Sequence determination of N-terminal and C-terminal blocked peptides containing N-alkylated amino acids and structure determination of these amino acid constituents by using fast-atom-bombardment/tandem mass spectrometry

Total structure and inhibition of tumor cell proliferation of laxaphycins

From a mixed assemblage of Lyngbya majuscula rich marine cyanobacteria, we isolated a series of cell growth inhibitory cyclic peptides. The structures of the two major components, laxaphycins A (1) and B (2), and of two minor peptides, laxaphycins B2 (3) and B3 (4), were determined by spectroscopic methods and degradative analysis. Absolute configurations of natural and nonproteinogenic amino acids were determined by a combination of hydrolysis, synthesis of noncommercial residues, chemical derivatization, and HPLC analysis. The organism producing the laxaphycins was identified as the cyanobacterium Anabaena torulosa. The antiproliferative activity of laxaphycins was investigated on a panel of solid and lymphoblastic cancer cells. Our results demonstrate that in contrast to laxaphycin A, laxaphycin B inhibits the proliferation of sensitive and resistant human cancer cell lines and that this activity is strongly increased in the presence of laxaphycin A. This effect appears to be due to an unusual biological synergism.

Semi-automated chromatographic procedure for the isolation of acetylated N-terminal fragments from protein digests

Several published procedures have been combined to develop a general strategy for the specific identification and isolation of the acetylated-N-terminal fragment from all other proteolytic fragments. This ruse can be divided into four steps: (i) succinylation of the substrate to block lysine NH2 groups; (ii) enzymatic digestion of the modified protein; (iii) automated phenylisothiocyanate derivatization of the protease derived fragments to block newly generated "free" N-termini; and (iv) reversed-phase high-performance liquid chromatography with on-line photodiode array spectroscopy. The individual phenylthiocarbamyl-peptide species exhibit an increased reversed-phase retention time and a greater UV (210-297 nm) profile compared to the corresponding control (-phenylisothiocyanate) digest. The N-terminal acetylated fragment shows neither a retention time shift nor an augmented UV profile. To validate each process step, synthetic peptides and acetylated-N-terminal proteins of known sequence were used as test samples. The desired fragment was isolated from three proteins and positively identified by electrospray mass spectrometry and amino acid composition. Proteins with other N-terminal blocking groups should be amenable to this procedure.

Low-energy collision-induced dissociation of protonated polyamino alcohol derivatives of peptides and N-terminal blocked peptides

This investigation reports the low-energy collision-induced dissociation of the protonated molecules of polyamino alcohols, formed by chemical reduction of synthetic peptides and N-terminal blocked peptides, in order to evaluate its potential for peptide sequence determination. The --CH2--NH-- cleavage with charge retention on the N-terminus was prominent, and the entire sequence of ions produced in this manner was observed. Some of the sequence ions arising from --CH2--NH-- cleavage accompanied by migration of two hydrogens, and --NH--C alpha H-- cleavage with charge retention on the C-terminus, also occurred prominently. In addition, a great number of internal fragment ions were produced in relatively high abundance; these provided supplementary sequence information and were, in some cases, critical in determining the sequence. The usefulness of this method was exemplified by its application to the sequence determination of bacterial lipopeptides.

Adipokinetic hormone controls lipid metabolism in adults and carbohydrate metabolism in larvae of Manduca sexta

The peptide hormone which controls activation of fat body glycogen phosphorylase in starving larvae of Manduca sexta was isolated from larval corpora cardiaca and sequenced by FAB tandem mass spectrometry. It was found to be identical with Manduca AKH. This, together with earlier observations, demonstrates that in M. sexta AKH controls glycogen phosphorylase activation in starving larvae while in adults it controls lipid mobilization during flight. Larval corpora cardiaca contain about 10 times less AKH than the corpora cardiaca of adults. The corpora cardiaca of M. sexta appear to contain only one AKH.

Tandem mass spectrometry for the structural determination of backbone-modified peptides

A variety of backbone-modified peptides were desorbed by fast atom bombardment and collisionally activated. These peptide modifications involve the replacement of a normal [CONH] peptide linkage with such groups as thiomethylene ether (CH2S), thioamide (CSNH), methyleneamine (CH2NH), and thiomethylene sulfoxide (CH2SO) moieties. Modified linear peptides decompose to give fragmentations characteristic of the modifications as well as typical peptide bond fragments. The presence of a replacement group in cyclic peptides can induce new fragmentations. The presence of other functional groups, such as an exocyclic N-terminal residue, however, can dominate the observed fragmentations. Upon collisional activation, unmodified linear peptides fragment to give N-terminal ions as the most abundant daughter ions. In comparison, ψ[CH2NH] and ψ[CH2S ] modified linear peptides decompose to give prominent C-terminal sequence ions. The ψ[CH2SO] modified linear peptides, however, fragment into both N- and C-terminal ions of high relative abundance. Depending on the modification, daughter ions or internal fragment ions are observed that are characteristic of the amide bond replacement. Useful structural information can therefore be obtained.

