Isolation and structure of a covalent cross-link adduct between mitomycin C and DNA

A DNA cross-link adduct of the antitumor agent mitomycin C (MC) to DNA has been isolated and characterized; the results provide direct proof for bifunctional alkylation of DNA by MC. Exposure of MC to Micrococcus luteus DNA under reductive conditions and subsequent nuclease digestion yielded adducts formed between MC and deoxyguanosine residues. In addition to the two known monoadducts, a bisadduct was obtained. Reductive MC activation with Na2S2O4 (sodium dithionite) leads to exclusive bifunctional alkylation. The structure of the bisadduct was determined by spectroscopic methods that included proton magnetic resonance, differential Fourier transform infrared spectroscopy, and circular dichroism. Formation of the same bisadduct in vivo was demonstrated upon injection of rats with MC. Computer-generated models of the bisadduct that was incorporated into the center of the duplex B-DNA decamer d(CGTACGTACG)2 indicated that the bisadduct fit snugly into the minor groove with minimal distortion of DNA structure. A mechanistic analysis of the factors that govern monofunctional and bifunctional adduct formation is presented.

Mutations in the WRN gene in mice accelerate mortality in a p53-null background

Werner's syndrome (WS) is a human disease with manifestations resembling premature aging. The gene defective in WS, WRN, encodes a DNA helicase. Here, we describe the generation of mice bearing a mutation that eliminates expression of the C terminus of the helicase domain of the WRN protein. Mutant mice are born at the expected Mendelian frequency and do not show any overt histological signs of accelerated senescence. These mice are capable of living beyond 2 years of age. Cells from these animals do not show elevated susceptibility to the genotoxins camptothecin or 4-NQO. However, mutant fibroblasts senesce approximately one passage earlier than controls. Importantly, WRN(-/-);p53(-/-) mice show an increased mortality rate relative to WRN(+/-);p53(-/-) animals. We consider possible models for the synergy between p53 and WRN mutations for the determination of life span.

Reductive metabolism and alkylating activity of mitomycin C induced by rat liver microsomes

Mitomycin C, an antitumor antibiotic, is rapidly metabolized in the presence of rat liver microsomes. NADPH and anaerobic conditions are required for the process. The products isolated after reexposure to air are 2,7-diaminomitosene derivatives. Specifically, in the presence of inorganic phosphate, 1,2-cis- and -trans-2,7-diaminomitosene 1-phosphates, 1,2-cis- and -trans-2,7-diamino-1-hydroxymitosenes, and 2,7-diaminomitosene are formed. The last substance is a new mitomycin C derivative, and proof for its structure is presented. Mytomycin C has been previously postulated to be an alklating agent requiring reduction for activity (Iyer, V. N., & Szybalski, W. (1964) Science (Washington, D.C.) 145, 55]. The 1-phosphates above represent the first chemically characterized bioreductive alkylation products of the drug. 5'-Uridylic acid is alkylated analogously under these conditions, to give cis- and trans-2,7-diaminomitosene 1-(5'-uridylate), while the phosphodiester UpU and uridine itself are inert. Hydrogen gas/PtO2 gives the same results as microsomes/NADPH. The formation of the observed compounds indicates that enzymatic (or chemical) reduction of the quinone system of mitomycin C induces ring opening of the aziridine function, generating a reactive center at the C1 position as previously postulated by others (ibid.). The second alkylating center, also postulated, is not evident, however, under the conditions tested, indicating that the aziridine is the primary bioreductive alkylation function of mitomycin C. Identification of the products and mechanism of the microsomal anaerobic metabolism of mitomycin C are significant in view of the reported toxicity of the drug to anaerobic cancer cells.

Reaction of DNA with chemically or enzymatically activated mitomycin C: isolation and structure of the major covalent adduct

