O6-methylguanine-DNA methyltransferase and human cancer chemotherapy

Two kinds of human tumor cell strains having different activity of O6-methylguanine-DNA methyltransferase (O6-MT) were transplanted into nude mice. Then the mice were injected intraperitoneally with bifunctional alkylating agent 1-(4-amino-2-methyl-5-pyrimidinyl) methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (ACNU). The tumors with low O6-MT activity were quickly suppressed or cured. The result suggests that some tumors, if provisionally determined with low O6-MT activity, might be efficiently cured by treatment with ACNU. This probably opens a new way for human cancer chemotherapy.

Conformational transition of fructose-1,6-bisphosphatase: structure comparison between the AMP complex (T form) and the fructose 6-phosphate complex (R form)

A structure of the neutral form of fructose-1,6-bisphosphatase complexed with AMP has been determined by the molecular replacement method and refined at a 2.5-A resolution to a crystallographic R factor of 0.169. The root-mean-square errors of the structure from standard geometry are 0.013 A for bond lengths and 2.99 degrees for bond angles. Comparison of the AMP complex with the F6P complex shows that dimer C3-C4 twists about 19 degrees about a molecular 2-fold axis when dimers C1-C2 of the R and T forms of the enzyme are superimposed one another and that a slight shift of about 1 A of the AMP domain partially compensates this twist. The R to T transition of the enzyme does not significantly change the conformation of the F6P-binding site. However, residues at the divalent metal site and the AMP site show significant positional shifts. If these results can be extended to substrate in place of F6P, they suggest that regulation of the enzyme by AMP may occur partly through effects on metal-ion affinity or position. AMP binds to the same sites of the T and R forms, but only half-occupancy was observed in the alkaline R form. Sequential binding of AMP, at least in pairs, is suggested as the unligated R form is converted to the T form. Two possible pathways are suggested for allosteric communication over about 28 A between the AMP site and the active site: one via helices H1, H2, and H3 and another via the eight-stranded beta-sheet.(ABSTRACT TRUNCATED AT 250 WORDS)

Crystal structure of the neutral form of fructose-1,6-bisphosphatase complexed with the product fructose 6-phosphate at 2.1-A resolution

The crystal structure of fructose-1,6-bisphosphatase (EC 3.1.3.11) complexed with the product fructose 6-phosphate (F6P) has been refined at 2.1-A resolution to an R factor of 0.177 with root-mean-square deviations of 0.014 A and 2.9 degrees from the ideal geometries of bond lengths and bond angles, respectively. The secondary structures but not the trace of the unligated enzyme have been slightly revised in the F6P complex at this higher resolution. Helix H4 in the unligated structure has been refined to a helix-like coil, and two very short 3(10) helices have been found, one in H4 and one in H5. F6P at 10 mM concentration in the absence of divalent metals in our study shows major binding at the active site and minor binding at the AMP site. The major site has almost equal full occupancy in the C1 and C2 chains of the crystallographic asymmetric unit, while the minor site shows occupancy only in the C1 chain at about 50%. The electron density in both (2Fo - Fc) and (Fo - Fc) maps calculated by omitting F6P slightly favors the beta anomer of D-F6P over the alpha anomer. Possible functions of the active-site residues are discussed, and candidates are suggested for site-directed mutagenesis.

Determination of eight elements in six human cancer cell lines and two human normal cell lines by PIXE

Gastric, hepatic, and pulmonary cancer cell lines, and the third passage of normal gastric and pulmonary cell lines were analyzed by proton induced X-ray emission (PIXE) method. The contents of element Sr, Ca, Fe, Zn, P, K, Cu, and As in the cell lines were determined. The Sr, Ca, Fe, Zn, and As contents in cancer cell lines were significantly lower than those in the normal cell lines (p less than 0.05), whereas there were no significant differences for the P, K, and Cu contents (p greater than 0.1). The results suggest that the need of some essential elements has been diminished in cancer cell proliferation.

