Novel sfi1 alleles uncover additional functions for Sfi1p in bipolar spindle assembly and function

A variety of spindle and kinetochore defects have been shown to induce a mitotic delay through activation of the spindle checkpoint. With the aim of identifying novel mitotic defects we carried out a mad1 synthetic lethal screen in budding yeast. In this screen, four novel alleles of sfi1 were isolated. SFI1 is an essential gene, previously identified through its interaction with centrin/CDC31 and shown to be required for spindle pole body (SPB) duplication. The new mutations were all found in the C-terminal domain of Sfi1p, which has no known function, but it is well conserved among budding yeasts. Analysis of the novel sfi1 mutants, through a combination of light and electron microscopy, revealed duplicated SPBs <0.3 microm apart. Importantly, these SPBs have completed duplication, but they are not separated, suggesting a possible defect in splitting of the bridge. We discuss possible roles for Sfi1p in this step in bipolar spindle assembly.

Andreas Rett and benign familial neonatal convulsions revisited

In 1964 Andreas Rett published the first account of a family with benign familial neonatal convulsions (BFNC). The authors retraced Rett's family and report that the clinical and genetic features of this original family fit the currently accepted definitions of BFNC. They also consider the career of Dr. Rett, a researcher and social reformer as well as an advocate for the rights of children with developmental disabilities.

The mechanism of dienoyl-CoA reduction by 2,4-dienoyl-CoA reductase is stepwise: observation of a dienolate intermediate

The chemical mechanism of the 2,4-dienoyl-CoA reductase (EC 1.3.1.34) from rat liver mitochondria has been investigated. This enzyme catalyzes the NADPH-dependent reduction of 2,4-dienoyl-coenzyme A (CoA) thiolesters to the resulting trans-3-enoyl-CoA. Steady-state kinetic parameters for trans-2,trans-4-hexadienoyl-CoA and 5-phenyl-trans-2,trans-4-pentadienoyl-CoA were determined and demonstrated that the dienoyl-CoA and NADPH bind to the 2,4-dienoyl-CoA reductase via a sequential kinetic mechanism. Kinetic isotope effect studies and the transient kinetics of substrate binding support a random order of nucleotide and dienoyl-CoA addition. The large normal solvent isotope effects on V/K ((D)(2)(O)V/K) and V ((D)(2)(O)V) for trans-2,trans-4-hexadienoyl-CoA reduction indicate that a proton transfer step is rate limiting for this substrate. The stability gained by conjugating the phenyl ring to the diene in PPD-CoA results in the reversal of the rate-determining step, as evidenced by the normal isotope effects on V/K(CoA) ((D)V/K(CoA)) and V/K(NADPH) ((D)V/K(NADPH)). The reversal of the rate-determining step was supported by transient kinetics where a burst was observed for the reduction of trans-2,trans-4-hexadienoyl-CoA but not for 5-phenyl-trans-2,trans-4-pentadienoyl-CoA reduction. The chemical mechanism is stepwise where hydride transfer from NADPH occurs followed by protonation of the observable dienolate intermediate, which has an absorbance maximum at 286 nm. The exchange of the C alpha protons of trans-3-decenoyl-CoA, catalyzed by the 2,4-dienoyl-CoA reductase, in the presence of NADP(+) suggests that formation of the dienolate is catalyzed by the enzyme active site.

Inactivation of CMY-2 beta-lactamase by tazobactam: initial mass spectroscopic characterization

The CMY-2 beta-lactamase, a plasmid determined class C cephalosporinase, was shown to be susceptible to inhibition by tazobactam (K(i)=40 microM). The reaction product(s) of CMY-2 beta-lactamase with the beta-lactamase inhibitor tazobactam were analyzed by electrospray ionization/mass spectrometry (ESI/MS) to characterize the prominent intermediates of the inactivation pathway. The ESI/MS determined mass of CMY-2 beta-lactamase was 39851+/-3 Da. After inactivating CMY-2 beta-lactamase with excess tazobactam, a single species, M(r)=39931+/-3.0, was detected. Comparison of the peptide maps from tryptic digestion of the native enzyme and the inactivated beta-lactamase followed by LC/MS identified two 22 amino acid peptides containing the active site Ser64 modified by a fragment of tazobactam. These two peptides were increased in mass by 70 and 88 Da, respectively. UV difference spectra following inactivation revealed the presence of a new species with a 302 nm lambda(max). Based upon the increase in molecular mass of the tazobactam inactivated CMY-2 beta-lactamase, we propose that during the inactivation of this beta-lactamase by tazobactam an imine is formed. Tautomerization forms the spectrally observed enamine. Hydrolysis generates the covalently attached malonyl semialdehyde, its hydrate, or an enol. This work provides information on the mass of a stable enzyme intermediate of a class C beta-lactamase inactivated by tazobactam and, for the first time, unequivocal evidence that a cross-linked species is not required for apparent inactivation.

