites of exon 4. Based on their characteristics of hypoxanthine incorporation, the present mutant clones fit the model for the proposed functional domains of the HPRT protein.     

Intraspecies transfer via total cellular DNA of the gene for hypoxanthine phosphoribosyltransferase into cultured mouse cells

Summary Under selective growth conditions a revertant of mouse cells, defective in hypoxanthine phosphoribosyltransferase activity (HPRT, EC-No. 2.4.2.8), was isolated, which contained an electrophoretically abnormal form of HPRT activity. The specific HPRT activity in crude extracts of the revertant cells is about 30% of the level determined in normal wild type cells. The variant HPRT reacts with antiserum against normal mouse HPRT but the rate of heat inactivation of the variant activity is different from the wild type form. By isozyme and karyotype analyses of somatic cell hybrids between the revertant mouse cells and Chinese hamster cells we found that the abnormal HPRT activity is coded for by the mouse X-chromosome as expected for a mutation in the structural HPRT gene.DNA has been purified from the abnormal HPRT revertant cells and incubated with mouse A9 cells (HPRT^-). After growth in selective medium one clone was isolated which expressed the electrophoretically abnormal form of HPRT. Six clones showed the normal form of HPRT due to reversion of the defective HRRT locus in A9 cells. This result indicates DNA-mediated transfer of the mouse HPRT gene at a frequency of about 0.5×10^-7. A similar frequency has been found for transfer of the variant HPRT locus via isolated metaphase chromosomes to A9 recipient cells. When placed in non-selective media the DNA-mediated transferent cells gradually lost their ability to express the HPRT transgenome at a rate of about 6% per average cell generation.     

Hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency: Identification of point mutations in Japanese patients with Lesch-Nyhan syndrome and hereditary gout and their permanent expression in an HPRT-deficient mouse cell line

Two different single nucleotide transitions of hypoxanthine-guanine phosphoribosyltransferase (HPRT) were identified in a Japanese patient with Lesch-Nyhan syndrome (LNS) and a patient with hereditary gout. HPRT enzyme activities in the two patients were severely deficient, but the size and amount of mRNA were normal according to Northern analysis. Entire coding regions of HPRT cDNAs were amplified by PCR and sequenced. A G-to-A substitution at base 208 in exon 3, which predicted glycine 70 to arginine, was detected in the LNS patient (identical mutation with HPRT_Utrecht). A C-to-A substitution at base 73 in exon 2, which predicted proline 25 to threonine, was detected in the gout patient (designated HPRT_Yonago). We transfected normal HPRT cDNA, mutant cDNA with HRPT_Utrecht or mutant cDNA with HPRT_Yonago, respectively, to HPRT-deficient mouse cells and isolated permanent expression cell lines. The HPRT-deficient mouse cells had no detectable HPRT activity and a very low amount of HPRT mRNA. When the HPRT-deficient mouse cells were transfected with normal human cDNA, HPRT enzyme activity increased to 21.8% that of normal mouse cells. The mouse cells transfected with HPRT_Utrecht showed no increase in HPRT activity; however, when the mouse cells were transfected with HPRT_Yonago, the activity increased to 2.4% that of normal activity. The proliferative phenotypes of these cells in HAT medium and in medium containing 6-thioguanine were similar to those of skin fibroblasts from the patients. This series of studies confirmed that each of the two point mutations was responsible for the decreases in HPRT enzyme activity, and the proliferative phenotypes in HAT medium and medium containing 6-thioguanine.     