[Anaerobic infection of burns]

To analyze anaerobic infection 158 specimens were obtained from deep necrotic tissues and exudates under burned subeschar, secretion of eyes and blood. The results showed that more anaerobes were isolated from deep necrotic tissues (46.88%) and smelly exudates of subeschar (38.71%). Most of them were mixed with aerobes (97.1%). In 43 strains of anaerobes, Clostridium accounted for 18 strains (41.86%), and Bacteroides for 17 strains (39.53%). Fusiform nucleatum. Peptococcus, and Veillonell paryula were also differentiated. The recent total detectable rate of anaerobes (38.71%) from burn wounds was higher significantly than that of 1980-1983 (14.7%). It may be due to the exact selection of specimens and modification of cultural medium and equipment.

[Parenteral magnesium sulfate exerts no central anticonvulsant action]

It was said that parenteral magnesium (MS) had a central anticonvulsant action responsible for controlling seizures in eclampsia of pregnancy. The present study was carried out to examine this statement. In conscious rabbits, MS 214 mg.kg-1 iv quickly relieved them from convulsion induced by sc pentylenetetrazol, yet spikes of high frequency in electrocorticogram burst inceasingly. Judging from electrocorticogram changes, MS 220 mg.kg-1 iv did not raise the electroshock seizure threshold. In mice, MS 430 mg.kg-1 did not significantly increase the LD50 of ip pentylenetetrazol. However, in anesthetized rats, MS 250 mg.kg-1 ip lowered the hypertensive response to angiotensin amide and norepinephrine. These results indicated that parenterally administered MS exerted no demonstrable central anticonvulsant action, and its benefits gained in the treatment of eclampsia of pregnancy might derive from its peripheral action, such as reduction of vascular response to pressor substances.

Embryotoxicity and teratogenicity of DL111-IT, an early pregnancy-terminating agent, in the subsequent gestation following administration in rats

DL 111-IT is a new non-hormonal early pregnancy-terminating agent. The subsequent embryotoxic and teratogenic effects of DL111-IT were studied in the rats whose first pregnancy had been terminated by the agent. The secondary pregnancy of the treated rats were allowed to initiate at 30-, 45-, and 90-day intervals, respectively, after the termination of the first pregnancy. The toxicities primed with DL111-IT, such as resorbed embryos and nephrohydrosis fetuses, were found to be 8.0% and 8.5%, respectively, in the rats with pregnancy at 30-day interval. This is significantly different from 4.0% resorbed embryos and 2.4% nephrohydrosis fetuses in vehicle control (P less than 0.05). However, the rate of resorbed embryos decreased to 7.0% or 6.0% in the rats with pregnancy at 45- or 90-day intervals, respectively. No other significant embryotoxicity and teratogenicity were observed in those rats initiating their pregnancy 45 days later after first-pregnancy termination by DL111-IT. Thus, DL111-IT appears to have very low subsequent toxicity in the course of rat pregnancy cycles.

[4-aminopyridine induced rage reaction in mice]

Rage reaction was induced in mice by sc 4-aminopyridine (4-AP) 6 mg . kg-1. Mice appeared hyperreactive after 8-12 min and then squeaked and fought each other. These manifestations were most distinct in 10-30 min and subsided after 40-60 min. The occurrence of rage reaction on this dose level was around 90%. At higher doses 4-AP caused convulsions and death after evocation of rage reaction. The ED50 of 4-AP for eliciting rage reaction was 4.7 +/- 0.7 mg . kg-1 sc. No significant difference in induction of rage reaction was seen between male and female mice of different body weights. Both neuroleptic drugs (chlorpromazine, haloperidol, tarden and clozapine) and anxiolytic drugs (diazepam, chlordiazepoxide, and meprobamate) inhibited 4-AP-induced rage reaction in mice. Barbiturates, Chloral hydrate, methaqualone, morphine hydrochloride, aspirin, phenytoin sodium, diphenhydramine hydrochloride, atropine sulfate, and procaine hydrochloride did not affect rage reaction. The 4-AP-induced aggressive behavior, similar to that induced by electric footshock or isolation, has the merits of convenience to deal with and time saving. Hence we recommended it as a screening method for drugs with neuroleptic and anxiolytic activities.