Type II topoisomerases as targets for quinolone antibacterials: turning Dr. Jekyll into Mr. Hyde

Quinolones are a very important family of antibacterial agents that are widely prescribed for the treatment of infections in humans. Although the founding members of this drug class had little clinical impact, successive generations include the most active and broad spectrum oral antibacterials currently in use. In contrast to most other anti-infective drugs, quinolones do not kill bacteria by inhibiting a critical cellular process. Rather, they corrupt the activities of two essential enzymes, DNA gyrase and topoisomerase IV, and induce them to kill cells by generating high levels of double-stranded DNA breaks. A second unique aspect of quinolones is their differential ability to target these two enzymes in different bacteria. Depending upon the bacterial species and quinolone employed, either DNA gyrase or topoisomerase IV serves as the primary cytotoxic target of drug action. While this unusual feature initially stymied development of quinolones with high activity against Gram-positive bacteria, it ultimately opened new vistas for the clinical use of this drug class. In addition to the antibacterial quinolones, specific members of this drug family display high activity against eukaryotic type II topoisomerases, as well as cultured mammalian cells and in vivo tumor models. These antineoplastic quinolones represent a potentially important source of new anticancer agents and provide an opportunity to examine drug mechanism across divergent species. Because of the clinical importance of quinolones, this review will discuss the mechanistic basis for drug efficacy and interactions between these compounds and their topoisomerase targets.

Sites of hydroxyl radical reaction with amino acids identified by (2)H NMR detection of induced (1)H/(2)H exchange

Hydroxyl radical reacts with the aliphatic C-H bonds of amino acids by H atom abstraction. Under anaerobic conditions inclusion of a (2)H atom donor results in (1)H/(2)H exchange into these C-H bonds [Goshe et al. Biochemistry 2000, 39, 1761--1770]. The site of (1)H/(2)H exchange can be detected and quantified by (2)H NMR. Integration of the (2)H NMR resonances within a single spectrum permits the relative rate of H atom abstraction from each position to be determined. Analysis of the aliphatic amino acid spectra indicates that the methine and methylene positions were more reactive than the methyl positions. The (2)H NMR spectra of isoleucine and leucine show that H-atom abstraction distal to the alpha-carbon occurs preferentially. Significant (1)H/(2)H exchange was observed into the delta positions of proline and arginine and into the epsilon-methylene of lysine, indicating that a positive charge on a geminal N does not inhibit the (1)H/(2)H exchange. Comparisons of (2)H NMR integrations between amino acid spectra indicated that (1)H/(2)H exchange occurred in the following descending order: L > I > V > R > K > Y > P > H > F >M> T > A > [C, S, D, N, E, Q, G, W]. The extent of (1)H/(2)H exchange into methionine, N-glycyl-methionine, and methionine sulfoxide suggests that a prominent solvent exchange pathway involving hydroxyl radical mediated oxidation of methionine exists to account for the large (2)H incorporation into the gamma-methylene of methionine sulfoxide that is absent for N-glycyl-methionine. Analysis of the (1)H NMR spectra of the reactions with phenylalanine and tyrosine indicated that hydroxyl radical addition to the phenyl ring under the anaerobic reductive reaction conditions did not result in either exchange or hydroxylation.