The detection of promutagen activation by extracts of cells expressing cytochrome P450IA2 cDNA: preincubation dramatically increases revertant yield in the Ames test

Two slightly different protocols, the plate incorporation method and the preincubation method, are used in the Ames Salmonella mutagen test. Using a preincubation method, we recently demonstrated efficient activation of a number of food-derived promutagens by extracts of mammalian cells expressing cDNAs of rat-liver cytochrome P450IA2 and of a P450IA2-IA1 hybrid. We report here that, for 2-amino-3,4-dimethylimidazo[4,5-ƒ]quinoline (MeIQ), 1-aminoanthracene and several other promutagens, preincubation dramatically increased the number of revertant colonies in the Ames test when extracts of cytochrome P450IA2-containing transfected cells or low concentrations of rat-liver extracts were used as the source of activating enzymes. At higher concentrations of rat-liver extract protein, the effect of preincubation was less pronounced. The effect of preincubation was not due to the low protein concentrations in the assays since increasing the total protein concentration did not abolish the requirement for preincubation for the detection of MeIQ activation at low concentrations of rat-liver extract. In experiments where P450IA2 synthesized in transfected cells in culture is used to study promutagen activation, the plate incorporation protocol may seriously underestimate the capacity of cell extracts to activate promutagens. Thus, interlaboratory comparisons become difficult and unnecessarily large quantities of cell extract protein may be needed to detect promutagen activation. Whenever Ames test assays are carried out under conditions where P450 concentration limits revertant yield, it would be prudent to examine both the preincubation and plate incorporation protocol.     

The detection of promutagen activation by extracts of cells expressing cytochrome P450IA2 cDNA: preincubation dramatically increases revertant yield in the Ames test.

Two slightly different protocols, the plate incorporation method and the preincubation method, are used in the Ames Salmonella mutagen test. Using a preincubation method, we recently demonstrated efficient activation of a number of food-derived promutagens by extracts of mammalian cells expressing cDNAs of rat-liver cytochrome P450IA2 and of a P450IA2-IA1 hybrid. We report here that, for 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ), 1-aminoanthracene and several other promutagens, preincubation dramatically increased the number of revertant colonies in the Ames test when extracts of cytochrome P450IA2-containing transfected cells or low concentrations of rat-liver extracts were used as the source of activating enzymes. At higher concentrations of rat-liver extract protein, the effect of preincubation was less pronounced. The effect of preincubation was not due to the low protein concentrations in the assays since increasing the total protein concentration did not abolish the requirement for preincubation for the detection of MeIQ activation at low concentrations of rat-liver extract. In experiments where P450IA2 synthesized in transfected cells in culture is used to study promutagen activation, the plate incorporation protocol may seriously underestimate the capacity of cell extracts to activate promutagens. Thus, interlaboratory comparisons become difficult and unnecessarily large quantities of cell extract protein may be needed to detect promutagen activation. Whenever Ames test assays are carried out under conditions where P450 concentration limits revertant yield, it would be prudent to examine both the preincubation and plate incorporation protocol.     

Enumeration of 6-thioguanine-resistant tumour cells using flow cytometry and comparison with a microtitration cloning assay

Measurement of mutant frequency in tumour specimens has been hampered by low cloning efficiency in soft agar. A method was developed to detect cell proliferation using the thymidine analogue 5-bromo-2'-deoxyuridine (BrUdR). BrUdR incorporation was monitored by immunofluorescent staining of fixed cells using a monoclonal antibody highly specific for this nucleoside analogue. The 6-thioguanine (6TG) exposure conditions which inhibited DNA synthesis, as measured by BrUdR incorporation, in wild-type cells while allowing proliferation of spontaneous hypoxanthine-guanine phosphoribosyltransferase (HPRT) mutants were investigated using tumour cell lines. It was shown that exposure to 10(-5) M BrUdR for the equivalent of 1 cell cycle time did not affect growth of wild-type cells, nor did it affect the growth of HPRT- mutants in the presence of 6TG. Methods for rapid flow cytometric enumeration of BrUdR-labelled 6TG-resistant cells were developed using fluorescent microspheres as an internal standard. To validate the BrUdR mutation assay, the 6TG mutant frequency (MF) was measured in L1210 R/S, a mouse leukaemic cell line (BrUdR 6TG MF = 7.0 X 10(-5] and the results directly compared with those from a microtitration cloning assay (MF = 4.6 X 10(-5]. The results were similar and within the range reported for HPRT MF in mammalian cells.     

Transformation of the gene for hypoxanthine phosphoribosyltransferase.

