Surface-induced dissociation of peptide ions in Fourier-transform mass spectrometry

Peptide molecular ion species up to m/z 3055 introduced into a Fourier-transform mass spectrometer can be made to undergo extensive fragmentation by electrically floating the ion cell. The proportion of ions dissociated increases with increasing voltage, with 48 eV producing the highest absolute abundance of fragment ions above m/z 200. At this energy, spectra closely resemble those from photodissociation at 193 nm, indicating an internal energy deposition of 6-7 eV; change of product abundances with kinetic energy resembles a conventional breakdown curve. The precursor ions apparently are electrostatically attracted to strike screen wires across the ion cell entrance, producing daughter ions of low kinetic energy.

Metabolism-based covalent bonding of the heme prosthetic group to its apoprotein during the reductive debromination of BrCCl3 by myoglobin

The reductive metabolism of BrCCl3 by ferrous myoglobin leads to the alteration of the prosthetic heme to form products that can be dissociated from the protein and to those that are irreversibly bound to the protein. The major dissociable or soluble heme metabolites have recently been characterized. In this study, the irreversibly bound heme product was characterized by Edman degradation, amino acid analysis, and electronic absorption and mass spectrometry of peptides derived from the altered protein. It was found that the prosthetic heme was modified by a CCl2 moiety derived from BrCCl3 and was covalently bound to histidine residue 93, the normal proximal ligand to the heme-iron. The data are consistent with a mechanism by which the trichloromethyl radical reacts with the heme to form an intermediate that either can alkylate the proximal histidine residue or form soluble metabolites. The covalent bonding of the heme prosthetic moiety to the apoprotein likely leads to a change in the tertiary structure of the protein that may be responsible for its altered catalytic activity as well as its enhanced susceptibility to proteolysis. Similar processes may account, at least in part, for the covalent alteration of the heme prosthetic group of other hemoproteins caused by xenobiotics and endogenous substrates.

Analysis of tryptic peptides from the C-terminal region of alpha-crystallin from cataractous and normal human lenses

Antisera have been made to synthetic peptides corresponding to the expected tryptic fragments from the C-terminal region of human alpha A2 crystallin (T19 corresponds to residues 158-163; T20 corresponds to residues 164-173). These antisera were used on conjunction with a sensitive radioimmunoassay, to identify the elution times of peptides resolved on a C18 column from a tryptic digest of water soluble and water insoluble proteins from the human lens. Isolation and purification of the peptides reactive with the anti-peptide sera, followed by the use of tandem mass spectrometry to determine the amino acid sequences in the peptides, demonstrated that the antisera reacted specifically with the T19 and T20 sequences. Using the antisera specific for the T19 sequence, analysis of the peptides resolved from tryptic digests of individual lenses demonstrated no major differences between the elution profiles of five normal vs. ten cataractous lenses, while analysis of the same digests with the antiserum to the T20 sequence demonstrated major changes in reactivity and/or elution time of tryptic peptides from eight of the cataractous lenses analyzed. Together, these studies strongly suggest that during human cataract formation, covalent changes occur in the C-terminal region of the alpha A2 molecule. In addition, these studies provide the general methodology, whereby antisera specific for known sequences of a polypeptide chain, can be used to locate the sequences involved in covalent modification during the process of senile cataractogenesis of the human lens.

The complete sequence of the acidic subunit from Mojave toxin determined by Edman degradation and mass spectrometry

Mojave toxin, a heterodimeric, neurotoxic phospholipase complex from Crotalus scutulatus scutulatus, is one of a group of closely related rattlesnake toxins for which much structural information is still lacking. The complete amino-acid sequence of the acidic subunit from Mojave toxin was determined. The three individual peptide chains, derived from the acidic subunit by reductive alkylation, were separated by high-performance liquid chromatography. Fragmentations of the A and B chains were done using specific proteinases and the resulting peptide mixtures were fractionated by reverse-phase high-performance liquid chromatography. Sequence analyses on the intact chains and the fragments from digests were done by automated Edman degradation, carboxypeptidase Y degradation and triple-quadrupole and tandem-quadrupole Fourier-transform mass spectrometry. The sequence for each acidic subunit chain is very similar to the corresponding chain from the related neurotoxin complex, crotoxin, and overall the sequence is similar to the sequences of group I and II phospholipases A2. The N-terminus of the B chain is blocked by pyroglutamic acid. The existence of two distinct and closely related C chains was established. It is unlikely that the small sequence difference can account for the isoforms that are present in purified Mojave toxin and in unfractionated venom.