Novel inactivation of enoyl-CoA hydratase via beta-elimination of 5, 6-dichloro-7,7,7-trifluoro-4-thia-5-heptenoyl-CoA

5,6-Dichloro-7,7,7-trifluoro-4-thia-5-heptenoyl-CoA (DCTFTH-CoA) is an analogue of a class of cytotoxic 4-thiaacyl-CoA thioesters that can undergo a beta-elimination reaction to form highly unstable thiolate fragments, which yield electrophilic thioketene or thionoacyl halide species. Previous work demonstrated that the medium-chain acyl-CoA dehydrogenase both bioactivates and is inhibited by these CoA thioesters through enzyme-catalyzed beta-elimination of the reactive thiolate moiety [Baker-Malcolm, J. F., Haeffner-Gormley, L., Wang, L., Anders, M. W., and Thorpe, C. (1998) Biochemistry 37, 1383-1393]. This paper shows that DCTFTH-CoA can be directly bioactivated by the enoyl-CoA hydratase (ECH) with the release of 1,2-dichloro-3,3,3-trifluoro-1-propenethiolate and acryloyl-CoA. In the absence of competing exogenous trapping agents, DCTFTH-CoA effects rapid and irreversible loss of hydratase activity. The inactivator is particularly effective at pH 9.0, with a stoichiometry approaching 1 mol of DCTFTH-CoA per enzyme subunit. Modification is associated with a new protein-bound chromophore at 360 nm and an increase in mass of 89 +/- 5 per subunit. Surprisingly, ECH exhibiting less than 2% residual hydratase activity retains essentially 100% beta-eliminase activity and continues to generate reactive thiolate species from DCTFTH-CoA. This leads to progressive derivatization of the enzyme with additional UV absorbance, covalent cross-linking of subunits, and an eventual complete loss of beta-eliminase activity. A range of exogenous trapping agents, including small thiol nucleophiles, various proteins, and even phospholipid bilayers, exert strong protection against modification of ECH. Peptide mapping, thiol titrations, UV-vis spectrophotometry, and mass spectrometry show that inactivation involves the covalent modification of Cys62 and/or Cys111 of the recombinant rat liver ECH. These data suggest that enoyl-CoA hydratase is an important enzyme in the bioactivation of DCTFTH-CoA, in a pathway which does not require involvement of the medium-chain acyl-CoA dehydrogenase.

Association of mammographically defined percent breast density with epidemiologic risk factors for breast cancer (United States)

OBJECTIVE

Mammographically defined percent breast density is an important risk factor for breast cancer, but the epidemiology of this trait is poorly understood. Although several studies have investigated the associations between reproductive factors and density, few data are available on the associations of breast density and waist-to-hip ratio (WHR), physical activity, education, alcohol and smoking.

METHODS

We investigated the associations of known and suspected breast cancer risk factors with breast density in a large breast cancer family study. Information was collected on members of 426 families through telephone interviews, mailed questionnaires and mammography. Mammographic films on 1900 women were digitized and breast density was estimated in discrete five-unit increments by one radiologist. Analysis of covariance techniques were used and all analyses were performed stratified by menopausal status.

RESULTS

Similar to other reports, nulliparity, late age at first birth, younger age and lower body mass index were associated with increased percent density in both premenopausal and postmenopausal women, and hormone replacement therapy among postmenopausal women. Higher levels of alcohol consumption and low WHR were associated with increased percent density among both premenopausal and postmenopausal women (differences of 3-11% between high and low categories). However, smoking and education were inversely associated with percent density among premenopausal (p = 0.004 and p = 0.003, respectively) but not postmenopausal women (p = 0.52 and p = 0.90). Physical activity was not associated with percent density in either stratum (p values > 0.25). Combined, these factors explained approximately 37% of the variability in the percent density measure in premenopausal women and 19% in postmenopausal women.

CONCLUSIONS

Many of these factors may potentially affect breast cancer risk through their effect on percent breast density.

Inclusion of risk factor covariates in a segregation analysis of a population-based sample of 426 breast cancer families

