On the accumulation of D-aspartate in elastin and other proteins of the ageing aorta.

Ageing and degenerative changes of the human aorta are associated with medial thinning and a reduced dry weight content of elastin. The metabolic stability of cross-linked elastin was investigated by measuring the accumulation of D-aspartate with ageing in insoluble elastin isolated from human aorta. D-Aspartate accumulation in elastin was compared with D-aspartate accumulation in aortic collagen and an elastin bound glycoprotein fraction. The D-aspartate content of elastin, purified from infrarenal aorta; increased linearly with age from 3% of the total aspartate in youth to 13% in the mid 80s. In contrast the D-aspartate content of aortic collagen remained invariant (3-5% of the total aspartate) from youth to old age. The apparent first order rate constant for the racemization of L-aspartate in elastin was 1.14 x 10(-3). The D-aspartate content of the elastin bound glycoproteins increased by only a small amount, from 3% in the mid 30s to 6% in the mid 80s. These results argue for the metabolic stability of aortic elastin as compared with the fibrillar collagens of the human aorta. Both the rate of racemization and the specific accumulation of D-aspartate in elastin, but not collagen, indicates that mature cross-linked elastin is not synthesized in the adult aorta.