The antitumor antibiotic mitomycin C is shown to form a covalent complex with calf thymus DNA under anaerobic conditions in the presence of either NADPH cytochrome c reductase/NADPH, xanthine oxidase/NADH, or the chemical reducing system H2/PtO2. Digestion of the complex with DNase I/snake venom diesterase/alkaline phosphatase yields a single mitomycin deoxyguanosine adduct as the major DNA alkylation product, identified as N2-(2'' beta,7''-diaminomitosen-1'' alpha-yl) 2'-deoxyguanosine (Structure 2). Two minor adducts, 2-5% each of the total adduct pool, are isolated and identified as the 1'' beta stereoisomer of 2 (Structure 3), and 10''-decarbamoyl-2 (Structure 7). The same results were obtained with M13 DNA and poly(dG-dC).poly(dG-dC); however, in the latter case, a minor adduct apparently possessing two deoxyguanosine and one mitomycin unit is isolated. Digestion of the covalent mitomycin-calf thymus DNA complex with nuclease P1 yields four dinucleotide adducts, all of which consist of 2 linked at its 3' end to each of the four possible 5' nucleotides (A, T, G, and C). Upon treatment of each dinucleotide adduct with snake venom diesterase/alkaline phosphatase, 2 is released along with the corresponding free nucleoside. In apparent conflict with the present results, previous reports from another laboratory have indicated that modification of calf thymus DNA by mitomycin C under conditions identical to those described here result in the isolation of three mitomycin C mononucleotide adducts possessing linkages of the drug to N2 and O6 of guanine and N6 of adenine. Evidence is shown suggesting that the latter adducts are actually three of the above four dinucleotide derivatives of 2 obtained independently by us and, thus, all of them in fact possess an identical N2-mitosenylguanine adduct moiety. Model-building studies indicate an excellent fit of the guanine N2-linked drug molecule inside the minor groove of B-DNA with no appreciable distortion of the DNA structure.

Mechanism of monofunctional and bifunctional alkylation of DNA by mitomycin C

The relative amounts of monofunctional and bifunctional alkylation products of DNA with mitomycin C (MC) depend on whether one or both masked alkylating functions of MC are activated reductively; adduct 8 is the result of one function and adducts 7 and 9, formed as a pair, are the result of both functions being activated [Tomasz, M., Lipman, R., Chowdary, C., Pawlak, J., Verdine, G. L., & Nakanishi, K. (1987) Science (Washington, D.C.) 235, 1204-1208]. To determine the mechanism governing this differential reactivity of MC with DNA, MC-Micrococcus luteus DNA complexes formed under varying conditions in vitro were digested to nucleosides and adducts. Adduct distribution, analyzed by high-performance liquid chromatography, served as the measure of monofunctional and bifunctional activation. H2/PtO2 and xanthine oxidase/reduced nicotinamide adenine dinucleotide (NADH) activated MC mostly monofunctionally, and Na2S2O4 activated the drug bifunctionally under comparable conditions. Excess MC selectively suppressed, but excess PtO2 selectively promoted, bifunctional activation by H2/PtO2; excess xanthine oxidase and/or NADH also had promoting effects. O2 tested in the Na2S2O4 system was inhibitory. 10-Decarbamoyl-MC acted strictly monofunctionally under all conditions. Monoadducts bound to DNA were converted to bis adducts upon rereduction. A mechanism with the following features was derived: (i) Activation of MC at C-1 and C-10 is sequential (C-1 first). (ii) A one-time reduction is sufficient for both. (iii) Activation of the second function may be selectively inhibited by kinetic factors or O2. (iv) 7 and 9 are coproducts of bifunctional activation; their ratio depends on the DNA base sequence. (v) Activation of the second function involves an iminium intermediate. Direct applications to the action of MC in vivo are discussed.

Mitomycin C-DNA adducts generated by DT-diaphorase. Revised mechanism of the enzymatic reductive activation of mitomycin C

Mitomycin C (MC) was reductively activated by DT-diaphorase [DTD; NAD(P)H:quinone oxidoreductase] from rat liver carcinoma cells in the presence of Micrococcus lysodeicticus DNA at pH 5.8 and 7.4. The resulting alkylated MC-DNA complexes were digested to the nucleoside level and the covalent MC-nucleoside adducts were separated, identified, and quantitatively analyzed by HPLC. In analogous experiments, two other flavoreductases, NADH-cytochrome c reductase and NADPH-cytochrome c reductase, as well as two chemical reductive activating agents Na2S2O4 and H2/PtO2 were employed as activators for the alkylation of DNA by MC. DTD as well as all the other activators generated the four known major guanine-N2-MC adducts at both pHs. In addition, at the lower pH, the guanine-N7-linked adducts of 2,7-diaminomitosene were detectable in the adduct patterns. At a given pH all the enzymatic and chemical reducing agents generated very similar adduct patterns which, however, differed dramatically at the acidic as compared to the neutral pH. Overall yield of MC adducts was 3-4-fold greater at pH 7.4 than at 5. 8 except in the case of DTD when it was 4-fold lower. Without exception, however, cross-link adduct yields were greater at the acidic pH (2-10-fold within the series). The ratio of adducts of bifunctional activation to those of monofunctional activation was 6-20-fold higher at the acidic as compared to the neutral pH. A comprehensive mechanism of the alkylation of DNA by activated MC was derived from the DNA adduct analysis which complements earlier model studies of the activation of MC. The mechanism consists of three competing activation pathways yielding three different DNA-reactive electrophiles 11, 12, and 17 which generate three unique sets of DNA adducts as endproducts. The relative amounts of these adducts are diagnostic of the relative rates of the competing pathways in vitro, and most likely, in vivo. Factors that influence the relative rates of individual pathways were identified.