The termini of the chlorella virus PBCV-1 genome are identical 2.2-kbp inverted repeats

The Chlorella virus PBCV-1 genome is a linear nonpermuted 333-kbp dsDNA molecule with covalently closed hairpin termini. The termini (minus the hairpin) are identical inverted repeats of at least 2185 bases after which the sequence diverges. The inverted repeats contain two small potential open reading frames and several direct repeats. However, neither the open reading frames nor the remainder of the inverted repeats are transcribed during PBCV-1 replication. Twenty-nine other Chlorella virus DNAs, of 36 tested, hybridized to the PBCV-1 terminal fragments.

Segmental fixation with U-shaped rod in the treatment of spondylolisthesis

From 1987 to 1990 the authors treated 20 cases of spondylolisthesis by an improved operative procedure including excision of the medial part of the superior articular processes of the slipped vertebra, excision of the soft tissue between the ununited isthmi, and excision of the ligamentum flavum between the intervertebral space above the slipped segment. In some instances the lower portion of the lamina over the slipped vertebra should be resected. A U-shaped rod was used to hold sublaminar fixation of two segments above and below the slipped vertebra, with the slipped vertebra spared. Utilizing the U-shaped rod as support, bone strips were placed along the lateral and anterior sides of the rod to bridge the gap between the laminae of the displaced vertebra. Other bone grafts were focused on the facet joints. The patients were allowed ambulation early postoperatively. 19 cases could be evaluated at preliminary follow-up. All showed satisfactory results.

[Application of DNA interstrand cross-link assay to estimate the sensitivity of tumor cells to alkylating agents]

Fluorometric method was modified and used to detect DNA interstrand crosslinks (ISC) in tumor cells after treatment with cisplatin. Linear dose-response curve was obtained. The difference of DNA ISC formation between HeLa S3 (containing O6-methylguanine methyltransferase, Mer+) and HeLa MR (devoid of O6-methyguanine methyltransferase, Mer-) cells was studied after treatment with alkylating agent 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (ACNU). The survival fraction was also observed in Mer+ and Mer- cells treated with ACNU. It seems that DNA ISC formation may be used as one of the possible criteria in estimating the sensitivity of tumor cells to bifunctional alkylating agent and forecasting the efficacy of tumor chemotherapy.

Comparative description of Pseudomonas cocovenenans (van Damme, Johannes, Cox, and Berends 1960) NCIB 9450T and strains isolated from cases of food poisoning caused by consumption of fermented corn flour in China

Phenotypic and genetic characteristics of Pseudomonas cocovenenans NCIB 9450T (T = type strain) and strains isolated from cases of food poisoning caused by consumption of fermented corn flour are compared. Our results show that these strains are members of the same species and conform to the description of Section II of the genus Pseudomonas. Because of small differences in substrate utilization patterns, the strains isolated in the People's Republic of China are thought to be biovars of P. cocovenenans.

[Effect of capsaicin on the release of substance P from spinal cord and blood pressure]

The role of substance P (SP) in the regulation of sympathetic outflow to the cardiovascular system was assessed. Intrathecal injection(ith) of capsaicin caused a release of SP from the spinal cord and resulted in an increase in blood pressure and heart rate accompanied by an elevation of plasma adrenaline and noradrenaline. This pressor response was blocked by ith SP antagonist D-Pro2, D-Phe7, D-Trp9-SP or antiserum against SP. The immunohistochemical study showed that the SP-like immunoreactivity in T-8 of the spinal cord was decreased as the amount of capsaicin administrated was increased. Spinal cord transection had no effect on pressor responses caused by ith 10 micrograms capsaicin. These results suggest that SP transmits excitatory information to the cardiovascular system via spinal sympathetic pathway.

