The chemistry of the collagen cross-links. Characterization of the products of reduction of skin, tendon and bone with sodium cyanoborohydride.

Reduction of tissues with sodium cyanoborohydride at pH7.4 gave results identical with those obtained by KBH4 treatment. On reduction with sodium cyanoborohydride at pH 4.4, however, a previously undetected basic compound was formed and was identified by mass spectrometry and chemical degradation techniques as dihydrohydroxymerodesmosine. Histidino-hydroxymerodesmosine was not present, and further analysis confirmed that reduced aldol, a mojor product of reduction with KBH4 at the lower pH, was also absent. These results, together with an analysis of the time course of the reduction, support previous assertions that histidino-hydroxymerodesmosine is an artifact [robins *Bailey (1973) Biochem. J. 135, 657-665] and suggests that the non-reduced form of hydroxymerodesmosine probably does not constitute a major intermolecular bond in vivo.