Identification of proctolin in the central nervous system of the horseshoe crab, Limulus polyphemus

A proctolin-like peptide was isolated from the prosomal CNS of the chelicerate arthropod, Limulus, and purified using size exclusion, ion exchange and high performance liquid chromatography. Coincident bioassay (cockroach hindgut) and radioimmunoassay were employed to identify fractions which contained proctolin-like material. Proctolin-like activity coeluted with synthetic proctolin with all three chromatographic techniques employed. When applied to either the Limulus heart or hindgut preparations, purified Limulus proctolin produced excitatory responses which were indistinguishable from those produced by the synthetic peptide. Purified samples of the Limulus proctolin-like peptide were subjected to Edman degradation and tandem mass spectrometry and the amino acid sequence of the Limulus peptide was determined to be identical to that of cockroach proctolin (H-Arg-Tyr-Leu-Pro-Thr-OH). The presence of proctolin in the Limulus CNS and its biological action on the isolated heart and hindgut suggest a physiological role for this peptide in the regulation of cardiac output and hindgut motility.

Fourier-transform mass spectrometry of large molecules by electrospray ionization

The multiply charged ions produced by electrospray ionization of peptides of molecular masses up to 29 kDa have been successfully introduced into a Fourier transform mass spectrometer of unique capabilities for tandem mass spectrometry, large ion dissociation, and resolution. Electrospray ionization places an unusually high number of charges on a peptide yielding mass/charge (m/z) values of 600-1500; in this range at normal operating pressures (approximately 10(-9) torr; 1 torr = 133.3 Pa) Fourier-transform mass spectrometry resolving power is greater than 100,000. Although only 10(-7) torr pressure has been obtained with the initial interface, the resulting resolving power of 5000 makes possible the resolution of isotopic peaks of multiply charged ions. Mass measuring accuracies of a few daltons for molecular masses up to 17 kDa have also been achieved.

The amino acid sequence of the sex steroid-binding protein of rabbit serum

The amino acid sequence of the sex steroid-binding protein (SBP or SHBG) of rabbit serum, specific for binding testosterone and 5 alpha-dihydrotestosterone, was determined using a complementary combination of mass spectrometric and Edman degradation techniques. The monomeric unit of the homodimeric protein is a single chain glycopeptide of 367 amino acid residues, with N-linked oligosaccharide side chains at Asn-345 and Asn-361 and disulfide bonds connecting Cys-158 to Cys-182 and Cys-327 to Cys-355. The polypeptide molecular weight of the monomer calculated from the sequence is 39,769. The molecular weight of the homodimer including 9% carbohydrate is 87,404. The sequence contains a relatively hydrophobic segment between Trp-241 and Leu-282, which includes many leucine residues in an alternating pattern. An amino acid sequence repeat is also located within that segment. Both of these patterns are present in human SBP and in the androgen-binding protein of rat epididymis. The sequence data indicate that the previously reported microheterogeneity of rabbit SBP in sodium dodecyl sulfate-polyacrylamide gel electrophoresis reflects variants generated by differential glycosylation of the monomer rather than different gene products. Seventy-nine percent of the amino acids of rabbit SBP are identical to those of human SBP; rabbit SBP thus joins human SBP and rat androgen-binding protein in one gene family that is distinct from the steroid hormone receptor superfamily. It appears that the problem of binding sex steroid hormones has been solved independently in two different gene families that contain completely different steroid-binding domains. Since the nonhomologous steroid-binding domains of both families of proteins recognize essentially the same steroid structure, it will be interesting to determine the structural basis of the two different protein designs that lead to similar steroid-binding specificity.