Although many segregation analyses of breast cancer have been published, few have included risk factor covariates. Maximum likelihood segregation analyses examining age-at-onset (model 1) and susceptibility (model 2) models of breast cancer were performed on 426 four-generation families originally ascertained between 1944 and 1952 through a breast cancer proband. Cancer status and risk factor data were collected through interviews of participants or surrogates. When segregation analyses were performed on 10,791 women, without estimation of any covariates, all hypotheses under both models were rejected. Model 1, which required estimation of fewer parameters than model 2, provided a better fit to the data according to Akaike's Information Criterion. Further segregation analyses were performed under model 1 on a subset of women with complete data on education, age at first birth (nulliparous women included), and alcohol use, covariates that were found to significantly (P<0.05) improve the fit over the addition of exam age alone in logistic regression models. All three covariates improved the fit of the models, as did year of birth, but at all stages of model building, all of the hypotheses were still rejected. After the allele frequency was fixed at 0.0033, a subset of families appeared to fit a dominant model. Using this model, risk estimates were calculated based on inferred genotype, age, and covariate values. The penetrance was estimated to be 0.15, much lower than previous estimates based on families ascertained through breast cancer probands with early onset. Moreover, the estimates of penetrance were not greatly influenced by incorporation of the measured risk factors.

Orientation of coenzyme A substrates, nicotinamide and active site functional groups in (Di)enoyl-coenzyme A reductases

The stereochemical course of reduction of dienoyl-coenzyme A (CoA) thiolesters catalyzed by the 2,4-dienoyl-CoA reductase from rat liver mitochondria was investigated. The configuration of the double bond in the 3-enoyl-CoA products was determined by (1)H NMR, and experiments to determine the stereochemical course of reduction at Calpha and Cdelta by use of 4-(2)H-labeled beta-nicotinamide adenine dinucleotide phosphate, reduced form (NADPH), were conducted in H(2)O and D(2)O. Defining the diastereoselectivity of the reaction, catalyzed by the Delta(3),Delta(2)-enoyl-CoA isomerase, facilitated the determination of the stereochemical course of reduction by 2, 4-dienoyl-CoA reductase. The absence of solvent exchange of the proton transferred during the Delta(3),Delta(2)-enoyl-CoA isomerase catalyzed equilibration of trans-2- and trans-3-enoyl-CoAs, coupled with the strong sequence homology to enoyl-CoA hydratase support the intramolecular suprafacial transfer of the pro-2R proton of trans-3-enoyl-CoA to the pro-4R position of trans-2-enoyl-CoA. The results indicate that the configuration of the double bond of the 3-enoyl-CoA product is trans and that a general acid-catalyzed addition of a solvent derived proton/deuteron occurs on the si face at Calpha of the dienoyl-CoA. The addition of the pro-4S hydrogen from NADPH occurs on the si face at Cdelta of trans-2, cis-4-dienoyl-CoA and on the re face at Cdelta of trans-2, trans-4-dienoyl-CoA. The stereochemical course of reduction of InhA, an enoyl-thiolester reductase from Mycobacterium tuberculosis, was also determined by use of ¿4-(2)HNADH in D(2)O. The reduction of trans-2-octenoyl-CoA catalyzed by InhA resulted in the syn addition of (2)H(2) across the double bond yielding (2R,3S)-¿2, 3-(2)H(2)ŏctanoyl-CoA. In the crystal structure of the InhA ternary complex, the residue donating the proton to Calpha could not be identified ¿Rozwarski, D. A., Vilcheze, C., Sugantino, M., Bittman, R., and Sacchettini, J. C. (1999) J. Biol. Chem. 274, 15582-15589. The current results place further restrictions on the source of the proton and suggest the reduction is stepwise.

Action of quinolones against Staphylococcus aureus topoisomerase IV: basis for DNA cleavage enhancement

Topoisomerase IV is the primary cellular target for most quinolones in Gram-positive bacteria; however, its interaction with these agents is poorly understood. Therefore, the effects of four clinically relevant antibacterial quinolones (ciprofloxacin, and three new generation quinolones: trovafloxacin, levofloxacin, and sparfloxacin) on the DNA cleavage/religation reaction of Staphylococcus aureus topoisomerase IV were characterized. These quinolones stimulated enzyme-mediated DNA scission to a similar extent, but their potencies varied significantly. Drug order in the absence of ATP was trovafloxacin > ciprofloxacin > levofloxacin > sparfloxacin. Potency was enhanced by ATP, but to a different extent for each drug. Under all conditions examined, trovafloxacin was the most potent quinolone and sparfloxacin was the least. The enhanced potency of trovafloxacin correlated with several properties. Trovafloxacin induced topoisomerase IV-mediated DNA scission more rapidly than other quinolones and generated more cleavage at some sites. The most striking correlation, however, was between quinolone potency and inhibition of enzyme-mediated DNA religation: the greater the potency, the stronger the inhibition. Dose-response experiments with two topoisomerase IV mutants that confer clinical resistance to quinolones (GrlA(Ser80Phe) and GrlA(Glu84Lys)) indicate that resistance is caused by a decrease in both drug affinity and efficacy. Trovafloxacin is more active against these enzymes than ciprofloxacin because it partially overcomes the effect on affinity. Finally, comparative studies on DNA cleavage and decatenation suggest that the antibacterial properties of trovafloxacin result from increased S. aureus topoisomerase IV-mediated DNA cleavage rather than inhibition of enzyme catalysis.

