Collagen crosslinks: isolation of reduced N -hexosylhydroxylysine from borohydride-reduced calf skin insoluble collagen

Treatment of calf skin insoluble collagen with NaB 3 H 4 followed by acid hydrolysis and ion-exchange chromatography yields a new compound which is prominent in the chromatograms of several collagens. We now describe this compound as reduced N ϵ -hexosylhydroxylysine. It appears to arise by reduction of the Schiff base between the carbonyl moiety of a hexose and the ϵ-amino group of hydroxylysine; the postulated structure was derived from both high- and low-resolution mass spectrometry and by comparison with synthetic N ϵ -galactosylhydroxylysine. Conceivably, the unreduced compound may be a crosslink, uniting collagen and glycoproteins or proteoglycans in the connective tissue.