Proteolytic fragments of the nicotinic acetylcholine receptor identified by mass spectrometry: implications for receptor topography

A triple-state quadrupole or a tandem quadrupole Fourier-transform mass spectrometer was used to detect and sequence the peptides released by proteolytic cleavage of the acetylcholine receptor (AcChR) from Torpedo californica electroplax. Fragments in mass range up to 3479 daltons were characterized on the above instrumentation and used to determine proteolytically accessible sites on the receptor. These data were consistent with the cleavage points determined for membrane-bound fragments of the same AcChR samples using gas-phase microsequencing. Each subunit of the receptor is readily cleaved near the C-terminus in the region between the proposed transmembrane hydrophobic alpha-helices MIII and MIV. This region includes the putative regulatory phosphorylation sites and the amphipathic alpha-helix. Cleavage is also observed in the N-terminal domain, but occurs much more slowly than in the C-terminal region. No cleavage was detected in the middle third of the receptor, which includes the proposed transmembrane alpha-helices MI and MII. An evaluation of these data in terms of the transmembrane topography of the AcChR peptides is consistent with a synaptic or extracellular disposition for the region between MIII and MIV.

Amino acid sequence analysis of two mouse calbindin-D9k isoforms by tandem mass spectrometry. Protein modification by internal insertion of a single amino acid

Two forms of calbindin-D9k have sometimes been observed within a single tissue. Sequencing of these proteins has been complicated by the presence of blocked amino termini. Tandem mass spectrometry is a powerful tool for comparing related proteins, and its use does not depend upon an unblocked amino terminus. In the present studies, calbindin-D9k was purified from the intestines of mice (270 animals per purification) by use of gel permeation chromatography and two preparative electrophoresis steps in the presence and absence of EDTA. The purified protein appeared to be homogeneous following electrophoresis under nondenaturing conditions, but two components were identified by sodium dodecyl sulfate-gel electrophoresis and immunoblotting. Two forms of the protein were isolated by reverse-phase high performance liquid chromatography. In each of three preparations, the average ratio of the major:minor isoforms was 2:1. The major form contained 77 amino acids and lacked the amino-terminal serine found in 78-amino acid calbindins from rat and pig. The amino acid sequence was identical with the deduced sequence reported for rat intestinal calbindin-D9k in 73 of 77 positions. In the minor form, a glutamine was found in a location between Lys-43 and Ala-44 of the major form and between the two calcium binding sites of the protein. The minor form was otherwise identical with the major form, including the presence of a blocked amino terminus. The inserted glutamine was located at the site of an intron in the rat calbindin gene, suggesting the possibility that alternative splicing produced the two forms of calbindin-D9k. The functional significance of an inserted amino acid between the two calcium binding sites remains to be explored.

Complete amino acid sequence of a human monocyte chemoattractant, a putative mediator of cellular immune reactions

In a study of the structural basis for leukocyte specificity of chemoattractants, we determined the complete amino acid sequence of human glioma-derived monocyte chemotactic factor (GDCF-2), a peptide that attracts human monocytes but not neutrophils. The choice of a tumor cell product for analysis was dictated by its relative abundance and an amino acid composition indistinguishable from that of lymphocyte-derived chemotactic factor (LDCF), the agonist thought to account for monocyte accumulation in cellular immune reactions. By a combination of Edman degradation and mass spectrometry, it was established that GDCF-2 comprises 76 amino acid residues, commencing at the N terminus with pyroglutamic acid. The peptide contains four half-cystines, at positions 11, 12, 36, and 52, which create a pair of loops, clustered at the disulfide bridges. The relative positions of the half-cystines are almost identical to those of monocyte-derived neutrophil chemotactic factor (MDNCF), a peptide of similar mass but with only 24% sequence identity to GDCF. Thus, GDCF and MDNCF have a similar gross secondary structure because of the loops formed by the clustered disulfides, and their different leukocyte specificities are most likely determined by the large differences in primary sequence.

Structural characterization of toxic cyclic peptides from blue-green algae by tandem mass spectrometry

Combined use of chemical degradation, derivatization, and tandem mass spectrometry for rapid structural characterization of toxic cyclic peptides from blue-green algae at the nanomole level is described. Previously, all blue-green algal toxins were thought to belong to a family of seven-residue cyclic peptides, having the general structure cyclo-D-Ala-L-Xaa-erythro-beta-methyl-D-isoaspartic acid-L-Yaa-Adda-D-isoglutamic acid-N-methyldehydroalanine, where Xaa and Yaa represent variable amino acids of the L configuration and Adda is 3-amino-9-methoxy-2,6,8-trimethyl-10-phenyl-deca-4,6-dienoic acid. Structural characterization of two additional toxins indicates that further variability can exist within this family of naturally occurring toxic cyclic peptides. Isoaspartic acid and dehydroalanine can substitute for beta-methylisoaspartic acid and N-methyldehydroalanine, respectively.