Purified DNA from wild-type Chinese ovary (CHO) cells has been used to transform three hypoxanthine phosphoribosyltransferase (HPRT) deficient murine cell mutants to the enzyme positive state. Transformants appeared at an overall frequency of 5 x 10(-8) colonies/treated cell and expressed CHO HPRT activity as determined by electrophoresis. One gene recipient, B21, was a newly isolated mutant of LMTK- deficient in both HPRT and thymidine kinase (TK) activities. Transformation of B21 to HPRT+ occurred at 1/5 the frequency of transformation to TK+; the latter was, in turn, an order of magnitude lower than that found in the parental LMTK- cells, 3 x 10(-6). Thus both clonal and marker-specific factors play a role in determining transformability. The specific activity of HPRT in transformant extracts ranged from 0.5 to 5 times the CHO level. The rate of loss of the transformant HPRT+ phenotype, as measured by fluctuation analysis, was 10(-4)/cell/generation. While this value indicates stability compared to many gene transferents, it is much greater than the spontaneous mutation rate at the indigenous locus. The ability to transfer the gene for HPRT into cultured mammalian cells may prove useful for mutational and genetic mapping studies in this well-studied system.     

Genotoxicity of 3'-azido-3'-deoxythymidine in the human lymphoblastoid cell line, TK6: relationships between DNA incorporation, mutant frequency, and spectrum of deletion mutations in HPRT.

Perinatal treatment with 3'-azido-3'-deoxythymidine (AZT) has been found to reduce the rate of maternal-infant transmission of HIV; however, AZT is genotoxic in mammalian cells in vitro and induces tumors in the offspring of mice treated in utero. The purpose of the present study was to investigate the relationships between incorporation of AZT into DNA, and the frequency and spectrum of mutations at the HPRT locus of the human lymphoblastoid cell line, TK6, following in vitro exposures to AZT. Cells were cultured in medium containing 0 or 300 microM AZT for 1, 3, or 6 day(s) (n = 5/group). The effects of exposure duration on incorporation of AZT into DNA and HPRT mutant frequency were determined using an AZT radioimmunoassay and a cell cloning assay, respectively. AZT accumulated in DNA in a supralinear manner, approaching a plateau at 6 days of treatment (101.9 +/- 14.7 molecules AZT/10(6) nucleotides). After 3 days of AZT exposure, HPRT mutant frequency was significantly increased (1.8-fold, p = 0.016) compared to background (mutant frequency = 3.78 x 10(-6)). Multiplex PCR amplification of genomic DNA was used to determine the frequency of exon deletions in HPRT mutant clones from untreated cells versus AZT-treated cells. Molecular analyses of AZT-induced mutations revealed a significant difference in the frequency of total gene deletions (44/120 vs. 18/114 in controls, p = 0.004 by the Mann-Whitney U-statistic). In fact, the Chi-square test of homogeneity demonstrate that the differences between the control and AZT-treatment groups is attributed mainly to this increase in total gene deletion mutations (p = 0.00001). These data indicate that the primary mechanism of AZT mutagenicity in human TK6 cells is through the production of large deletions which occur as a result of AZT incorporation into DNA and subsequent chain termination. The data imply that perinatal chemoprophylaxis with AZT may put children of HIV-infected women at potential risk for genetic damage.     

Studies on cell communication with enucleated human fibroblasts

Metabolic cooperation, the correction of the mutant phenotype in cells deficient in hypoxanthine phosphoribosyltransferase (HPRT-) by intimate contact with normal cells (HPRT+), represents a form of cell communication that is easily studied with radioautography. In the present study it was found that the formation of cell junctions needed for communication does not require protein synthesis nor is it under the immediate control of the cell nucleus. Enucleated normal cells efficiently communicate with HPRT- mutant cells. The effectiveness of enucleated cells as donors in metabolic cooperation provides evidence that it is the transfer of small molecules, nucleotide, or nucleotide derivatives that is responsible for correction of the mutant phenotype. Karyoplasts (nuclei with small amounts of cytoplasm surrounded by a plasma membrane) are unable to efficiently communicate with intact cells. The utilization of [3H]hypoxanthine by communicating mixtures of HPRT+ and HPRT- human cells is not significantly different than in the normal cells alone. Metabolic cooperation, as studied involves a redistribution of purine-containing compounds among communicating cells.     