Fast atom bombardment mass spectra of various peptides from sarcoplasmic calcium-binding protein

The sarcoplasmic calcium-binding protein of the crayfish Astacus leptodactylus is a calciprotein. The primary structure of this protein has been determined. Tryptic digestion of the denaturated protein followed by high-performance liquid chromatographic separation identified essentially eight peptides. The structure of these peptides has been confirmed after amino acid analysis by fast atom bombardment spectra in positive mode.

Collisionally activated decomposition of modified peptides using a tandem hybrid instrument

Analyses are described of small peptides and related compounds using a tandem hybrid mass spectrometer of BEQQ geometry. Collisionally activated decomposition of [M + H]+ ions, generated by fast atom bombardment, was performed in the radio frequency (rf)-only quadrupole. Interpretation of fragmentation was greatly facilitated by analysis of labeled analogs, obtained by 18O exchange of carboxyl oxygens. N-Acetylation was also valuable although significant changes in fragmentation resulted from derivatization. Daughter ion spectra of [M + H]+ ions of angiotensin III showed diagnostic fragmentations throughout the peptide chain.

The determination of the amino acid sequence in the fast atom bombardment mass spectra of dipeptides

The positive ion fast atom bombardment mass spectra of dipeptides all exhibit the MH+ ion as the most abundant one. Direct identification of the N-terminal amino acid in the mass spectrum is not possible because of the absence of its acylium ion. Fortunately the identification of the C-terminus is often possible because of the occurrence of the protonated C-terminal amino acid as a result of MH+ ion fragmentation. In all cases, including those where the abundance of this C-terminal ion is too low for identification, the metastable ion spectrum of the MH+ ion provides sufficient information for an unequivocal sequence determination.

Amino acid sequences of hemoglobins I and II from root nodules of the non-leguminous Parasponia rigida-rhizobium symbiosis, and a correction of the sequence of hemoglobin I from Parasponia andersonii

The amino acid sequence of hemoglobins I (pI 6.15 as oxyhemoglobin) and II (pI 5.64 as oxyhemoglobin) from the nitrogen-fixing root nodules of Parasponia rigida have been determined by protein sequencing. The sequence of hemoglobin I (pI 6.16, as oxyhemoglobin) from Parasponia andersonii was re-examined and the corrected primary structure, now in agreement with that predicted from the DNA sequence, is reported. The three Parasponia hemoglobins contain 161 amino acid residues (Mr approximately equal to 18,700 including the heme) with a single cysteine residue and five methionine residues. The N-terminal serine is blocked by an acetyl group. The primary structure of the Parasponia hemoglobins is highly conserved. Hemoglobins I from the two species of Parasponia are identical; both show microheterogeneity at position 30 (Asp/Glu substitution) and hemoglobin I from P. rigida shows microheterogeneity at position 150 (Ala/Val) while hemoglobin I from P. andersonii has only an Ala at 150. P. rigida hemoglobin II shows no microheterogeneity at these positions, having Asp and Val residues respectively, and it contains a single amino acid change of a Gln for an Arg at position 85, which accounts for the 0.5 unit difference in isoelectric point observed between hemoglobins I and II. The sequence data are consistent with allelic heterogeneity at a single locus rather than different genes.

Practical hints for peptide sequencing by soft ionization methods

Practical hints are summarized for the investigation of peptides by soft ionization methods based on liquid secondary ion mass spectrometry. The method aims: to purify the sample; to force a peptide to the surface by ion pair formation; to recover the sample used for mass spectral analysis; to distinguish between Lys and Gln by derivatization; to spot amino acids in the low mass region; and to improve sequence information by use of peptide fragments, not described previously.

Unequivocal structure assignment of C- versus N-alkylated amino acids using fast atom bombardment/tandem mass spectrometry

Structure assignment of the isomeric immonium ions 5 and 6, generated via FAB from N-isobutyl glycine and N-methyl valine, can be achieved by their collision induced dissociation characteristics.