Characterization of a benzyladenine binding-site peptide isolated from a wheat cytokinin-binding protein: sequence analysis and identification of a single affinity-labeled histidine residue by mass spectrometry

A wheat embryo cytokinin-binding protein was covalently modified with the radiolabeled photoaffinity ligand 2-azido-N6-[14C]benzyladenine. A single labeled peptide was obtained after proteolytic digestion and isolation by reversed-phase and anion-exchange HPLC. Sequencing by classical Edman degradation identified 11 of the 12 residues but failed to identify the labeled amino acid. Analysis by laser photodissociation Fourier-transform mass spectrometry of 10 pmol of the peptide independently confirmed the Edman data and also demonstrated that the histidine residue nearest the C terminus (underlined) was modified by the reagent in the sequence Ala-Phe-Leu-Gln-Pro-Ser-His-His-Asp-Ala-Asp-Glu.

Tandem mass spectrometry reveals that three photosystem II proteins of spinach chloroplasts contain N-acetyl-O-phosphothreonine at their NH2 termini

Photosystem II cores of spinach contain four phosphoproteins (8.3, 32, 34, and 44 kDa). Tryptic digestion of core particles released four phosphopeptides which were purified by affinity chromatography on Fe3+-chelating Sepharose and reverse-phase high pressure liquid chromatography. One peptide, derived from the 8.3-kDa protein, has been found to be the NH2 terminus of the psbH gene product (Michel, H. P., and Bennett, J. (1987) FEBS Lett. 212, 103-108). The other three peptides were found to be blocked at the NH2 terminus. We now report the use of tandem mass spectrometry to obtain the sequence of the three other peptides, to locate the phosphorylated residue, and to identify the blocking group. The three peptides correspond to the NH2 termini of D1, D2, and CPa-2; and each begins with N-acetyl-O-phosphothreonine. Comparison with sequences deduced from cloned genes indicates that D1 and D2 have lost their initiating N-formylmethionyl residues. The result for D1 contradicts the view that translation of D1 begins at the second AUG of the mRNA (Bloom, M., Brot, N., Cohen, B. N., and Weissbach, H. (1986) Methods Enzymol. 118, 309-315) and supports the view that processing of pre-D1 to its mature form involves loss of amino acids from the COOH terminus (Marder, J. B., Goloubinoff, P., and Edelman, M. (1984) J. Biol. Chem. 259, 3900-3908). In contrast, CPa-2 is processed at the NH2 terminus by cleaving off the first 14 amino acids. These results also establish that the NH2 termini of D1, D2, and CPa-2 are exposed to the stromal side of the thylakoids.

Amino acid sequence analysis of the neuronal type II calmodulin-dependent protein kinase by tandem mass spectrometry

The primary structure of the neuronal Type II calmodulin-dependent protein kinase has been examined by protein sequence analysis and compared to cDNA-derived sequence. Tandem mass spectroscopic analysis was used for the sequence determination. Comparison with published cDNA sequence data for the alpha subunit revealed that the difference between the alpha- and beta-subunits lay in two insertions into the sequence for the alpha-subunit and a short alpha-specific sequence. The N-terminal amino acid of the alpha subunit which is blocked to Edman degradation has been tentatively identified as N-acetyl-alanine.

Site-directed chemical modification of horse cytochrome c results in changes in antigenicity due to local and long-range conformational perturbations

Comparative binding studies with peptide fragments of the whole antigen, or with evolutionarily related intact proteins with varying degrees of sequence homology, have been used extensively to map antigenic sites on proteins to the resolution of single amino acid residues. These methods are limited, however, since high affinity antibodies will often not react with peptides and evolutionarily related proteins are available for only a few antigens. In this study we use site-directed chemical modification of horse cytochrome c to identify residues involved in the binding sites of four monoclonal antibodies specific for this protein. Thus, we have N-formylated the single tryptophan found in horse cytochrome c at position 59 and N-carbethoxylated one of the histidyl residues, which was determined to be at position 26 by the analysis of proteolytic cleavage fragments of the modified protein using liquid secondary ion-mass spectrometry on triple quadropole or tandem quadropole Fourier transform instruments. We discuss the impact of these modifications on the antigenicity of horse cytochrome c with regard to the conformational perturbations introduced by such modifications and with reference to our previous studies on the binding sites of these antibodies using other methodologies (Jemmerson, R., and Paterson, Y. (1986) BioTechniques 4, 18-31).

