Marked Longevity of Human Lung Parenchymal Elastic Fibers Deduced from Prevalence of D-Aspartate and Nuclear Weapons-related Radiocarbon

Normal structure and function of the lung parenchyma depend upon elastic fibers. Amorphous elastin is biochemically stable in vitro, and may provide a metabolically stable structural framework for the lung parenchyma. To test the metabolic sta-bility of elastin in the normal human lung parenchyma, we have (a) estimated the time elapsed since the synthesis of the protein through measurement of aspartic acid racemization and (b) modeled the elastin turnover through'measurement ofthe preva-lence of nuclear weapons-related "C. Elastin purified by a new technique from normal lung parenchyma was hydrolyzed; then the prevalences of D-aspartate and "4C were measured by gas chromatography and accelerator-mass spectrometry, respec-tively. D-aspartate increased linearly with age; K,,.,p (1.76 x 10-3yr-') was similar to that previously found for extraordi-narily stable human tissues, indicating that the age of lung par-enchymal elastin corresponded with the age of the subject. Ra-diocarbon prevalence data also were consistent with extraordi-nary metabolic stability of elastin; the calculated mean carbon residence time in elastin was 74 yr (95 % confidence limits, 40-174 yr). These results indicate that airspace enlargement char-acteristic of "aging lung " is not associated with appreciable new synthesis of lung parenchymal elastin. The present study provides the first tissue-specific evaluation of turnover of an extracellular matrix component in humans and underscores the potential importance of elastin for maintenance of normal lung structure. Most importantly, the present work provides a foun-dation for strategies to directly evaluate extracellular matrix injury and repair in diseases of lung (especially pulmonary em-physema), vascular tissue, and skin. (J. Clin. Invest. 1991