Recognition between mitomycin C and specific DNA sequences for cross-link formation

An extensive series of oligodeoxyribonucleotides was reacted with reductively activated mitomycin C (MC), and the resulting cross-linked drug-oligonucleotide complexes were isolated by reverse-phase HPLC and characterized by nucleoside and MC-nucleoside adduct analysis. HPLC also served for assay of the yield of cross-linked oligonucleotides. AT-rich duplex oligonucleotides, containing a single central CG.CG, gave high yields of cross-links between the two guanines while those having GC.GC, instead, gave none. In another series, the central sequences CGC.GCG and CGC.ICG both yielded 50% cross-link while CGC.GCI was completely resistant. Cross-linking was conducted also in two steps: Oligonucleotides substituted monofunctionally by MC at guanine at either a CG or GC sequence were annealed with their complementary strands followed by reductive reactivation of the bound MC to form a cross-link. The CG oligomers were cross-linked quantitatively while the GC ones were again resistant. These results show unambiguously that the MC cross-link is absolutely specific to the CG.CG duplex sequence, confirming our previous finding [Chawla, A.K., Lipman, R., & Tomasz, M. (1987) in Structure and Expression, Volume 2: DNA and Its Drug Complexes (Sarma, R.H., & Sarma, M.H., Eds.) Adenine Press, Guilderland, NY]. Evidence is presented that this specificity is due to the specific orientation of the monofunctionally attached MC in the minor groove. Superimposed on the CG.CG requirement, a four-base-pair sequence preference was observed at PuCGPyr.PuCGPyr sequences. This suggests that the guanine N2 atom of GpPyr is more reactive toward the drug than that of GpPu, due to the favorable effect of the negative dipole of the O2 of the Pyr on the reaction; in accordance, GpT was more reactive than GpC.(ABSTRACT TRUNCATED AT 250 WORDS)

Recognition of specific DNA sequences by mitomycin C for alkylation

Synthetic oligodeoxyribonucleotides were reacted with mitomycin C (MC) under conditions which restricted MC to monofunctional alkylating activity. The yields of monofunctional alkylation of oligonucleotides with variable sequence were determined by enzymatic digestion of the reaction mixture to unreacted nucleosides and the product of alkylation, a MC-deoxyguanosine adduct (2), followed by quantitative analysis by HPLC. The relative yields of 2 reflected relative monoalkylation reactivities. They were compared in a series of oligonucleotides having the sequence 5'-NGN' in which the 5'-base was varied while the 3'-base was kept constant as T. Under Na2S2O4 activation conditions a striking enhancement of the yield was observed at the 5'-CG sequence: 36%, compared to 2% at 5'-AG and 4.1% at 5'-TG. The 5'-GG sequence also showed enhanced reactivity although to a lesser extent (14.7%). The enhancements were specific to the duplex state of the oligonucleotides. Using NADPH:cytochrome c reductase as the reducing agent gave similar results. MC activated by acidic pH also displayed 5'-CG alkylation specificity. 10-Decarbamoyl-MC activated by Na2S2O4 showed the same 5'-CG specificity as MC. Replacement of deoxyguanosine by deoxyinosine in the opposite strand at a 5'-CG site abolished the enhancement of alkylation. Such replacement at a 5'-GG site had a similar effect. It was found that the base 3' to the guanine had only a relatively modest modulating effect on the enhanced reactivity of the G at the 5'-CG sequence. This 3'-base effect appeared to be independent of the 5'-base of the 5'-NGN' triplet. The order of reactivity is 3'-(C greater than T greater than A).(ABSTRACT TRUNCATED AT 250 WORDS)

Reaction of acid-activated mitomycin C with calf thymus DNA and model guanines: elucidation of the base-catalyzed degradation of N7-alkylguanine nucleosides

