Comparison of the C-terminal Amino-acid Sequence of Two Horse Immunoglobulins IgG and IgG(T)

Abstract

IT was observed that treatment of the heavy chain of normal rabbit immunoglobulin G (IgG) with cyanogen bromide in 70 per cent formic acid split the chain into sections suitable for amino-acid sequence studies. Cyanogen bromide reacts with methionine residues converting them to homoserine with simultaneous splitting of the peptide chain and leaving the homoserine residue C-terminal¹. Particularly the C-terminal octadecapeptide was easily isolated in high yield and its sequence was determined². The heavy chain of human IgG from normal serum³ and a pathological serum⁴ both had the same C-terminal sequence and this differed from that of the rabbit in only two of the eighteen residues, suggesting that there might be a characteristic pattern for all mammalian IgG. The heavy chain of horse IgG has been investigated by the same technique and compared with the heavy chain of the horse T protein. The latter immunoglobulin is barely detectable in normal serum but increases very considerably in the serum of a horse strongly immunized with protein antigens such as diphtheria or tetanus toxoid and it may contain almost all the antibody^5,6. This protein has a high carbohydrate content and it has been suggested that it might be the equine equivalent of human IgA. From a study of the antigenic cross-reactions of the whole proteins and their subunits, however, this seemed unlikely⁷.