Aggregation of collagen exposed to UVA in the presence of riboflavin: a plausible role of tyrosine modification.

Riboflavin-sensitized photodynamic modification of collagen led to significant formation of cross-linked molecules. Sodium azide or 1,4-diazabicyclo(2,2,2)octane, which are known to be singlet oxygen quenchers, and catalase could not inhibit the modification. Surprisingly, the collagen modification was accelerated in the presence of superoxide dismutase. The aggregation was accompanied by the loss of tyrosine and histidine residues in the collagen. An inhibitory effect of dissolved oxygen on the modification of collagen was observed. Similarly, the loss of tyrosine residues in the irradiated collagen was inhibited in the presence of dissolved oxygen. Dityrosine formation was also observed with the loss of tyrosine. These results indicate that photodynamic modification of tyrosine probably contributes to the riboflavin-sensitized cross-linking of collagen through the formation of dityrosine.