Mitomycin C (MC, 1) forms covalent adducts under acidic activating conditions (pH approximately 4) with deoxyguanosine, d(GpC), and guanine residues of calf thymus DNA. In the case of deoxyguanosine, five adducts arise from a common precursor, N7-(2'' beta, 7''-diaminomitosen-1''-yl)-2'-deoxyguanosine (10a; not isolated), which hydrolyzes spontaneously via two pathways: scission of the glycosidic bond to form N7-(2'' beta, 7''-diaminomitosen-1'' alpha-yl)guanine (5) and its 1'' beta-isomer (6) and imidazolium ring opening to generate three 2,6-diamino-4-hydroxy-5-(N-formyl-2'' beta, 7''-diaminomitosen-1'' beta-yl)pyrimidine (FAPyr) derivatives that are substituted at N6 by isomeric 2'-deoxyribose units [i.e., 1' beta-furanose (7), 1' alpha-furanose (8), and 1' beta-pyranose (9)]. The structures of 5-9 were determined by spectroscopic methods. The same five adducts were obtained from d(GpC), but only the guanine adducts 5 and 6 were formed in DNA. Adducts 7-9 interconvert during high-performance liquid chromatography (HPLC). The unexpected isomerization of the deoxyribose moiety of the initially formed 1' beta-furanose adduct 7 to those of 8 and 9 occurs upon imidazolium ring opening, as discerned by the course of imidazolium cleavage of the simple models N7-ethyl- and N7-methylguanosine and N7-methyl-2'-deoxyguanosine. All ring-opened N7-alkylguanosine derivatives studied here exist as a mixture of distinct N-formyl rotamers, manifested by multiple interconverting peaks on HPLC and in the 1H NMR spectra. In the UV spectra of such derivatives, a new and diagnostic maximum at 218 nm (at pH 7) is observed. Acid-activated MC is found to alkylate preferentially the Gua-N7 position in deoxyguanosine or d(GpC), in contrast to reductively activated MC, which preferentially alkylates the Gua-N2 position. This finding is explained by the different electronic structures of acid- and reduction-activated MC. In DNA, the N7 specificity of acid-activated MC is partially offset by steric factors.

NMR and computational characterization of mitomycin cross-linked to adjacent deoxyguanosines in the minor groove of the d(T-A-C-G-T-A).d(T-A-C-G-T-A) duplex

Two-dimensional homonuclear and heteronuclear NMR and minimized potential energy calculations have been combined to define the structure of the antitumor agent mitomycin C (MC) cross-linked to deoxyguanosines on adjacent base pairs in the d(T1-A2-C3-G4-T5-A6).d(T7-A8-C9-G10-T11-A12) duplex. The majority of the mitomycin and nucleic acid protons in the MC-X 6-mer complex have been assigned from through-bond and through-space two-dimensional proton NMR studies in aqueous solution at 5 and 20 degrees C. The C3.G10 and G4.C9 base pairs are intact at the cross-link site and stack on each other in the complex. The amino protons of G4 and G10 resonate at 9.36 and 8.87 ppm and exhibit slow exchange with solvent H2O. The NMR experimental data establish that the mitomycin is cross-linked to the DNA through the amino groups of G4 and G10 and is positioned in the minor groove. The conformation of the cross-link site is defined by a set of NOEs between the mitomycin H1" and H2" protons and the nucleic acid imino and amino protons of G4 and the H2 proton of A8 and another set of NOEs between the mitomycin geminal H10" protons and the nucleic acid imino and amino protons of G10 and the H2 proton of A2. Several phosphorus resonances of the d(T-A-C-G-T-A) duplex shift dramatically on mitomycin cross-link formation and have been assigned from proton-detected phosphorus-proton two-dimensional correlation experiments. The proton chemical shifts and NOEs establish fraying at the ends of the d(T-A-C-G-T-A) duplex, and this feature is retained on mitomycin cross-link formation. The base-base and base-sugar NOEs exhibit similar patterns for symmetry-related steps on the two nucleic acid strands in the MC-X 6-mer complex, while the proton and phosphorus chemical shifts are dramatically perturbed at the G10-T11 step on cross-link formation. The NMR distance constraints have been included in minimized potential energy computations on the MC-X 6-mer complex. These computations were undertaken with the nonplanar five-membered ring of mitomycin in each of two pucker orientations. The resulting low-energy structures MX1 and MX2 have the mitomycin cross-linked in a widened minor groove with the chromophore ring system in the vicinity of the G10-T11 step on one of the two strands in the duplex.(ABSTRACT TRUNCATED AT 400 WORDS)