Identification of the sites of hydroxyl radical reaction with peptides by hydrogen/deuterium exchange: prevalence of reactions with the side chains

Hydroxyl radical-effected protium/deuterium ((1)H/(2)H) exchange into the C-H bonds present in peptides has been used to identify the site of hydrogen atom abstraction by hydroxyl radical. Radiolysis of anaerobic, N(2)O-saturated D(2)O solutions containing peptide and dithiothreitol generates a hydroxyl radical that mediates (1)H/(2)H exchange into the side chains of peptides of up to 66 atom % excess (2)H. The (1)H/(2)H exchange is determined by measuring the isotope ratio, [M + H + 1](+)/[M + H](+), of the peptide using electrospray ionization-mass spectrometry. The (1)H/(2)H exchange within each residue of the peptide was determined by measuring the isotope ratio of each isolated dansyl amino acid following hydrolysis and derivatization. Generation of 0.40 mM hydroxyl radical effected (1)H/(2)H exchange into each of the five different residues of (Ala(2))-leucine enkephalin (YAGFL). The propensity of the residues to undergo exchange was L > Y > A congruent with F > G, independent of whether they were radiolyzed separately or as the peptide. The minimal exchange into glycine suggests that reaction of hydroxyl radical with the side chain hydrogens predominates over reaction with the polypeptide alpha-hydrogens. The ability of radiolysis to effect (1)H/(2)H exchange into a larger peptide, SNEQKACKVLGI, was also demonstrated.

Quinolones inhibit DNA religation mediated by Staphylococcus aureus topoisomerase IV. Changes in drug mechanism across evolutionary boundaries

Quinolones are the most active oral antibacterials in clinical use and act by increasing DNA cleavage mediated by prokaryotic type II topoisomerases. Although topoisomerase IV appears to be the primary cytotoxic target for most quinolones in Gram-positive bacteria, interactions between the enzyme and these drugs are poorly understood. Therefore, the effects of ciprofloxacin on the DNA cleavage and religation reactions of Staphylococcus aureus topoisomerase IV were characterized. Ciprofloxacin doubled DNA scission at 150 nM drug and increased cleavage approximately 9-fold at 5 microM. Furthermore, it dramatically inhibited rates of DNA religation mediated by S. aureus topoisomerase IV. This inhibition of religation is in marked contrast to the effects of antineoplastic quinolones on eukaryotic topoisomerase II, and suggests that the mechanistic basis for quinolone action against type II topoisomerases has not been maintained across evolutionary boundaries. The apparent change in quinolone mechanism was not caused by an overt difference in the drug interaction domain on topoisomerase IV. Therefore, we propose that the mechanistic basis for quinolone action is regulated by subtle changes in drug orientation within the enzyme.drug.DNA ternary complex rather than gross differences in the site of drug binding.

Fifty-year follow-up of cancer incidence in a historical cohort of Minnesota breast cancer families