[A rat model for the study of Pseudomonas aeruginosa enterogenous infection after severe scald]

Eighty eight healthy Wistar rats were used in this study. Pseudomonas aeruginosa (serotype IX, labelled with fluorescein isothiocyanate) were fed in suspension (10(9)/ml, 1ml/100g body weight) following a 25-28% full-thickness burn created on the back of animals. The animals were sacrificed at eight different time phases from 1-72 hours after burns. The labelled bacteria were traced in internal organs, the quantitative bacterial cultures of visceral tissues and blood cultures were performed coincidently. The results showed that the labelled bacteria introduced into GI tract could be demonstrated from the internal organs and blood circulation.

Strain difference in pulmonary vascular responsiveness to hypoxia in rats

The difference in pulmonary vascular response to hypoxia between Hilltop Sprague-Dawley (HT) rats and Wistar (W) rats was studied. Effects of inhibitor of leukotriene (LT) synthesis or prostaglandin (PG) synthesis on hypoxic pulmonary vasoconstriction (HPV) and chronic pulmonary hypertension were observed, and variations in plasma TXB2 and 6-keto-PGF1 alpha during hypoxia were determined. The results showed that in rats of both strains LTs are the major mediator of HPV, which is also mediated by vasoconstrictive PGs in HT rats, while modulated by vasodilative PGs in W rats. This might be the crucial mechanism responsible for the higher pulmonary vascular responsiveness in HT rats. Differences in the modulating effect of histamine and in the structural feature of pulmonary arteriole might be contributing factors as well.

Crystal structure of fructose-1,6-bisphosphatase complexed with fructose 6-phosphate, AMP, and magnesium

The crystal structure of fructose-1,6-bisphosphatase (EC 3.1.3.11) complexed with fructose 6-phosphate, AMP, and Mg2+ has been solved by the molecular replacement method and refined at 2.5-A resolution to a R factor of 0.215, with root-mean-square deviations of 0.013 A and 3.5 degrees for bond lengths and bond angles, respectively. No solvent molecules have been included in the refinement. This structure shows large quaternary and tertiary conformational changes from the structures of the unligated enzyme or its fructose 2,6-bisphosphate complex, but the secondary structures remain essentially the same. Dimer C3-C4 of the enzyme-fructose 6-phosphate-AMP-Mg2+ complex twists about 19 degrees relative to the same dimer of the enzyme-fructose 2,6-bisphosphate complex if their C1-C2 dimers are superimposed on one another. Nevertheless, many interfacial interactions between dimers of C1-C2 and C3-C4 are conserved after quaternary structure changes occur. Residues of the AMP domain (residues 6-200) show large migrations of C alpha atoms relative to barely significant positional changes of the FBP domain (residues 201-335).

Schwann cells and myasthenia gravis. Preferential uptake of soluble and membrane-bound AChR by normal and immortalized Schwann cells, and immunogenic presentation to AChR-specific T line lymphocytes

The normal neuromuscular synapse is formed by the intimate association of nerve endings, postsynaptic end-plate foldings in the muscle fiber, and nonmyelinating Schwann cells (SC) sealing the synaptic ramifications. Because SC have been recognized recently to have an immunogenic potential inducible to present protein autoantigens to autoimmune T lymphocytes, and considering their close proximity to the acetylcholine receptor (AChR)-bearing postsynaptic membranes, presentation of soluble and membrane vesicle-bound AChR to appropriate T cells was investigated. Short-term monolayer cultures of SC isolated from neonatal rat sciatic nerves, as well as cells of an immortalized SC line of similar origin, were fully able to present the relevant molecular epitopes to major histocompatibility complex (MHC) compatible AChR-specific T line lymphocytes immunogenically. Presentation of AChR was restricted by RT1.B (I-A) MHC class II products. Both types of cultured rat SC were inducible to expression of MHC class I and II products, and they were able to phagocytose AChR-enriched membrane vesicles preferentially. In contrast, phagocytosis of latex particles by SC was negligible. These data qualify perisynaptic SC as potential presenter cells of autoimmunogenic AChR in myasthenia gravis. Thus, SC may play a critical and as-yet unpredicted regulatory role in the cellular pathogenesis of myasthenia gravis.