Solution structure of the monoalkylated mitomycin C-DNA complex

Mitomycin C (MC) is a potent antitumor antibiotic which alkylates DNA through covalent linkage of its C-1" position with the exocyclic N2 amino group of guanine to yield the [MC]dG adduct at the duplex level. We report on the solution structure of the monoalkylated MC-DNA 9-mer complex where the [MC]dG5 adduct is positioned opposite dC14 in the d(A3-C4-[MC]G5-T6).d(A13-C14-G15-T16) sequence context. The solution structure was solved based on a combined NMR-molecular dynamics study including NOE intensity based refinement. The formation of the [MC]dG adduct occurs with retention of the Watson-Crick alignment at the [MC]dG5.dC14 base-pair and flanking pairs in the complex. The MC ring is positioned in the minor groove with its indoloquinone aromatic ring system at a approximately 45 degrees angle relative to the helix axis and directed towards the 3'-direction on the unmodified strand. The MC indoloquinone chromophore is asymmetrically positioned in a slightly widened minor groove so that its plane is parallel to and stacked over the d(C14-G15-T16) segment on the unmodified strand with its other face exposed to solvent. The MC five-membered ring adopts an envelope pucker with its C-2" atom displaced from the mean plane and directed away from the unmodified strand. We observe conformational perturbations in the DNA 9-mer duplex on formation of the monoalkylated MC complex. Specifically, the base-pairs are displaced by approximately -3.0 A towards the major groove on positioning the MC in the minor groove. This perturbation is accompanied by base stacking patterns similar to those observed in A-DNA while the majority of the sugars adopt puckers characteristic of B-DNA. Conformational perturbations as monitored by helix twist, sugar pucker pseudorotation and glycosidic torsion angles are also observed for the d(T6-C7-I8).d(C11-G12-A13) segment that is adjacent to but does not overlap the MC binding on the 9-mer duplex. We note that the O-10" atom on the carbamate side-chain of MC forms an intermolecular hydrogen bond with the exocyclic amino group of dG15 in two of the three refined structures of the complex. The solution structure of the complex containing this intramolecular hydrogen bond readily explains both the previously observed d(C-G).d(C-G) sequence requirement for cross-linking and the observed, somewhat less stringent, requirement of the same sequence for the initial monoalkylation step.(ABSTRACT TRUNCATED AT 400 WORDS)

Bending of DNA by the mitomycin C-induced, GpG intrastrand cross-link

Mitomycin C (MC) forms interstrand and intrastrand cross-link adducts and monoalkylation products (monoadducts) with DNA. Each of the three types of adducts was incorporated site-specifically into both a 15-mer and a 21-mer oligodeoxyribonucleotide duplex. The adduct-containing duplexes were 32P-phosphorylated and ligated to form multimers, which were then analyzed for anomalous electrophoretic mobility by nondenaturing polyacrylamide gel electrophoresis, using the method of Koo and Crothers [(1988) Proc. Natl. Acad. Sci. U.S.A. 85, 1763-1767] in order to detect DNA curvature caused by the adducts. The intrastrand cross-link adduct was found to induce a 14.6 +/- 2.0 degrees DNA bend per lesion (minimum value) while no DNA bending was detected for either the interstrand cross-link or the monoadduct. Molecular mechanics modeling indicated that the possible origin of the bend lies in a considerable deviation from parallel of the normals to the best planes of the intrastrand cross-linked guanines, due to a shorter than normal distance between their N2 atoms forced upon them by the cross-link. The observed bending by the MC intrastrand lesion may be the cause of the increased flexibility of MC-modified DNA, localized to distinct regions, as observed in earlier work by hydrodynamic methods and electron microscopy. The MC adduct-caused DNA bend may serve as a recognition site for certain DNA-binding proteins.

Plasma immunoreactive beta-endorphin levels in depression. Effect of electroconvulsive therapy

Immunoreactive (ir) plasma beta-endorphin level was assayed in ten symptomatic patients with a unipolar major depressive disorder and in 16 psychiatrically normal controls matched for age and sex. Plasma ir-beta-endorphin level in depressed patients was similar to that in controls. All depressed patients was similar to that in controls. All depressed patients had a transient, approximately threefold increase in ir-beta-endorphin after each use of electroconvulsive therapy (ECT). The increase of plasma ir-beta-endorphin level after ECT parallels the transient elevation of adrenocorticotropic hormone level reported by others and probably reflects a hypothalamic response to ECT.