A family history of breast cancer is well established as a risk factor for the disease. Because family history is a dynamic rather than a static characteristic, longitudinal studies of entire families can be very instructive in quantifying the significance of risk classification. The Minnesota Breast Cancer Family Study is a historical cohort study of relatives of a consecutive series of 426 breast cancer cases (probands) identified between 1944 and 1952. The incidence of cancer and the measurement of risk factors in sisters, daughters, granddaughters, nieces, and marry-ins was determined through telephone interviews and mailed questionnaires. Ninety-eight percent of eligible families were recruited, and 93% of members participated. A total of 9073 at-risk women were studied: 56% were biological relatives of the case probands, whereas the others were related through marriage. Through 1996, 564 breast cancers were identified in nonprobands. Compared to the rate of breast cancer among marry-ins (188 cases), sisters and daughters of the probands were at a 1.9-fold greater age-adjusted risk (128 cases; 95% confidence interval, 1.4-2.4); granddaughters and nieces were at a 1.5-fold greater risk (248 cases, 95% confidence interval, 1.2-1.8). The breast cancer risk since 1952 was not distributed equally across families: although all biological relatives had a family history of breast cancer, 166 families (39%) experienced no additional cases. Most of the cases occurred among a subset of families: 21 families had 5 breast or ovarian cancers, 8 had 6, 2 had 7, and 4 had > or =8. There was no evidence of significantly increased risk for cancer at other sites, including the ovaries, cervix, uterus, colon, pancreas, stomach, or lymphatic tissue, although there was some evidence that stomach cancer in previous generations may help define the susceptible subset. These families contain four to five generations of validated occurrences of cancer, thus minimizing the uncertainty of genetic risk inherent in a disease with a late and variable age at onset. The patterns of breast cancer in these multigeneration families is consistent with the influence of autosomal dominant susceptibility in a subset, low penetrance genes in another, and purely environmental influences in the remainder.

Regulation of the biosynthesis of N-acetylglucosaminylpyrophosphoryldolichol, feedback and product inhibition

The assembly of the core oligosaccharide region of asparagine-linked glycoproteins proceeds by means of the dolichol pathway. The first step of this pathway, the reaction of dolichol phosphate with UDP-GlcNAc to form N-acetylglucosaminylpyrophosphoryldolichol (GlcNAc-P-P-dolichol), is under investigation as a possible site of metabolic regulation. This report describes feedback inhibition of this reaction by the second intermediate of the pathway, N-acetylglucosaminyl-N-acetylglucosaminylpyrophosphoryldolichol (GlcNAc-GlcNAc-P-P-dolichol), and product inhibition by GlcNAc-P-P-dolichol itself. These influences were revealed when the reactions were carried out in the presence of showdomycin, a nucleoside antibiotic, present at concentrations that block the de novo formation of GlcNAc-GlcNAc-P-P-dolichol but not that of GlcNAc-P-P-dolichol. The apparent K(i) values for GlcNAc-P-P-dolichol and GlcNAc-GlcNAc-P-P-dolichol under basal conditions were 4.4 and 2.8 microM, respectively. Inhibition was also observed under conditions where mannosyl-P-dolichol (Man-P-dol) stimulated the biosynthesis of GlcNAc-P-P-dolichol; the apparent K(i) values for GlcNAc-P-P-dolichol and GlcNAc-GlcNAc-P-P-dolichol were 2.2 and 11 microM, respectively. Kinetic analysis of the types of inhibition indicated competitive inhibition by GlcNAc-P-P-dolichol toward the substrate UDP-GlcNAc and non-competitive inhibition toward dolichol phosphate. Inhibition by GlcNAc-GlcNAc-P-P-dolichol was uncompetitive toward UDP-GlcNAc and competitive toward dolichol phosphate. A model is presented for the kinetic mechanism of the synthesis of GlcNAc-P-P-dolichol. GlcNAc-P-P-dolichol also exerts a stimulatory effect on the biosynthesis of Man-P-dol, i.e. a reciprocal relationship to that previously observed between these two intermediates of the dolichol pathway. This network of inhibitory and stimulatory influences may be aspects of metabolic control of the pathway and thus of glycoprotein biosynthesis in general.

Cloning, expression, and purification of the functional 2,4-dienoyl-CoA reductase from rat liver mitochondria

The mitochondrial 2,4-dienoyl-CoA reductase (EC 1.3.1.34) is an auxiliary enzyme for the beta-oxidation of unsaturated fatty acids. Import of this enzyme into the mitochondria requires a mitochondrial signal sequence at the amino terminus of the polypeptide chain which is processed/removed once inside the mitochondria. The cDNA of the full-length 2,4-dienoyl-CoA reductase was previously cloned as pRDR181. PCR methodologies were used to subclone the gene encoding the functional 2,4-dienoyl-CoA reductase from pRDR181. The PCR product was inserted into a pET15b expression vector and overexpressed in Escherichia coli. The soluble expressed protein can be separated into high- and low-activity fractions. The low-activity fraction can be converted to the high specific activity form by thermal annealing, suggesting it is a metastable misfolded form of the enzyme. Using ion-exchange and affinity chromatography, the enzyme has been purified to homogeneity and exhibits a single band on Coomassie blue-stained SDS-PAGE. The molecular mass of 32,413 Da determined by electrospray ionization-mass spectrometry indicates that the amino-terminal methionine had been removed. The Michaelis constants for trans-2, trans-4-hexadienoyl-CoA and NADPH were determined to be 0.46 and 2.5 microM, respectively; a turnover number of 2.1 s(-1) was calculated.