[Opportunistic infection and systemic dissemination in burns]

A total of 303 strains of opportunistic bacteria were isolated from our burned patients during April, 1980 to December, 1987. Among which Pseudomonas aeruginosa accounted for 161 strains (54.1%), Serratia accounted for 56 strains (18.5%), just the two species accounted for 72.6% in total. Among twenty commonly used antibiotics, Amikacin and Polymyxin-B were comparatively sensitive. Further reviewing the drug sensitivity of the two species, we found the sensitivity rates were variable among the strains isolated from different sources. To Polymyxin-B, strains isolated from wound surfaces were of 86.1% and 90.7% respectively, from subeschar tissues or visceral organs were of 53.8% and 39.3%, from blood stream were of 44.4% and 40%. It seemed that the drug resistance of the invading organisms was stronger than that of surface ones. It suggested that the Pseudomonas aeruginosa and Serratia play an important role in infection of burned patients.

[Determination of blood endotoxin in severely burned patients and its clinical significance]

Since October 1985 to June 1987, in 12 severely burned patients with endotoxemia was observed by using a quantitative endotoxin assay of limulus amoebocyte lysate (LAL) test with a chromogenic substrate. Among the 12 patients, 4 died and 8 survived. The average age of dead group was 31.8 years (19-45 years), mean TBSA was 63% (58-80%) and mean I 18.5%. The survival group average age of survival group was 27.5 years (18-39 years), mean TBSA was 58% (18-85%) and mean I 24.4% (6-56%). The plasma endotoxin concentrations of burn patients in dead group were 105-571 pg/ml, significantly higher than that of survival group (30-240 pg/ml) and healthy human (6.44 +/- 1.96 pg/ml). It was found that the increase of endotoxemia was closely related to burn wound sepsis, positive of blood culture, systemic disseminated septicaemia. Systemic use of sensitive antibiotics may increase the level of blood endotoxin in severe gram-negative bacillus infection. Polymixin-B is an exception.

[Common pathogens of surgical infections and their drug sensitivities]

328 bacterial strains were isolated from the surgical patients admitted to the Southwestern Hospital, and their drug sensitivities were tested. Among them, there were 158 strains of aerobic bacteria (47.9%), including 61 strains of E. coli, 21 strains of Proteus, 12 strains of P. aeruginosa, and 170 strains of anaerobic bacteria (52.1%), including 53 strains of B. melaninogenicus, 48 strains of B. fragilis, as well as 19 strains of bacteroides and 24 strains of Fusobacterium. Ceftazidime was highly effective against gram-negative bacteria, such as E. coli, P. aeruginosa, Serratia and Klebsiella. Metronidazole was effective against B. fragilis, Bacteroides and Fusobacterium. Combination of amikacin or gentamycin and metronidazole were more effective.

Molecular structure of the human asparagine synthetase gene

The human gene for asparagine synthetase has been isolated and the molecular organization has been determined by mapping and DNA sequencing of intron-exon boundaries. The gene spans 35 kb and contains 13 exons. The structure of the human gene has a high degree of similarity to that of the hamster asparagine synthetase gene, with identical positions for all but one of the intron regions. The 5' upstream region of this gene, like other housekeeping genes, lacks conventional TATA and CAAT boxes. Comparisons of the 5' upstream sequences of the human and hamster genes show limited similarity; however, both have a very high G + C content which may play a role in expression through DNA methylation.