Reassignment of the guanine-binding mode of reduced mitomycin C

Mitomycin C (1) is a clinically used antitumor antibiotic that binds covalently to deoxyribonucleic acid under reductive or acidic catalysis. We have determined the structures of the adducts resulting from attack of reductively activated 1 on the dinucleoside phosphate d(GpC) to be N2-(2'' beta, 7''-diaminomitosen-1''alpha-yl)-2'-deoxyguanosine (2) and its 1'' beta-isomer (3). This represents a revision of the previously reported structures for these adducts in that the mitomycin residue is linked to the N2- rather than O6-position of 2'-deoxyguanosine. This revision is the result of applying to the mitomycin case a newly developed general method that leads to unambiguous assignment of the linkage position in complex alkylated guanosines. The method as described here takes advantage of the resolution enhancement gained by calculation of the second derivatives of absorbance Fourier transform infrared spectra. In addition, we present 1H NMR data that corroborate the assigned structures of 2 and 3 and that should serve as a useful reference for future investigations into the binding of mitomycin C to DNA. The convenient synthesis of adducts 2 and 3 from deoxyguanosine and mitomycin C reported here should facilitate such investigations as well. Furthermore, we demonstrate a useful acetylation procedure for adducts and metabolites of mitomycin C that furnishes spectroscopically superior chemical derivatives (e.g., triacetates 4 and 5, derived from acetylation of adducts 2 and 3).

Psychiatric aspects of hypertension

The physiological response to stress falls mainly on the cardiovascular system, adrenergic stimulation resulting in peripheral vasoconstriction and an increase in systemic blood pressure. The analogous effects that occur in anxiety-provoking situations immediately suggest an association, whether causal or effectual, between raised blood pressure and anxiety. It has also been suggested that hypertensive individuals exhibit more aggressive traits than others and that these may be hidden or suppressed, becoming manifest by abnormal elevation of the blood pressure (Smirk, 1957).

Duplex oligodeoxyribonucleotides cross-linked by mitomycin C at a single site: synthesis, properties, and cross-link reversibility

Oligodeoxyribonucleotides cross-linked by reductively activated mitomycin C (MC) were prepared and purified for the first time. The cross-linked products were structurally characterized by nucleoside and MC-nucleoside adduct analysis. Optimal conditions were established for the cross-linking reaction, resulting in high yields, typically in the 20-50% range. Nuclease digests of the cross-linked oligonucleotides yielded the same bifunctional MC-deoxyguanosine adduct as that previously isolated from DNA exposed to MC in vitro and in vivo [Tomasz et al. (1987) Science 235, 1204]. The cross-linked oligonucleotides displayed broad thermal melting profiles, greatly increased Tm, and complex circular dichroism spectra. Phosphodiester linkages at the cross-link were resistant to spleen exonuclease, nuclease P1, and TaqI and ClaI restriction endonucleases; snake venom diesterase action was uninhibited. The cross-links are stable to heat at neutral pH but are removed by treatment in hot piperidine or by the reducing agents Na2S2O4 and dithiothreitol. Mechanisms are proposed for these reactions. These studies define optimal methods for introducing mitomycin cross-links into DNA fragments at a specific site, providing a versatile tool to study the effects of the MC cross-links on DNA structure and function.

Effects of caloric restriction on expression of testicular cytochrome P450 enzymes associated with the metabolic activation of carcinogens

Previous work demonstrated that microsomal cytochrome P4502A1 (CYP2A1) is expressed in rat testicular Leydig cells. The present study investigates the effects of diet, age, and strain on rat testicular CYP2A1 expression and assesses the potential role of testicular CYP2A1 in the metabolic activation of carcinogens. In ad libitum-fed 18-week-old Fischer 344 rats, testicular CYP2A1 immunoreactive protein and testosterone 7alpha-hydroxylase activity (7alpha-TOHase) exhibited a circadian variation with a daytime maximum and a night-time minimum (82.2 +/- 42.0 and 21.9 +/- 4.5 pmol 7alpha-hydroxytestosterone/min/mg protein, respectively). Caloric restriction (to 60% of ad libitum consumption), which reduces the severity of Leydig cell tumors in rats, decreased expression of both CYP2A1 and testicular 7alpha-TOHase >80% and eliminated their circadian variation. Conversely, caloric restriction induced a circadian rhythm in testicular 7-benzyloxyresorufin-O-dealkylase activity. Testicular microsomes from ad libitum-fed rats having peak diurnal 7alpha-TOHase activity had significantly greater (30%) microsome-mediated aflatoxin B1-DNA binding activity compared to microsomes prepared from nocturnal phase ad libitum-fed or calorically restricted rats which expressed low 7alpha-TOHase activity. In 12-month-old Fischer 344 rats, high CYP2A1 expression was correlated with severe Leydig cell hyperplasia (r = 0.80), whereas CYP2A immunoreactive protein and 7alpha-TOHase were expressed at lower levels in Sprague-Dawley than in Fischer 344 rats and were undetectable in pig, monkey, and human testes. These are strains/species that do not exhibit significant Leydig cell hyperplasia. This suggests that caloric intake, strain, and circadian factors may all mediate testicular CYP2A1 expression in the rat and that CYP2A1 may in turn influence carcinogen activation and pathological status in the testis.

