Covalent structure of the gamma chain of the A subunit of cholera toxin

Characterisation of cholera toxin by liquid chromatography--electrospray mass spectrometry

Cholera toxin, one of the toxins that may be generated by various strains of the bacterium Vibrio cholerae, can be considered as a substance possibly used in biological warfare. The possibilities of characterising the toxin by liquid chromatography electrospray mass spectrometry (LC-ES-MS) were investigated. The toxin can be detected by flow-injection (FIA) ES-MS of a dialysed solution and observation of the charge envelope signals of its A-unit and B-chain protein; sufficient information for identification by the molecular mass of either protein could be obtained for quantities in the order of 10 fmol. Confirmatory analysis was carried out by 2-mercaptoethanol reduction and FIA-ES-MS detection of the product proteins or by tryptic digest LC-ES-MS with ion chromatogram detection of most of the tryptic fragments of the A-unit and B-chain from the singly, doubly or triply charged ion signals. The confirmatory tryptic digest LC-ES-MS analysis could be achieved with quantities as low as 1 pmol. Possible biovariations in the toxin can mostly be determined by sequencing, where the amino acid composition of tryptic fragments of the A1-chain, T5 and T15, and of the B-chain, T1, T4 and T5, cover all known biovariations. Partial sequencing of cholera toxin, originating from a classical strain, O1/569B, was achieved by LC-ES-MS/MS of most tryptic fragments larger than three amino acid residues.

Covalent binding of ethylene dibromide and its metabolites to albumin

The present study was undertaken to determine covalent binding of [1,2-14C]ethylene dibromide (EDB) to albumin under in vivo and in vitro conditions. For the in vivo covalent binding, 25 mg/kg body weight of [1,2-14C]EDB was given daily to male rats for 12 consecutive days and the animals were sacrificed at 24 h following the last dose. Blood was withdrawn from inferior vena cava in heparinized tubes and plasma was separated, dialyzed against ice-cold 10 mM phosphate buffer (pH 7.4) and then subjected to size-exclusion high-performance liquid chromatography (SE-HPLC). A major radioactive peak eluted at an elution volume corresponding to 65,000 dalton molecular mass was found to be associated to albumin at a level of 0.14 nmol equivalent EDB/mg protein. For the in vitro covalent binding, human plasma or purified albumin was incubated with [1,2-14C]EDB in the presence of phenobarbital-treated rat liver microsomes and NADPH-generating system for 2 h at 37 degrees C. The 100,000 x g supernatant of the incubation mixture was dialyzed extensively and analyzed as described for the in vivo studies. Approximately 0.28 nmol equivalent EDB/mg protein was found to be associated to albumin (about 2-fold higher than the in vivo binding). Binding of 14C-label to albumin under in vivo and in vitro conditions was further supported by the affinity chromatography of albumin fraction isolated by SE-HPLC. Reversed-phase HPLC analysis of pronase digest of the albumin obtained from in vitro studies indicated formation of several amino acid adducts of EDB and/or its metabolites. Structure elucidation of such amino acid adducts will be helpful in developing a relatively non-invasive method of measuring the EDB exposure.

Primary and secondary structural analyses of glutathione S-transferase pi from human placenta

The primary structure of glutathione S-transferase (GST) pi from a single human placenta was determined. The structure was established by chemical characterization of tryptic and cyanogen bromide peptides as well as automated sequence analysis of the intact enzyme. The structural analysis indicated that the protein is comprised of 209 amino acid residues and gave no evidence of post-translational modifications. The amino acid sequence differed from that of the deduced amino acid sequence determined by nucleotide sequence analysis of a cDNA clone (Kano, T., Sakai, M., and Muramatsu, M., 1987, Cancer Res. 47, 5626-5630) at position 104 which contained both valine and isoleucine whereas the deduced sequence from nucleotide sequence analysis identified only isoleucine at this position. These results demonstrated that in the one individual placenta studied at least two GST pi genes are coexpressed, probably as a result of allelomorphism. Computer assisted consensus sequence evaluation identified a hydrophobic region in GST pi (residues 155-181) that was predicted to be either a buried transmembrane helical region or a signal sequence region. The significance of this hydrophobic region was interpreted in relation to the mode of action of the enzyme especially in regard to the potential involvement of a histidine in the active site mechanism. A comparison of the chemical similarity of five known human GST complete enzyme structures, one of pi, one of mu, two of alpha, and one microsomal, gave evidence that all five enzymes have evolved by a divergent evolutionary process after gene duplication, with the microsomal enzyme representing the most divergent form.

