Uniformity and Species‐Specific Features of the N‐Terminal Amino‐Acid Sequence of Porcine Immunoglobulin λ‐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.