Association of an aminoacyl-tRNA synthetase with a putative metabolic protein in archaea

Aminoacyl-tRNA synthetases are essential enzymes that catalyze attachment of amino acids to tRNAs for decoding of genetic information. In higher eukaryotes, several synthetases associate with non-synthetase proteins to form a high-molecular mass complex that may improve the efficiency of protein synthesis. This multi-synthetase complex is not found in bacteria. Here we describe the isolation of a non-synthetase protein from the archaeon Methanocaldococcus jannaschii that was copurified with prolyl-tRNA synthetase (ProRS). This protein, Mj1338, also interacts with several other tRNA synthetases and has an affinity for general tRNA, suggesting the possibility of forming a multi-synthetase complex. However, unlike the non-synthetase proteins in the eukaryotic complex, the protein Mj1338 is predicted to be a metabolic protein, related to members of the family of H(2)-forming N(5),N(10)-methylene tetrahydromethanopterin (5,10-CH(2)-H(4)MP) dehydrogenases that are involved in the one-carbon metabolism of the archaeon. The association of Mj1338 with ProRS, and with other components of the protein synthesis machinery, thus suggests the possibility of a closer link between metabolism and decoding in archaea than in eukarya or bacteria.

Calorie restriction modulates age-dependent changes in the retinas of Brown Norway rats

The present study examined the effect of a 40% reduction in caloric intake (CR) versus ad libitum (AL) feeding on retinal aging. CR- and AL-fed Brown Norway (BN) rats were obtained at 12, 24 and 30 months of age from the National Center for Toxicological Research (NCTR). Age-dependent declines in outer nuclear layer (ONL=photoreceptor) cell densities, ONL height, inner nuclear layer (INL) cell densities, and thicknesses of the inner retina and whole retina were quantified in thick sections at six loci across the circumference of the sensory retina (four peripheral, two central). Data were analyzed by repeated measures, general linear models. Aging in both diet groups was associated with declines in ONL cell density, ONL height, peripheral INL cell density and total retinal thickness (P< or =0.05). However, ONL cell densities, ONL height and retinal thickness were significantly greater in the CR versus AL diet group at all three ages (P< or =0.005). CR was also associated with a trend for greater peripheral INL cell density (P=0.06) and with greater INL thickness at 30 months (Bonferroni P=0.03). Elevated ONL cell densities in the CR-12 cohort relative to the AL-12 cohort could be explained by diet-associated differences in retinal length, i.e. delayed retinal growth in response to CR. Enhanced ONL cell density, ONL height, INL cell density, INL thickness and total retinal thickness in the CR-30 cohort appear to be as a result of reduced rates of retinal cell loss between 24 and 30 months. However, the protective effect of CR in retinas of older animals may also reflect the initial growth-associated enhancements which were observed in 12 month-old animals. The rat retina may provide a useful model for elucidating the neuroprotective mechanism(s) of CR.

Structure of Francisella tularensis peptidyl-tRNA hydrolase

The rational design of novel antibiotics for bacteria involves the identification of inhibitors for enzymes involved in essential biochemical pathways in cells. In this study, the cloning, expression, purification, crystallization and structure of the enzyme peptidyl-tRNA hydrolase from Francisella tularensis, the causative agent of tularemia, was performed. The structure of F. tularensis peptidyl-tRNA hydrolase is comparable to those of other bacterial peptidyl-tRNA hydrolases, with most residues in the active site conserved amongst the family. The resultant reagents, structural data and analyses provide essential information for the structure-based design of novel inhibitors for this class of proteins.

