Photolysis and ozonolysis of desmosine and elastolytic peptides

This paper describes the combined effects of photolysis and ozonolysis upon the main interchain crosslinks of elastin, desmosine and isodesmosine. Photolysis of purified (iso)desmosines in solution leads to the cleavage of the pyridinium rings to give lysine and an analogue of glutaconic aldehyde which is deformylated to a stable compound absorbing at 242 nm. This photoproduct is subsequently fragmented by ozone into glutamic, alpha-aminoadipic and possibly alpha-amino-delta-oxocaproic acids. However the yields of these different compounds are very low because we observed that numerous competing side reactions (polymerisation, recyclisation) accompany the photoozonolytic decomposition of the pyridinium rings of free (iso)desmosines. The results are clearer when reticulated elastolytic peptides are photoozonolysed. The (iso)desmosines, covalently linked in these peptides, are cleaved into lysine, glutamic and alpha-aminoadipic acids (in a ratio 2:1) with yields corresponding to 80-90% of those expected from the decomposition of the (iso)desmosines originally present in the peptide fraction. We have also observed that ozonolysis alone degraded another crosslink present in these peptides, the aldol condensation product, resulting in the production again of glutamic and alpha-aminoadipic acids in amounts consistent with the known concentrations of this particular crosslink in elastin. Finally we noted that the complete photoozonolytic degradation of the (iso)desmosines present in a semi purified reticulated elastolytic fraction resulted in a shift of the size distribution of these peptides toward lower values. It is not certain however that this shift, indicative of a freeing of the polypeptide chains from their original three dimensional network, is due uniquely to the cleavage of the (iso)desmosines. Indeed we have observed that tyrosine and phenylalanine were also degraded during photoozonolysis. Not knowing the mechanism of this degradation it is impossible to rule out the possibility that concomitant cleavages of peptide bonds did occur.