Lesch-Nyhan mutation: the influence of population density on purine phosphoribosyltransferase activities and exogenous purine utilization in monolayer cultures of skin fibroblasts

In extracts of normal and Lesch-Nyhan (LN) heterozygous skin fibroblast monolayer cultures, hypoxanthine-guanine (H-G) and adenine (A) phosphoribosyltransferase (PRT) activities are correlated. These activities vary concomitantly during the life cycle of a culture: Peak activities occur during exponential growth. The ratio of H-GPRT to APRT activity can manifest heterozygosity at the LN locus when H-GPRT activity, per se, is close to and unreliable different from the normal range. If 2 or 8 × 10⁴ cells are planted per 35 mm petri dish, a strain with clearly heterozygous levels of H-GPRT activity in its cell extract may not reveal its genotype by simultaneous measurement of adenine and hypoxanthine incorporation into its acid-insoluble fraction one day after subculture. In contrast, a 1:1 coculture of normal and mutant cells yields the ratio of adenine to hypoxanthine incorporation expected from the behavior of each cell type separately. In heterozygous cultures at the higher population density, the incorporation of hypoxanthine relative to adenine is 28% greater than in those at the lower population density. A 1:1 mixture of normal and mutant cells exhibited a 20% increase in the relative incorporation of hypoxanthine over the same four fold increase in population density. These observations appear to provide a quantitative expression of the phenomenon of metabolic cooperation between contacting H-GPRT-negative and H-GPRT-positive cells.     

Uneven distributions of naïve and memory T cells in the CD4 and CD8 T-cell populations derived from a single stem cell in an atomic bomb survivor: implications for the origins of the memory T-cell pools in adulthood

The processes that lead to the establishment and maintenance of memory T-cell pools in humans are not well understood. In this study, we examined the emergence of na?ve and memory T cells in an adult male who was exposed to an atomic bomb radiation dose of approximately 2 Gy in 1945 at the age of 17. The analysis presented here was made possible by our earlier observation that this particular individual carries a hematopoietic stem cell mutation at the hypoxanthine phosphoribosyltransferase (HPRT) locus that is almost certainly a result of his exposure to A-bomb radiation. Our key finding is that we detected a very much higher HPRT mutant frequency in the naive (CD45RA(+)) cell component of this individual's CD4 and CD8 T-cell populations than in the memory (CD45RA(-)) cell component of his CD4 and CD8 T-cell populations. This stands in marked contrast to our finding that HPRT mutant frequencies are fairly similar in the na?ve CD45RA(+) and memory CD45RA(-) components of the CD4 and CD8 T-cell populations of three unexposed individuals examined concurrently. In addition we found that the HPRT mutant frequencies were about 30-fold higher in the na?ve (CD45RA(+)) CD4 T cells of the exposed individual than in his memory (CD45RA(-)) cell populations, but that the effect was a little less striking in his CD8 cell populations, where the HPRT mutant frequencies were only about 15-fold higher in his na?ve T-cell pools than in his memory T-cell pools. We further found that 100% of the HPRT mutant cells in both his CD4 and CD8 na?ve cell subsets appeared to have originated from repeated divisions of the initial HPRT mutant stem cell, whereas only 4 of 24 and 5 of 6 mutant cells in his CD4 and CD8 memory cell subsets appeared to have originated from that same stem cell. The most straightforward conclusion may be that the great majority of the T cells produced by this individual since he was 17 years old have remained as na?ve-type T cells, rather than having become memory-type T cells. Thus the T cells that have been produced from the hematopoietic stem cells of this particular A-bomb-exposed individual seldom seem to enter and/or to remain in the memory T-cell pool for long periods. We speculate that this constraint on entry into memory T-cell pools may also apply to unirradiated individuals, but in the absence of genetic markers to assist us in obtaining evidential support, we must await clarifying information from radically different experimental approaches.     

