Uniformity and Species-Specific Features of the N-Terminal Amino-Acid Sequence of Porcine Immunoglobulin lamba-Chains

Cleavage with cyanogen bromide was employed to investigate the N-terminal amino acid sequence of the λ-chains of normal porcine immunoglobulin. The N-terminal homoserine-containing nonapeptide was isolated from all fractions of the λ-chains in a yield of 60% of theory. Its amino acid sequence was determined by mass spectrometry and was found to be uniform. Other homoserine-containing fragments were obtained from the cyanogen bromide hydrolyzate of the λ-chains with split disulfide bonds. Partial amino acid sequence of these fragments provided evidence that methionine in position 9 is present in more than 60% of variants and obviously has an invariant character. Variants of the λ-chains exist (by estimate not more than 15% of the total λ-chains) which, in addition to the invariant methionine residue in position 9, possess a variable methionine residue in the section between positions 46 and 51. The data obtained indicate that the sequence of 23 amino acid residues from the N-terminus of porcine λ-chains is uniform with the exception of a single replacement (in position 13). Moreover, in comparison with the sequence of λ-chains of man, mouse and birds, the sequence displays several species-specific replacements and a deletion of threonine which occupies position 5 in the chains of the other species. In the pig, the λ-chains represent the predominant type of light chains (some 70%). Hence it follows that some 50% of the light chains in this species possess a uniform amino acid sequence in the N-terminal section. These facts are difficult to reconcile with the germline theories of genetic control so that mechanisms of somatic diversification of immunoglobulins must be envisaged.