Aplysia californica neurons R3-R14: primary structure of the myoactive histidine-rich basic peptide and peptide I

The R3-R14 neurons of the marine mollusc Aplysia are neuroendocrine cells that express a gene encoding peptides I, II and histidine-rich basic peptide (HRBP), a myoactive peptide that excites Aplysia heart and enhances gut motility in vitro. Peptide II has been chemically characterized (35), but the complete primary structures of peptide I and HRBP have not been established by amino acid sequence analysis. HRBP, peptide I, and the prohormone (proHRBP) were therefore purified from acid extracts of Aplysia californica neural tissue using sequential gel filtration and reverse-phase high-performance liquid chromatography and chemically characterized. Amino acid sequence analysis demonstrated that HRBP was a 43-residue peptide whose sequence was: less than Glu-Val-Ala-Gln-Met-His-Val-Trp-Arg-Ala-Val-Asn-His-Asp-Arg-Asn-His-Gly- Thr-Gly - Ser-Gly-Arg-His-Gly-Arg-Phe-Leu-Ile-Arg-Asn-Arg-Tyr-Arg-Tyr-Gly-Gly-Gly- His-Leu - Ser-Asp-Ala-COOH. Compositional and sequence analyses of peptide I and proHRBP demonstrated that peptide I was a 26-residue peptide with the following sequence: NH2-Glu-Glu-Val-Phe-Asp-Asp-Thr-Asp-Val-Gly-Asp-Glu-Leu-Thr-Asn-Ala- Leu-Glu-Ser-Val-Leu-Thr-Asp-Phe-Lys-Asp-COOH. These results demonstrated that the pro-HRBP sequence predicted by nucleotide sequence analysis of a cDNA clone (24) was in fact synthesized in R3-R14 neurons. Hydrophilicity and hydrophobicity profiles of preproHRBP, combined with charge distribution profiles and predictive secondary structural analysis, showed that cleavage at dibasic sequences was strongly associated with peaks of hydrophilicity in alpha-helical regions of the preprohormone.

Differential expression of alpha, mu and pi classes of isozymes of glutathione S-transferase in bovine lens, cornea, and retina

Isozyme characterization of glutathione S-transferase (GST) isolated from bovine ocular tissue was undertaken. Two isozymes of lens, GST 7.4 and GST 5.6, were isolated and found to be homodimers of a Mr 23,500 subunit. Amino acid sequence analysis of a 20-residue region of the amino terminus was identical for both isozymes and was identical to GST psi and GST mu of human liver. Antibodies raised against GST psi cross-reacted with both lens isozymes. Although lens GST 5.6 and GST 7.4 demonstrated chemical and immunological relatedness, they were distinctly different as evidenced by their pI and comparative peptide fingerprint. A corneal isozyme, GST 7.2, was also isolated and established to be a homodimer of Mr 24,500 subunits. Sequence analysis of the amino-terminal region indicated it to be about 67% identical with the GST pi isozyme of human placenta. Antibodies raised against GST pi cross-reacted with cornea GST 7.2. Another corneal isozyme, GST 8.7, was found to be homodimer of Mr 27,000 subunits. Sequence analysis revealed it to have a blocked amino-terminus. GST 8.7 immunologically cross-reacted with the antibodies raised against cationic isozymes of human liver indicating it to be of the alpha class. Two isozymes of retina, GST 6.8 and GST 6.3, were isolated and identified to be heterodimers of subunits of Mr 23,500 and 24,500. Amino-terminal sequence analysis gave identical results for both retina GST 6.8 and GST 6.3. The sequence analysis of the Mr 23,500 subunit was identical to that obtained for lens GSTs. Similarly, sequence analysis of the Mr 24,500 subunit was identical to that obtained for the cornea GST 7.2 isozyme. Both the retina isozymes cross-reacted with antibodies raised against human GST psi as well as GST pi. The results of these studies indicated that all three major classes of GST isozymes were expressed in bovine eye but the GST genes were differentially expressed in lens, cornea, and retina. In lens only the mu class of GST was expressed, whereas cornea expressed alpha and pi classes and retina expressed mu and pi classes of GST isozymes.