Characterization of DNA synthesis and DNA-dependent ATPase activity at a restrictive temperature in temperature-sensitive tsFT848 cells with thermolabile DNA helicase B.

A temperature-sensitive mutant defective in DNA replication, tsFT848, was isolated from the mouse mammary carcinoma cell line FM3A. In mutant cells, the DNA-dependent ATPase activity of DNA helicase B, which is a major DNA-dependent ATPase in wild-type cells, decreased at the nonpermissive temperature of 39 degrees C. DNA synthesis in tsFT848 cells at the nonpermissive temperature was analyzed in detail. DNA synthesis measured by incorporation of [3H]thymidine decreased to about 50% and less than 10% of the initial level at 8 and 12 h, respectively. The decrease in the level of thymidine incorporation correlated with a decrease in the number of silver grains in individual nuclei but not with the number of cells with labeled nuclei. DNA fiber autoradiography revealed that the DNA chain elongation rate did not decrease even after an incubation for 10 h at 39 degrees C, suggesting that initiation of DNA replication at the origin of replicons is impaired in the mutant cells. The decrease in DNA-synthesizing ability coincided with a decrease in the level of the DNA-dependent ATPase activity of DNA helicase B. Partially purified DNA helicase B from tsFT848 cells was more heat sensitive than that from wild-type cells. Inactivation of DNA-dependent ATPase activity of DNA helicase B from mutant cells was considerably reduced by adding DNA to the medium used for preincubation, indicating that the DNA helicase of mutant cells is stabilized by binding to DNA.     

Gene amplification as a mechanism of reversion at the HPRT locus in V79 Chinese hamster cells

Spontaneous phenotypic revertants of hypoxanthine phosphoribosyl-transferase (HPRT) temperature-sensitive V79 Chinese hamster cells were selected by plating a temperature-sensitive mutant in HAT medium at 39 degrees C. The incidence of such revertants was approximately 2 X 10(-4) per cell. The majority of the revertants examined had increases of between three- and tenfold in their specific activity of the enzyme, and they were able to grow continuously in the presence of HAT medium at 39 degrees C. When the revertants were cultivated in the absence of HAT, they recovered their HAT-sensitive phenotype and their lowered level of HPRT. Three of the revertants were examined for their temperature inactivation profiles, and all were found to have profiles identical to the ts parent, and quite different from the V79 wild type. The kinetic properties of the cell lines were studied: the Km for both PRPP and hypoxanthine was significantly different in the temperature-sensitive cells but was not significantly altered in the revertants with respect to the ts mutants. A specific antibody to Chinese hamster brain HPRT was employed in immunoprecipitation experiments. By measuring the point at which the immunoprecipitation of the antibody to HPRT was overcome by increasing concentrations of cell supernatant, it was possible to estimate the relative amount of enzyme molecules in the cell lines. From these data, it could be concluded that the revertants overproduced an enzyme with the same immunological properties as the ts line. Southern blots of the Hind III restricted DNA from the ts mutant and two revertant cell lines were examined with an HPRT cDNA probe. This established that the HPRT gene was amplified twofold in one of the revertants, and threefold in the other. However, if the revertants were reintroduced into nonselective medium, the gene copy number declined to one. Finally, northern blots of RNA extracted from the various cell lines demonstrated that the HPRT mRNA was augmented 1.5-fold in one revertant and 1.4-fold in the other. Reintroduction into non-selective medium resulted in a decline in mRNA level for the second mutant, whereas the first mutant appeared to be stabilized. We conclude that gene amplification and concomitant amplification of messenger RNA and enzyme levels are mechanisms of phenotypic reversion at the HPRT locus in Chinese hamster cells.     