Protein cross-links: universal isolation and characterization by isotopic derivatization and electrospray ionization mass spectrometry

A general method of unequivocally identifying and obtaining sequence information on cross-linked peptides derived by proteolytic digestion of cross-linked proteins has been developed. The method is based on isotopic labeling of alpha-amino groups with 2, 4-dinitrofluorobenzene (DNFB) coupled with electrospray ionization mass spectrometry. Proteins containing covalent cross-link(s) are reductively methylated to convert lysine residues to dimethyl lysine. The methylated protein is partially hydrolyzed and the liberated alpha-amino termini are derivatized with an equimolar mixture of DNFB and [(2)H(3)]DNFB. Dinitrophenyl (DNP)-labeled peptides may be fractionated into mono- and bis-DNP pools by chromatography on phenyl media. The bis-DNP peptides are further separated by reverse-phase HPLC and analyzed by electrospray ionization mass spectrometry. The molecular ions of cross-linked peptides are unambiguously identified as 1:2:1 triplets in the mass spectrum resulting from the binomial distribution of isotopic label in the bis-DNP derivative. Sequence information can be elucidated from the unique product ion patterns which are generated from in-source fragmentation at an elevated cone voltage. Analysis of the disulfide cross-linked peptide (VTCG)(2) was undertaken as a proof of concept and the generality of the method was demonstrated by isolating and sequencing the isopeptide bond of polyubiquitin.

Role of glutamate 144 and glutamate 164 in the catalytic mechanism of enoyl-CoA hydratase

The role of two glutamate residues (E164 and E144) in the active site of enoyl-CoA hydratase has been probed by site-directed mutagenesis. The catalytic activity of the E164Q and E144Q mutants has been determined using 3'-dephosphocrotonyl-CoA. Removal of the 3'-phosphate group reduces the affinity of the substrate for the enzyme, thereby facilitating the determination of K(m) and simplifying the analysis of the enzymes' pH dependence. k(cat) for the hydration of 3'-dephosphocrotonyl-CoA is reduced 7700-fold for the E144Q mutant and 630000-fold for the E164Q mutant, while K(m) is unaffected. These results indicate that both glutamate residues play crucial roles in the hydration chemistry catalyzed by the enzyme. Previously, we reported that, in contrast to the wild-type enzyme, the E164Q mutant was unable to exchange the alpha-proton of butyryl-CoA with D(2)O [D'Ordine, R. L., Bahnson, B. J., Tonge, P. J. , and Anderson, V. E. (1994) Biochemistry 33, 14733-14742]. Here we demonstrate that E144Q is also unable to catalyze alpha-proton exchange even though E164, the glutamate that is positioned to abstract the alpha-proton, is intact in the active site. The catalytic function of each residue has been further investigated by exploring the ability of the wild-type and mutant enzymes to eliminate 2-mercaptobenzothiazole from 4-(2-benzothiazole)-4-thiabutanoyl-CoA (BTTB-CoA). As expected, reactivity toward BTTB-CoA is substantially reduced (690-fold) for the E164Q enzyme compared to wild-type. However, E144Q is also less active than wild-type (180-fold) even though elimination of 2-mercaptobenzothiazole (pK(a) 6.8) should require no assistance from an acid catalyst. Clearly, the ability of E164 to function as an acid-base in the active site is affected by mutation of E144 and it is concluded that the two glutamates act in concert to effect catalysis.

