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Studies on the antigenic properties of reduced and carboxymethylated egg-white lysozyme Thompson, Karen E.

Abstract

This thesis involved a study of the antigenic properties of reduced and S-carboxymethylated egg-white lysozyme (CM-lysozyme). Since work on the antigenic properties of native lysozyme is currently in progress in two other laboratories, it was thought that a comparative study on CM-lysozyme might elucidate the role of primary and tertiary structure in determining antigenicity in proteins. This molecule was chosen mainly because the complete amino acid sequence, and the X-ray crystallography of the molecule have been completed. This makes it possible to correlate information on antigenic determinants with their orientation on the crystalline structure. The native molecule is relatively resistant to enzymatic digestion, but the reduced and S-carboxymethylated derivative, lacking its disulfide bridges, and consequently its rigid tertiary structure, is readily digested. The first experiments involved attempts to isolate fragments of the CM-lysozyme molecule exhibiting haptenic activity. Trypsin was used, since this is a specific endopeptidase, cleaving proteins or peptides at the carboxyl of lysine and arginine residues. Only one tryptic peptide thus isolated (T-11) exhibited haptenic activity when various fractions were tested for their ability to inhibit either precipitation or complement fixation between CM-lysozyme and its homologous antiserum. There was no cross-reactivity observed between CM-lysozyme and antiserum directed against native lysozyme. Likewise, the tryptic digest of CM-lysozyme or any of the peptides isolated from it, did not inhibit the immune reaction of native lysozyme with its homologous antiserum. Further experiments were carried out in attempts to pinpoint the region of this large 23 amino acid peptide, T-11, which was responsible for haptenic activity. From sequential degradation of the peptide from both the C- and N-terminal, using enzymatic and chemical methods, the antigenic region was narrowed to the N-terminal portion of T-11. Two synthetic peptides, prepared by the solid phase method, comprising the N-terminal decapeptide of T-11, and the decapeptide plus arginine at the N-terminal, both showed haptenic activity by inhibition of precipitation between CM-lysozyme and its homologous antiserum. The degree of inhibition with the two synthetic peptides was comparable but somewhat less efficient than the whole T-11 peptide.

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