The bag cell egg-laying hormones of Aplysia brasiliana and Aplysia californica are identical

Egg laying in the marine molluscan genus Aplysia is elicited by an egg-laying hormone (ELH) which induces ovulation and acts on central neurons to effect egg-laying behavior. ELH, isolated from the A. californica bag cells, and three ELH-related peptides, isolated from the A. californica atrial gland, have been chemically characterized, yet relatively little is known about homologous peptides in other Aplysia species. In these studies, the primary structure of A. brasiliana ELH was determined. Bag cell clusters were extracted in an acidic solution, and the peptides purified by sequential gel filtration and reversed-phase HPLC; ELH was identified by bioassay. Amino acid compositional and sequence analyses demonstrated that the neurohormone was a 36-residue peptide whose sequence was identical to that of A. californica ELH: NH2-Ile-Ser-Ile-Asn-Gln-Asp-Leu-Lys-Ala-Ile-Thr-Asp-Met-Leu-Leu-Thr-Glu- Gln-Ile- Arg-Glu-Arg-Gln-Arg-Tyr-Leu-Ala-Asp-Leu-Arg-Gln-Arg-Leu-Leu-Glu-Lys-COOH .

Purification and characterization of unique glutathione S-transferases from human muscle

Results of studies designed to investigate the origin of the diversity of glutathione S-transferase (GST) isozymes in human tissues indicated that human muscle has at least three forms of GST with pI values of 5.0, 5.1, and 5.2 that are distinct from GST isozymes characterized so far. The major muscle isozyme which was expressed in all the six samples analyzed in this study was a unique GST of pI 5.2 that was designated as GST zeta. It had a blocked N-terminal and did not correspond to any of the known three classes (alpha, mu, or pi) of human GST as evidenced by its immunological properties and substrate specificities. The N-terminal regions of human muscle GST 5.1 and 5.0 had identical amino acid sequences except at residue 5, but demonstrated significant differences in amino acid composition and substrate specificities. These two isozymes showed homology with the mu class of human GST in their N-terminal region and were also immunologically related to the mu class of human GST although their subunit molecular weight values (Mr 23,000) were lower than that reported for GST psi. The substrate specificities of these isozymes were also significantly different from those of other human GST isozymes characterized so far. Significantly, muscle tissue did not express the alpha class of GST isozymes; however, two other isozymes were identified, GST 4.8 and GST 4.5, which had identical N-terminal amino acid sequences that were similar to that reported for the pi class of human GST. GST 4.8 was present in all six samples analyzed in this study whereas GST 4.5 was present in only two of these samples, indicating a possibility of polymorphism at the GST3 locus. This study indicated the occurrence of at least three distinct isozymes in muscle tissue, providing further evidence for tissue specific expression of GST isozymes in humans.

