Lysine derived cross-links are present in a non-elastin, proline-rich protein fraction of Iguana iguana eggshell.

1. Eggshell of Iguana iguana contains a variety of lysine-derived cross-linking amino acids apparently identical with some that occur in elastin. 2. Quantification of desmosine by amino acid analysis and radioimmunoassay gave estimates of 217 ng per mg and 109 ng per mg (dry weights), respectively. 3. [3H]NaBH4 reduction of eggshell protein established presence of lysine-derived aldehydes, allysine and allysine-aldol. 4. Enzymatic and chemical treatments indicate these cross-links occur in a non-elastin eggshell protein. 5. Iguana eggshell protein is unusually proline-rich, containing almost 440 proline residues per thousand.