Genetic anticipation and breast cancer: a prospective follow-up study

Genetic anticipation is characterized by an earlier age of disease onset, increased severity, and a greater proportion of affected individuals in succeeding generations. The discovery of trinucleotide repeat expansion (TRE) mutations as the molecular correlate of anticipation in a number of rare Mendelian neurodegenerative disorders has led to a resurgence of interest in this phenomenon. Because of the difficulties presented to traditional genetics by complex diseases, the testing for genetic anticipation coupled with TRE detection has been proposed as a strategy for expediting the identification of susceptibility genes for complex disorders. In the case of breast cancer, a number of previous studies found evidence consistent with genetic anticipation. It is known that a proportion of such families are linked to either BRCA1 or BRCA2, but no TRE mutations have been identified. It has been shown that the typical ascertainment employed in studies purporting to demonstrate genetic anticipation combined with unadjusted statistical analysis can dramatically elevate the type I error. We re-examine the evidence for anticipation in breast cancer by applying a new statistical approach that appears to have validity in the analysis of anticipation to data ascertained from a recent follow-up of a large prospective cohort family study of breast cancer. Using this approach, we find no statistically significant evidence for genetic anticipation in familial breast cancer. We discuss the limitations of our analysis, including the problem of adequate sample size for this new statistical test.

Isotope edited product ion assignment by alpha-N labeling of peptides with [2H3(50%)]2,4-dinitrofluorobenzene

An isotopic modification of Sanger's method for identifying peptide N-termini has been developed to assist peptide sequencing by tandem mass spectrometry. Tryptic peptides, such as Val-His-Leu-Thr-Pro-Val-Glu-Lys, are derivatized with an equimolar mixture of 2,4-dinitrofluorobenzene and [2H3]2,4-dinitrofluorobenzene. Under optimized derivatization conditions, the alpha-amino group could be derivatized while the epsilon-amine of the lysine side chain and the imidazole of histidine remained underivatized. The alpha-dinitrophenyl modified peptides were characterized by electrospray ionization-tandem mass spectrometry (ESI-MS/MS) and liquid chromatography (LC)-ESI-MS. The [M + H]+ ions showed a doublet pattern with a delta m/z of 3 and the [M + 2H]2+ ions were recognized as doublets with a delta m/z of 1.5. MS/MS was employed where both isotopic [M + 2H]2+ ions were alternately subjected to collision-induced dissociation in the second quadrupole. Fragmentation in the ionization source generated identical product ion patterns that were observed during fragmentation in the second quadrupole. In the product ion mass spectra, the N-terminal a and b ions (no c ion observed) are doublets with a delta m/z of 3 or 1.5, while the C-terminal y and z ions (no x ion observed) are singlets appearing at identical masses. Thus, the product ions containing the N-terminus derivatized with a dinitrophenyl group are unequivocally distinguished from the product ions containing the C-terminus. The dinitrophenyl modification generally enhanced the production of a and b ions without diminishing y and z ion yields.

Hydroxyl radical-induced hydrogen/deuterium exchange in amino acid carbon-hydrogen bonds

Hydroxyl radicals produced by radiolysis under anaerobic conditions in the presence of dithiothreitol and D2O have been shown to be capable of inducing hydrogen/deuterium (1H/2H) exchange in carbon-hydrogen bonds of amino acids. When the solution is saturated with N2O, a 1H/2H exchange efficiency of 38% (based on the G value of 5.6 x 10(-7) mol J(-1) for hydroxyl radical) was determined by measuring the amino acid isotope ratio [M+H+1]+/[M+H]+ using electrospray ionization-mass spectrometry. The incorporation of 2H was proportional to the amount of hydroxyl radical generated and required the presence of dithiothreitol. Using standard anaerobic reaction conditions with dithiothreitol and N2O, incorporations of 2H of 3% and 8% into L-valine (100 microM, 35 microM DTT) and L-leucine (100 microM, 31 microM DTT), respectively, were achieved after a dose of 89 Gy and, using d8-DL-valine (100 microM, 35 microM DTT) with H2O as the solvent, approximately 2% incorporation of protium was detected. Additionally, 1H/2H exchange into the peptide (Ala2)-leucine enkephalin produced 6% incorporation of 2H. These results directly demonstrate the ability of sulfhydryl groups to mediate the chemical repair of proteins through hydrogen-atom donation to an amino acid carbon-centered radical, thus providing a means of isotopically labeling solvent-accessible amino acid residues of peptides and proteins.