Elastic fibers comprise elastin and other proteins termed as elastin-associated proteins. The nature of association between the elastin and elastin-associated-protein is not known. We have isolated elastic fibers from 5-month-old porcine aorta and lung parenchyma using urea, dithiothreitol and 1% sodium dodecyl sulfate at 55 degrees C. Lysyl-derived crosslinks were stabilized by sodium borohydride reduction. The methionine residues and associated peptides were decreased by CNBr treatment. Limited proteolysis of elastic fibers obtained by this procedure by trypsin (TPCK) and chymotrypsin (TLCK), removed 3% and 11% of the elastic fibers from aorta and lung, respectively. Limited elastase digestion solubilized a further 12 to 14% of the elastic fiber from aorta and lung samples, respectively. The insoluble residue remaining after elastase digestion had amino acid composition similar to alkali extracted elastin and to tropoelastin. The material solubilized by chymotrypsin and trypsin contained lysinonorleucine, whereas desmosine crosslinks were present only in the elastase digests. Aorta and lung elastic fibers differ in their structures as indicated by quantitative differences in limited proteolysis. These results strongly indicate that elastin and elastin-associated proteins interact strongly through lysyl-derived crosslinks.