Prolonged inactive phases during the discontinuous pattern of prematurity in the electroencephalogram of very-low-birthweight infants

A consecutive cohort of 46 very-low-birthweight infants, who had routine electroencephalograms (EEGs) while in the neonatal intensive care unit, were studied. Two infants were lost to follow-up and were excluded, leaving a study population of 44 infants. Their mean birthweight was 945 +/- 166 g, gestation 27.1 +/- 1.7 weeks. Thirteen infants died before discharge. The remaining 31 had a mean corrected age of 26.1 +/- 8.7 months at the time of the last visit. Three groups were distinguished: normal survivors, handicapped survivors and non-survivors. The longest inactive phase encountered in the discontinuous EEG was the yardstick of the study with 3 subdivisions: less than 20 sec. 20-29 sec and equal to or more than 30 sec. The data showed a relative increase in poor outcome with increasing duration of inactivity (P less than 0.05) and, conversely, a favorable outcome with the absence of 20 sec or longer inactivity (P less than 0.001). All infants received therapeutic doses of phenobarbital during the early part of their illness which could have affected their EEG. Mean phenobarbital levels, obtained close to the time of the EEG, however, did not differ significantly and failed to show a significant relationship between the duration of the inactive phase and drug level observed. Inactive phases greater than or equal to 30 sec were more common (P less than 0.01) in infants with intraventricular hemorrhage. We conclude: although full recovery and normal outcome have been documented, prolonged isoelectric phases beyond 30 sec are more common in infants with fatal outcome.

Chlorella viruses isolated in China

Plaque-forming viruses of the unicellular, eucaryotic, exsymbiotic, Chlorella-like green algae strain NC64A, which are common in the United States, were also present in fresh water collected in the People's Republic of China. Seven of the Chinese viruses were examined in detail and compared with the Chlorella viruses previously isolated in the United States. Like the American viruses, the Chinese viruses were large polyhedra and sensitive to chloroform. They contained numerous structural proteins and large double-stranded DNA genomes of at least 300 kilobase pairs. Each of the DNAs from the Chinese viruses contained 5-methyldeoxycytosine, which varied from 12.6 to 46.7% of the deoxycytosine, and N6-methyldeoxyadenosine, which varied from 2.2 to 28.3% of the deoxyadenosine. Four of the Chinese virus DNAs hybridized extensively with DNA from the American virus PBCV-1, and three hybridized poorly.

B-T lymphocyte interactions in experimental autoimmune myasthenia gravis: antigen presentation by rat/mouse hybridoma lines secreting monoclonal antibodies against the nicotinic acetylcholine receptor

Many, but not all rat/mouse B hybridoma cells, producing monoclonal antibodies against determinants on the nicotinic acetylcholine receptor (AChR) of the electric organ of Torpedo californica, were able to immunologically present antigen to AChR-specific, Ia-compatible CD4+ T lymphocyte lines. Most of the hybridomas presented AChR in a privileged manner, i.e. they present AChR even more effectively than macrophages or dendritic cells. However, in the presentation of antigens other than AChR, they were inferior to macrophages. Moreover, some hybridomas were able to present AChR not only in soluble state, but also in membrane vesicles. Privileged presentation of AChR by hybridomas depended on the reactivity of the secreted immunoglobulins with epitopes of the AChR alpha chain, and on the expression of major histocompatibility complex class II antigens on the hybridoma cell surface. There was, however, no quantitative correlation between the actual AChR presentation capacity of one clone and the density of its surface Ia. Neither fine specificity nor isotype of hybridoma immunoglobulin are critical in determining privileged AChR presentation. We postulate that different hybridomas vary in their ability to take up soluble and particulate antigen, to process and to re-express them on the cellular membrane. This capacity may determine their efficiency to present antigen to T cells.