Analysis of mutational effects at the HPRT locus in human G(0) phase lymphocytes irradiated in vitro with gamma rays

The mutational effects of ionising radiation at the hypoxanthine-guanine phosphoribosyl transferase (HPRT) locus were studied in human peripheral blood G(0) phase lymphocytes irradiated in vitro with gamma rays. The presence of radiation induced mutants was assessed by selecting the HPRT mutants every week on the basis of 6-thioguanine resistance up to 1 month after irradiation. A dose-related increase of 14.25x10(-6) mutants/Gy was measured after an expression time of 7 days. After 2 weeks from culture starting the fraction of clonable cells in irradiated and control cell populations decreased, limiting the measurements of mutant frequency. The mutational spectrum of the HPRT gene was determined by PCR analyses in a total of 99 mutant clones derived from irradiated lymphocytes. The independent origin of mutant clones carrying the same mutation was assessed by analysing the TCR gamma gene rearrangements. The results showed a dose-related increase of deletion mutants up to 3Gy, whereas point mutation frequency increased only up to 2Gy. Two preferentially deleted regions were identified; one involving the HPRT exon 3, and another one the 3'-terminal and the 3'-flanking region of the gene. One complex mutation involving a non-contiguous deletion of exons 2-5 and 7/8 was observed among the mutants isolated after 3Gy irradiation.     

Endogenous non-cyclooxygenase metabolites of arachidonic acid modulate growth and mRNA levels of immediate-early response genes in rat mesangial cells

The role of endogenous arachidonic acid and its metabolites as mediators of cell growth was studied in rat mesangial cells. Inhibitors of the cytochrome P450 monooxygenase and lipoxygenase systems (nordihydroguaiaretic acid (NDGA), SK&F 525A, and ketoconazole) significantly reduced serum-stimulated cell growth as determined by cell counts and incorporation of [3H]thymidine. Inhibition of cyclooxygenase or lipoxygenases alone had no effect on cell growth. Stimulation with arginine vasopressin, epidermal growth factor, or phorbol myristate acetate increased [3H]thymidine incorporation and mRNA levels of the immediate-early response genes c-fos and Egr-1. These increases in [3H]thymidine incorporation and mRNA levels were reduced by NDGA and ketoconazole. NDGA, SK&F 525A, and ketoconazole had no effect on cellular ATP levels. Indomethacin had no effect upon cell growth. 14,15-Epoxyeicosatrienoic acid potentiated the effect of arginine vasopressin to enhance [3H]thymidine incorporation. Reverse-phase high pressure liquid chromatography analysis of lipid extracts from cells prelabeled with [3H]arachidonic acid resulted in the detection of a radioactive peak which eluted with lipoxygenase and monooxygenase products, with the same retention time as vicinal dihydroxyeicosatrienoic acids. This peak increased after stimulation with arginine vasopressin or epidermal growth factor and was reduced by preincubation with NDGA. Furthermore, analysis of unlabeled cell extracts by gas chromatography-mass spectrometry revealed the presence of a compound with epoxyeicosatrienoic acid-like characteristics. These results indicate that mesangial cells in culture likely produce products of the cytochrome P450 monooxygenase system that are important endogenous mediators of the growth response to mitogenic agents.     

Peptide Synthesis by Extracts from Bacillus subtilis Spores

Cell-free peptide synthesis by extracts from vegetative cells and spores of Bacillus subtilis was analyzed and compared. The initial rate of phenylalanine incorporation in a polyuridylate-directed system was found to be in a similar range for the two extracts. However, spore extracts frequently incorporated less total phenylalanine as did the vegetative cell system. Optimal conditions for amino acid incorporation by spore extracts were found to be similar to those of vegetative cell extracts. Polyphenylalanine synthesis was stimulated by preincubation of both extracts prior to the addition of polyuridylic acid (poly U) and labeled phenylalanine. Both systems showed a dependence on an energy-generating system and were inhibited by chloramphenicol and puromycin. Ribonuclease, but not deoxyribonuclease, inhibited the reaction significantly. The presence of methionine transfer ribonucleic acid (tRNA(F)) and methionyl-tRNA(F) transformylase was demonstrated in spore extracts. An analysis of several aminoacyl-tRNAs in spores revealed that the relative amounts of these tRNAs were similar to those found in vegetative cells. Only lysine tRNA was found to be present in relatively greater amounts in spores. These results indicate that dormant spores of B. subtilis contain the machinery for the translation of genetic information.     

