Primary structure of trypsin inhibitors from Sicyos australis

Three trypsin inhibitors from Sicyos australis, have been isolated, purified and sequenced. Following protein extraction with ammonium sulphate, the mixture of inhibitors was separated from other proteins by trypsin-affinity chromatography. Subsequent purification of the individual inhibitors was accomplished by reversed-phase HPLC. The primary structures of each inhibitor were elucidated by a combination of protein sequencing and electrospray ionization mass spectrometry (ESI-MS) and tandem mass spectrometry (MS-MS) on both the untreated and the reduced and S-carboxymethylated inhibitors. All three inhibitors show extensive sequence similarity with inhibitors from cultivated Cucurbitaceae species, although there are a number of novel residues present. One of the inhibitors has a blocked N-terminus (pyroglutamic acid) and the use of MS-MS was crucial to the elucidation of its primary structure. ESI-MS was further used to characterize the non-covalent complex between one of the inhibitors and trypsin.

Electrospray tandem mass spectrometry of nucleotides

The tandem mass spectra (MS/MS) following electrospray ionization of nucleotides (from 2- to 6-mers) have been examined under a range of collision and ionization conditions. In general, abundant peaks due to the free bases and phosphate groups are observed, along with a limited number of sequence ions. A number of strategies for increasing the extent of the observed fragmentation were investigated. Increasing the skimmer potential of the ion source, thereby increasing the initial internal energy of the incident ions, did not significantly enhance the sequence-specific fragmentation, although the relative intensities of fragment ions increased. In MS/MS spectra of different adducts (i.e. [M + H]+, [M-H]-) of dinucleotides the intensity of the ions due to the bases (BH2+ or B-) is dependent on both the base position (i.e. either 3' or 5') and the nature of the adduct ion. These data show that MS/MS spectra of [M-H]- ions of dinucleotides generated by electrospray ionization may be equally as diagnostic as MS/MS spectra obtained following fast-atom bombardment. With [M + Li]+ ions, more sequence ions were observed compared to the corresponding protonated and deprotonated species. For oligonucleotide hexamers, MS/MS spectra of the [M-2H]2- ions show fragment ions due to loss of bases followed by cleavage of the 3' C--O bond in the sugar from which the base is lost (i.e. w ions); however, a strong preference for loss of any particular base is not observed. Finally, MS/MS spectra of 5'-ApCpC-3' and 5'-d(AGGCCT)-3' obtained on a magnetic sector orthogonal acceleration time-of-flight mass spectrometer demonstrate the potential utility of this instrumental configuration for MS/MS analysis of oligonucleotides.

The observation of a hedamycin-d(CACGTG)2 covalent adduct by electrospray mass spectrometry

Covalent binding of the antitumour antibiotic hedamycin to the self-complementary hexadeoxyribonucleotide 5'-CACGTG-3' has been investigated by electrospray ionization mass spectrometry (ESI-MS). Ions due to double-stranded forms of the free 5'-CACGTG-3' and the hedamycin-5'- CACGTG-3' adduct have been observed in ESI mass spectra and their identity has been confirmed by resolution of individual charge states in ESI-MS spectra. Clear evidence that specific base-paired associations are being observed in ESI-MS is provided by the results of a titration experiment involving alkylated and non-alkylated complementary strands. This work demonstrates the potential of this powerful new tool for studying ligand-DNA binding.

Electrospray ionization mass spectrometry of covalent ligand-oligonucleotide adducts: evidence for specific duplex ion formation

Electrospray ionization mass spectrometry (ESI-MS) has been used to examine the covalent binding of the anti-tumour agents cisplatin and hedamycin with self-complementary oligonucleotides 5'-TACGTA-3', 5'-CACGTG-3', 5'-AGGCCT-3' or 5'-CGTACG-3' as models for binding to cellular DNA. The observation of duplex forms of oligonucleotide adducts of these compounds in the gas phase has been found to correlate with the stability of the adducts in solution. Hedamycin, which both intercalates into and alkylates DNA, enhances the stability of duplex ions in ESI mass spectra. In contrast, the binding of cisplatin is known to destabilise duplexes in solution and only weak double-stranded peaks are observed in the ESI spectra of cisplatin-oligonucleotide adducts. Results of titration experiments with the hedamycin-5'-CACGTG-3' adduct and complementary and non-complementary oligo-nucleotides provide strong evidence that the observed duplex ions are the result of specific base-paired associations in the gas phase, rather than non-specific interactions. Finally, estimates of the extent of cisplatin binding to different sequences based on ESI mass spectra of crude reaction mixtures are found to correlate well with data obtained by reversed-phase high-performance liquid chromatography. This work demonstrates the considerable potential of ESI-MS as a tool for characterization of the interactions of antitumour agents with DNA.