Mouse liver dihydrodiol dehydrogenases. Identity of the predominant and a minor form with 17 beta-hydroxysteroid dehydrogenase and aldehyde reductase

A major and a minor form of dihydrodiol dehydrogenase were co-purified with 17 beta-hydroxysteroid dehydrogenase and aldehyde reductase, respectively, to apparent homogeneity from liver cytosol of male ddY mice. The activities of dihydrodiol dehydrogenase and testosterone dehydrogenase or aldehyde reductase of the two enzyme forms comigrated electrophoretically. The major form of the enzyme oxidized 17 beta-hydroxysteroids and nonsteroidal alicyclic alcohols and reduced 17-ketosteroids and various synthetic carbonyl compounds, showing higher affinity for steroids than for xenobiotics. The activity of this enzyme form toward benzene dihydrodiol and testosterone exhibited identical thermostability and susceptibility to inhibition by quercitrin, SH-reagents, nonsteroidal estrogens and anti-inflammatory agents. On the other hand, the minor form of the enzyme, which oxidized benzene dihydrodiol but not 17 beta-hydroxysteroids, also reduced various aldehydes well and was specifically inhibited by barbiturates and sorbinil. These results indicate that the major form of dihydrodiol dehydrogenase is identical to 17 beta-hydroxysteroid dehydrogenase and the minor enzyme form to aldehyde reductase.

O6-methylguanine methyltransferase activity and sensitivity of Japanese tumor cell strains to 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)- 3-nitrosourea hydrochloride

Using 40 tumor cell strains derived from various organs of Japanese tumor patients and also 12 normal cell strains, we have measured the activity of O6-methylguanine-DNA methyltransferase (MT), which can repair O6-methylguanine produced in DNA by alkylating agents. Then, the lethal sensitivities of the strains to the anti-tumor drug 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-)2-chloroethyl)-3-nitrosourea hydrochloride (ACNU) were measured. The MT activity was assayed by measuring the 3H radioactivity transferred from the substrate DNA containing [methyl-3H]-O6-methylguanine to acceptor molecules in the cell extracts. Extracts from the majority of tumor strains and all normal cell strains contained substantial MT activity of varying degree, while the extracts of 6 tumor strains showed virtually undetectable MT activity. Hence these 6 strains were assigned as Mer-, the phenotype which is characterized by the inability to repair O6-methylguanine in DNA due to the lack of MT. The Mer- tumor strains were much more sensitive to ACNU than the rest of Mer+ strains, as measured by colony-forming ability. Furthermore, with all the tumor and normal strains tested, a good correlation was observed between MT activity and cellular resistance to ACNU. These results indicate that the frequency of Mer- strain is about 15% among Japanese tumor cell strains so far analyzed, and further suggest that MT may be the only system able to repair lethal DNA damage induced by ACNU.

Comparative analysis of O6-methylguanine methyltransferase activity and cellular sensitivity to alkylating agents in cell strains derived from a variety of animal species

Using 26 cultured cell lines derived from 17 different animal species, we have measured both the activity of O6-methylguanine (O6-MeG) methyltransferase (MT) in cell extracts and the sensitivity of the strains to the lethal effects of the alkylating agents, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea (ACNU). The MT activity was assayed by measuring the amount of 3H radioactivity transferred from methyl-[3H]-labeled O6-MeG in DNA to acceptor protein molecules in the extracts. In all the 21 mammalian cell strains, lethal sensitivity to ACNU as measured by colony-forming ability correlated well with cellular MT activity, indicating that the major lethal ACNU damage is reparable by the MT. On the other hand, MNNG sensitivity did not necessarily correlate with the MT activity.

Expression of the truncated E. coli O6-methylguanine methyltransferase gene in repair-deficient human cells and restoration of cellular resistance to alkylating agents

We have constructed a truncated E. coli O6-methylguanine methyltransferase (MT) gene (ada gene) to express the MT activity for O6-methylguanine and O4-methylthymine but not for methylphosphotriester in human cells and transferred it into Mer- HeLa MR cells. The transfectant cells expressed the truncated E. coli MT were resistant to alkylating agents as same as the transfectant cells with the intact ada gene in cell killing, sister-chromatid exchange induction and host-cell reactivation of adenovirus 5. These results strongly suggest that methylphosphotriester may not contribute to the biological effect of alkylating agents in human cells.