Methionine derivatives, mass

C5a is inactivated by the myeloperoxidase-H202 system, which oxidises a methionine residue (Met 70) on the molecule group A streptococcal endo-proteinases also abolish chemotactic activity of C5a and related compounds. Neutrophil lysosomal enzymes (e.g. elastase and cathepsin G) also destroy C5a chemotactic activity, but as these proteases are inhibited by the serum antiproteinases, a -antiproteinase and a2-macroglobulin, the physiological role of neutrophilic proteases in the inactivation of C5a is questionable. Two chemotactic factor inactivators have been found in human serum an a-globulin that specifically and irreversibly inactivates C5-derived chemotactic factors, and a / -globulin that inactivates bacterial chemotactic factors. These activities are heat labile (destroyed by treatment at 56 °C for 30 min) and are distinct from those attributable to anaphylatoxin inactivator. An apparently specific inhibitor of C5-derived chemotactic activity has also been described in human synovial fluid and peritoneal fluid. This factor (molecular mass of 40 kDa) is heat stable and acts directly on C5a. 

These derivatizations are highly selective, and may thus allow PSD measurements to be carried out on peptides after modification. Such a protocol would significantly enhance our ability to derive sequence information from PSD spectra, because the mass shifts observed in fragments help locate the particular residue within the peptide, and also confirm assignments of fragments as arising from N- or C-terminal regions. In addition to derivatizations that may modify the C- and N-termini and the derivatization of tyrosine residues, we have carried out oxidation of methionine residues with sufficient specificity to enable measurement of PSD spectra. 

However, recent advances in trypsin cleavage and subsequent mass spectrometric identification of peptide fragments, coupled with the development of photoactive derivatives of propafenones, have enabled Chiba and colleagues to identify specific amino acids labeled during cross-linking [197]. Quantitative analysis of photolabeling indicates major sites for reactivity within TM3 (methionine 197), TMS (methionine 311), TMS (methionine 769), and TMll (methionine 951) (see Table 1.1 and Ref. [197]). Minor peaks were identified within several other TM a-helices including TMl and 12 [197] (Table 1.2). 

Extensive studies have been performed to disclose the biosynthetic pathway of microcystin and their lower mass analogs, the nodularins (53,85). One of the major questions was the origin of the Adda residue. The methyl substimtion pattern was indicative of incorporation of either propionate or acetate followed by methylation via S-adenosylmethionine. Although both propionate and methionine were found to be incorporated, the pattern of labelled metabolites was clearly indicative of an acetate-plus-methionine sequence for Cl through C8. The remainder of Adda presumably derives from phenylalanine via phenylacetic acid. The other subunits are for the most part derived from predictable pathways. According to the biosynthetic intermediates isolated the assembly of the linear penta- and octapeptides occurs with the Adda unit as N-terminal residue and Arg as C-terminus. Cyclization apparently represents the last step, since conversion of N-methyl-serine and -threonine to Mdha and Mdhb, respectively, occurs in earlier steps. 

Figure. 5. Formylation of HIV-1 p24 . (A) As shown in the upper spectrum, the major peaks of [M + H] at m/z 2016.17 represent the N-terminal tryptic peptide (Proi-Argig) of HIV-Ilav-1 p24 derived from silver staining gel. The other peaks of [M + H] at m/z 2032.13 and 2044.19 could be assigned to the methionine-oxidized form and the formylated form of the N-terminal tryptic peptide (Proi-Argig), respectively. As shown in the lower spectrum, these peaks were also detected in the spot corresponding to HIV-1lav-i p24 derived from Coomassie brilliant blue R-250 staining gel. The molecular mass of 28.02 corresponding to the formyl group was deleted. (B) The formyl group was sequentially cleaved with 0.6 N hydrochloric acid as described in Section 2.4.1.

P. Roepstorff, K. Norris, S, Severinsen, K. Brunfeldt, Mass spectrometry of peptide derivatives. Temporary protection of methionine as sulfoxide during permethylation FEBS Lett. 9 235-238(1970)... 

The mass spectrum of the bis-TMS-derivative of methionine exhibits a molecular ion at m/e 293 with an intensity of about 5 % of that of the base peak at m/e 176, the latter arising by fission of the Cj-C2-bond ... 

Figure 5.11 ESI product ion mass spectra of (a) the three-fold charged precursor ion [M+3H] of m/z 1182 of T4 derived from the 20kDa splice variant of hGH ( the cysteine was derivatized to its acrylamide adduct), and (b) the two-fold charged precursor ion [M+2H] of m/z 481.1 of the modified peptide T2 comprising an isoleucine (bold) instead of a methionine at position 14 of hGH (corresponding to position 6 of T2), both measured on an LTQ system.

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