Basis for differential cellular sensitivity to 8-azaguanine and 6-thioguanine

Cellular resistance to the cytotoxic purine analogues 8-azaguanine (AG) and 6-thioguanine (TG) is usually mediated by a mutation leading to the loss or reduction in hypoxanthine phosphoribosyltransferase (HPRT) activity. However, stable AG-resistant variants have often been shown to contain wild-type levels of HPRT, while cellular resistance to TG is always accompanied by a profound deficiency in HPRT activity. Such AG-resistant, HPRT-positive cells are still sensitive to TG. To investigate the basis of this differential sensitivity, we examined the inhibition of the HPRT activity by AG and TG in whole cells, in cell-free extracts, and with purified mouse HPRT. In addition, the relative incorporation and utilization of AG and TG by L929 cells were determined under a variety of culture conditions. Results show that, compared to TG, AG is generally a very poor substrate for HPRT. Incorporation of radioactive AG by HPRT-positive cells was extremely sensitive to the free purine concentrations in the medium, so that under the usual culture conditions employing undialyzed serum, cellular uptake and utilization was minimal even when relatively high levels of AG were present. In contrast, the incorporation of radioactive TG was comparable to that of a natural substrate, hypoxanthine. These results indicate that the differential cellular sensitivity to AG and TG is due to the difference between these two guanine analogues as substrates of HPRT. Additional data indicate also that cellular resistance to TG is mediated exclusively by HPRT deficiency, but resistance to very high levels of AG may result through at least two other mechanisms not involving HPRT deficiency. These observations may help resolve some of the conflicting data in the literature, and demonstrate that TG is a better selective agent for the HPRT-deficient phenotype.     

Regulation of purine de novo synthesis in cultured human fibroblasts: the role of P-ribose-PP.

Procedures for assaying the rate of purine de novo synthesis in cultured fibroblast cells have been compared. These were (i) the incorporation of [(14)C]-glycine or [(14)C]formate in alpha-N-formylglycinamide ribonucleotide (an intermediate in the purine synthetic pathway) and (ii) the incorporation of [(14)C]-formate into newly synthesised cellular purines and purines excreted by the cell into the medium. Fibroblast cells, derived from patients with a deficiency of hypoxanthine phosphoribosyltransferase (HPRT-) (EC 2.4.2.8) and increased rates of purine de novo synthesis, were compared with fibroblasts from healthy subjects (HPRT+). Fetal calf serum, which was used to supplement the assay and cell growth medium, was found to contain sufficient quantities of the purine base hypoxanthine to inhibit purine de novo synthesis in HPRT+ cells. This inhibition was the basis of differentiation between HPRT- and HPRT+ cells. In the absence of added purine base, both cell types had similar capacities for purine de novo synthesis. This result contrasts with the increased rates of purine de novo synthesis reported for a number of human HPRT- cells in culture but conforms recent studies made on human HPRT- lymphoblast cells. The intracellular concentration and utilisation of 5-phosphoribosyl-1-pyrophosphate (P-Rib-PP), a substrate and potential controlling factor for purine de novo synthesis, were determined in HPRT- and HPRT+ cells. The rate of utilisation of P-Rib-PP in the salvage of free purine bases was far greater than that in purine de novo synthesis. Although HPRT- cells had a 3-fold increase in P-Rib-PP content, the rate of P-Rib-PP generation was similar to HPRT+ cells. Thus, in fibroblasts, the concentration of P-Rib-PP appears to be critical in the control of de novo purine synthesis and its preferential utilisation in the HPRT reaction limits its availability for purine de novo synthesis. In vivo, HPRT+ cells, in contrast to HPRT- cells, may be operating purine de novo synthesis at a reduced rate because of their ability to reutilise hypoxanthine.