Sequence homology in the metalloproteins; purple acid phosphatase from beef spleen and uteroferrin from porcine uterus

The primary structures of purple acid phosphatase and uteroferrin, two iron-binding glycoproteins isolated from beef spleen and porcine uterine fluids, respectively, have been examined by a combination of tandem mass spectrometry and classical Edman sequencing methods. Reported here are amino acid sequence data covering more than 90% of the primary structures for these two proteins. The sequence data reveal an unexpectedly high degree of homology, greater than 90%, for these two proteins.

Tandem quadrupole Fourier-transform mass spectrometry of oligopeptides and small proteins

Modifications to the newly developed tandem quadrupole Fourier-transform mass spectrometer have made it possible to record mass spectra on oligopeptides and small proteins in the mass range between 2 and 13 kDa.

Protein sequencing by tandem mass spectrometry

Methodology for determining amino acid sequences of proteins by tandem mass spectrometry is described. The approach involves enzymatic and/or chemical degradation of the protein to a collection of peptides which are then fractionated by high-performance liquid chromatography. Each fraction, containing as many as 10-15 peptides, is then analyzed directly, without further purification, by a combination of liquid secondary-ion/collision-activated dissociation mass spectrometry on a multianalyzer instrument. Interpretation of collision-activated dissociation mass spectra is described, and results are presented from a study of soluble peptides produced by treatment of apolipoprotein B with cyanogen bromide and trypsin.

Sequence analysis of polypeptides by collision activated dissociation on a triple quadrupole mass spectrometer

A new approach to the direct sequencing of oligopeptides in complex mixtures is described. Mixtures of [2Ho]/[2H3]-N-acetylated and N,O-permethylated peptides are analyzed by collision activate dissociation on a triple quadrupole mass spectrometer using isobutane chemical ionization. Analysis of the collision activated dissociation spectra enables peptide sequences to be deduced. Use of electron capture negative chemical ionization for the sequence analysis of neuropeptides at the picomole level is also described.

Studies of negative ions by collision-induced decomposition and hydrogen-deuterium exchange techniques

Development of two new techniques for studying the gas phase chemistry of negative ions is reported. Collision induced dissociation (CID) of (M-1)- ions has been accomplished in a newly constructed triple stage quadrupole mass spectrometer. This instrument was assembled by adding two additional Finnigan quadrupole mass filters to a Finnigan Model 3200 CI mass spectrometer. Generation of (M-1)- ions is accomplished by allowing OH- and sample to react under CI conditions in the ion source. The first quadrupole mass filter, Q1, is then employed to selectively pass the (M-1)- ion into a second quadrupole filter containing argon or neon at 10(-3) torr. On collision with the inert gas the (M-1)- ions dissociate into fragments which are then mass analyzed in the third quadrupole filter, CID spectra of (M-1)- ions from twelve carbonyl compounds are presented in this paper. Ion molecule isotope exchange reactions in the CI ion source can be used to count the number of hydrogen atoms in many different chemical environments. Collisions between sample (M-1)- ions and deuterium-labeled reagent gases (ND3, D2O, EtOD) facilitate incorporation of deuterium into the negative ion if the basicities of the sample and reagent anions are similar. Thus it is possible to selectively incorporate deuterium into many organic samples by controlling the exothermicity of the acid base, ion-molecule chemistry.

Poisoning with 4-aminopyridine: report of three cases

Four-aminopyridine is an acutely toxic avicide considered by the manufacturer to be a bird "repellant" because only a small number of birds are acutely poisoned, become disoriented, and emit a distress cry frightening other members of the flock. Four-aminopyridine dramatically enhances transmission at the neuromuscular junction and other synapses, and has been employed clinically in the treatment of prolonged paralysis caused by antibiotics and muscle relaxants, and in the Eaton-Lambert syndrome. In this paper we report the results of an acute poisoning misadventure in three adult males. We review the animal toxicology, summarize the neurophysiological research using 4-AP as a potassium channel blocker, comment on clinical applications, and outline the management of overdose with this agent.