Cholera toxin A subunit: functional sites correlated with regions of secondary structure

The A subunit of cholera toxin contains the ADP-ribosyltransferase activity in its major constituent polypeptide A1 (Mr 23,000) which is responsible for the elevation of cAMP typically observed with most mammalian cell types after exposure to the toxin. The primary structure of the A subunit, recently established by sequence analyses, is presented and used as the basis for the secondary structure prediction according to the method of Chou and Fasman. The results indicated the presence of 27% alpha-helix, 25% beta-structure, 12% beta-turn, and 36% random coil. The majority of the beta-structure consisted of six strands located in the NH2-terminal portion of the molecule (residues 33-106) covering one-half of the region corresponding to the A1 polypeptide portion. The beta-sheet domain led immediately into the active site region characterized by the alternating structures of beta-pleated sheet and alpha-helix (residues 95-140) similar to that reported for other NAD+ binding proteins. The presence of this structural feature in the region was confirmed by the use of another predictive method (J. Garnier et al., J. Mol. Biol. 1978, 120, 97-120). In addition, two regions (residues 14-18 and 200-214), previously identified to contain binding sites for the B subunit as evidenced by chemical modification and monoclonal antibody studies, were found to be in alpha-helix configuration.

Post-translational heterogeneity of the human vitamin D-binding protein (group-specific component)

The vitamin D-binding protein in human serum (the group-specific component) is an alpha 2-globulin which is genetically polymorphic in all populations studied. Previous work (J. Svasti and B. H. Bowman (1978) J. Biol. Chem. 253, 5188-5194, and J. Svasti, A. Kurosky, A. Bennett, and B. H. Bowman (1979) Biochemistry 18, 1611-1617) has shown that the electrophoretic variations of the proteins controlled by two allelic genes, Gc1 and Gc2, are due to at least three amino acid substitutions between Gc1 and Gc2 (Svasti et al. (1979] and to heterogeneity in the Gc1 phenotype arising from carbohydrate dissimilarities. Gc1 migrates electrophoretically as two protein bands, while Gc2 migrates cathodally as a single band. This study demonstrates a post-translational glycosylation difference occurring in a single area of the Gc1 sequence which accounts for the heterogeneity observed previously. The glycosylation site, a threonine residue, appears to be in a sequence which differs between Gc1 and Gc2. The O-glycosidic bond, which is typical of mucins, is rare in plasma proteins. The cyanogen bromide fragment containing the galactosamine-containing carbohydrate in Gc1 was partially sequenced through 20 residues from the amino terminus. No detectable galactosamine could be found in the homologous cyanogen bromide fragment in Gc2. A new purification procedure for the vitamin D-binding protein in human plasma has been developed. Three chromatographic steps provide purified protein.

Prosimian hemoglobins I. The primary structure of the beta-globin chain of Lemur catta

The primary structure of the beta chain from the hemoglobin of a prosimian primate, Lemur catta, has been determined by automated Edman degradation of intact polypeptide chain and fragments produced by tryptic, cyanogen bromide and acid cleavage, and by homology with the sequence of Lemur fulvus. The sequence presented here differs from the human beta-globin sequence at 26 sites. This is the same degree of divergence previously reported for the beta-globin chain of Lemur fulvus. The sequences of the two congeneric lemuroid beta-globin chains are surprisingly divergent, differing at 18 sites. Of the 26 positions where L. catta differs from Homo sapiens, 7 are at positions with defined function. Of these 7 positions, 4 (2-Phe, 54-Ile, 94-Val, 112-Ile) are unique to L. catta among the primate beta-globin chains of established sequence. Residue 112-Ile is consistent with the prediction of Beard and Goodman (19) of an isoleucyl residue in this position in the ancestral primate beta-globin chain.

Micellar gangliosides mediate the lipid insertion of cholera toxin protomer A

The topology of the interaction of cholera toxin with ganglioside and detergent micelles was studied with the technique of hydrophobic photolabelling. Cholera toxin alpha and gamma polypeptide chains appear to penetrate into the hydrophobic core of ganglioside micelles. Micelles of SDS cause the labelling also of the beta polypeptide chains, while Triton X-100 micelles have little ability to mediate the labelling of the toxin. The specific reduction of the alpha-gamma disulfide bond allows the penetration of the alpha polypeptide chain into Triton X-100 micelles, but does not affect the interaction of cholera toxin with either ganglioside or SDS micelles. Thus, ganglioside micelles appear to cause a conformational change of the native toxin, such as to induce the penetration of the alpha chain into the micelle hydrophobic core.