Characterization of conformation independent antigenic determinants in the triple-helical part of calf and rat collagen.

About 20 per cent of the antibodies in rabbit antisera to native calf or rat collagen exhibited affinity for denatured rabbit collagen and could be isolated by immunoadsorption. Such antibodies reacted with the unfolded α1-chain as well as with the α2-chain of collagen. Inhibition experiments suggested that the two kinds of polypeptide chains are not completely equivalent in their antigenic determinants. These determinants were not significantly influenced by a treatment of native collagen with pronase, a procedure known to remove short, non-helical sequences at both ends of the molecule. The results suggested that the antigenic determinants are conformation independent. They are, however, mainly located in the middle region of collagen, having a rather complex conformational structure. Cyanogen bromide cleavage of collagen did not impair the serologic activity of these determinants but with one exception none of the individual cyanogen bromide peptides possessed the full activity of the entire α-chain. However, most of the peptides could be agglutinated by the antibodies when put onto tanned red cells. Inhibition studies of these agglutination reactions clearly demonstrated that virtually all of the peptides carry unique antigenic determinants, which occasionally are shared by a few other peptides. Additional evidence for heterogeneity was obtained by further cleavage of the cyanogen bromide peptides with proteases. The minimum number of antigenic determinants thus estimated in calf collagen was nine. Evidence is provided that their structure in most cases does not correspond to sequences of the type Gly-Pro-X.