Reductive alkylation of DNA by mitomycin A, a mitomycin with high redox potential

The mitomycins are a group of antitumor antibiotics that covalently bind to DNA upon reductive activation. Mitomycin A (1b; MA) is more toxic than its clinically useful mitomycin C (1a; MC). The greater toxicity of mitomycin A has been previously attributed to its higher reduction potential. In this report, the DNA alkylation products of reductively activated MA were isolated and characterized by conversion to the known 7-amino mitosene-deoxyguanosine adducts. The three major adducts formed were identified as a monoadduct, N2-(2"beta-amino-7"-methoxymitosen-1"alpha-yl)- 2'-deoxyguanosine (5), a decarbamoyl monoadduct, N2-(2"beta-amino-10"-decarbamoyl-7"-methoxymitosen-1"alpha-y l)-2'- deoxyguanosine (6), and a bisadduct, N2-(2"beta-amino-10"-deoxyguanosin-N2-yl-7-methoxymitosen-1" alpha- yl)-2'-deoxyguanosine (7). Under all reductive activation conditions employed, MA selectively alkylated the 2-amino group of guanine in DNA, like MC. In addition, both MA and MC alkylated DNA and cross-linked oligonucleotides to a similar extent. However, variations in the reductive activation conditions (H2/PtO2, Na2S2O4, or enzymatic) affected the distribution of the three major MA adducts in a different manner than the distribution of MC adducts was affected. A mechanism is proposed wherein the 7-methoxy substituent of MA allows initial indiscriminate activation of either of the drugs' two electrophilic sites. While oxygen inhibited cross-linking by MC, similar aerobic conditions exhibited little influence on the cross-linking ability of MA. Hence, the greater toxicity of MA may be influenced by increased and nonselective activation and cross-link formation in both aerobic and anaerobic cells. This effect is a direct consequence of the higher redox potential of MA as compared to MC.

The major mitomycin C-DNA monoadduct is cytotoxic but not mutagenic in Escherichia coli

To determine the mutagenic and genotoxic properties of the major guanine N2-adduct formed by the antitumor drug mitomycin C, we have synthesized a decanucleotide, d(TTACG[MC]TATCT), containing the adduct, which was inserted into a gapped bacteriophage M13 genome. Analysis of the constructed genome indicated that 41% ligation of the adducted 10-mer occurred on both sides of the gap, whereas the control 10-mer ligated with 34% efficiency. After transfection of the adducted single-stranded M13 DNA into Escherichia coli, the adduct was found to be highly genotoxic. Viability of the adducted genome in a repair-competent strain was only 7%, which increased to 12% and 15% upon induction of SOS by irradiating the cells with 254-nm light at 20 and 50 J/m2, respectively. Even lower viability of 2%, 4.6%, and 0.2% was observed in uvrA, uvrB, and uvrC strains, respectively, which increased up to 10-fold with SOS. An examination of the surviving phage populations revealed that the adduct was not detectably mutagenic. No mutants from the repair-proficient strain were detected after analysis of more than 2500 progeny phage. Only 0.2% of the survivors were mutants in the uvrA strain. It is uncertain, however, if they were induced by the adduct, since all the mutants showed untargeted mutations. We conclude that the major guanine N2-adduct formed by mitomycin C is cytotoxic but not appreciably mutagenic in E. coli.

Metabolic, psychological, and health correlates of dietary restraint in healthy postmenopausal women

BACKGROUND

Dietary restraint, a term used to describe the intentional control of food intake to prevent weight gain or promote weight loss, is commonly practiced by older adults, but little is known about its effects on physiology and metabolism.

METHODS

We therefore compared a wide range of parameters between groups of healthy non-obese postmenopausal women classified psychometrically as unrestrained eaters (body mass index [BMI] 23.8 +/- 0.6 [SEM] kg/m(2), n = 28) or restrained eaters (BMI 24.5 +/- 0.5, n = 39). Measurements were made of reported micronutrient intakes, cardiopulmonary function, hematology, body temperature, skin thickness, bone mass, and immune function; in addition, self-perceived health, mood, and some dimensions of eating behavior were assessed by questionnaire.

RESULTS

Macronutrient and micronutrient intakes were not significantly different between restrained and unrestrained eaters reporting energy intake to within 30% of predicted total energy expenditure. Restrained eaters had significantly lower hemoglobin (12.9 +/- 0.1 [SEM] vs 13.2 +/- 0.1 g/dl; p <.05), but values were within the normal range in both groups. In addition, restrained eaters scored significantly higher on the Eating Attitudes Test (p <.01) and drive-for-thinness (p <.001) and maturity fears (p <.05) subscores of the Eating Disorders Inventory, but values were again within the normal range. No other parameter differed significantly between groups.

CONCLUSIONS

In this normal-weight population, restrained eating was not associated with detrimental effects in a wide range of physiological, metabolic, and health characteristics. Further work is needed to determine the relevance of these results to the general population.