Structure of Two Histidine-containing Cross-Links from Collagen

Reduction of insoluble collagens from bone, skin, and tendon with NaB3H4 produced several radioactive substances which were isolated from acid hydrolysates of the protein. One of these substances, aldol-histidine, was most abundant in cow skin collagen and also appeared in reconstituted fibrils of purified cow skin tropocollagen. Another compound, histidino-hydroxymerodesmosine, was abundant in all of the collagens studied, and it appeared to be structurally related to aldol-histidine. The postulated structures of both compounds were obtained primarily by high and low resolution mass spectrometry and by high resolution nuclear magnetic resonance spectrometry. Ancillary data were provided by ultraviolet spectrometry and by colorimetric methods. Both compounds were present in partially purified, large peptides obtained from cyanogen bromide digests of insoluble collagen and of reconstituted collagen fibrils. These results suggest that aldol-histidine and histidino-hydroxymerodesmosine serve as cross-links in collagen, potentially uniting three or four polypeptide chains, respectively.