The turnover of mitochondria in a variety of tissues of young adult and aged rats.

Among the most controversial hypotheses of aging are those which involve the progressive accumulation of errorbearing or altered macromolecules with advancing age. The effect of low levels of error or of alterations in only one or a small number of the many macromolecular mitochondrial components might be amplified by the highly integrated process of mitochondrial biosynthesis and observed as a change in turnover rates. The turnover rates of mitochondria from a variety of tissues of young adult (12-month-old) and aged (24-month-old) rats were measured by following the loss of radioactivity from proteins of purified mitochondrial preparations after initial labeling with 3H-leucine. Mitochondria from liver, brain, heart, and testes appeared to lose label as a homogeneous class with respect to rate. However, mitochondria from kidney, lung, and intestinal mucosa exhibited at least one additional exponential component. No significant differences were found for any tissue between the two age groups. Estimates for the half-lives in days obtained by combining both young and old sets of data are: liver, 9.3; testes, 12.6; heart, 17.5; brain 24.4; small intestine, 0.7 (first) and 17.6 (second); lung, 4 (first) and 16.6 (second); and kidney, 6 (first) and 10.9 (second). It is concluded that these data do not support the concept that errors in macromolecules are accumulated with age.