The relative metabolic inertia of tendon collagen in the rat

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Abstract
Glycine labelled with c14 in the methylene position was fed to 3 series of rats, old rats, young slowly growing adults and rapidly growing young ones. The animals were killed at varying intervals and samples of liver, muscle and of collagen from white tendons were obtained. 2,4-Dinitrophenylglycine from these 3 sources was crystallized after chromatographic separation and the radioactivity detd. The specific activity of glycine from liver protein showed a very high initial activity which decreased rapidly during the subsequent 35-50 days. The results indicate that the half-life of the mixed liver proteins is 4-5 days. With muscle the initial activity was lower than in liver and there was a steady but slow fall with time. After 2-3 weeks the activity of muscle glycine equalled that of liver and later exceeded it. The glycine from the collagen of old rats showed a very low radioactivity. In young adults the activity was slightly higher, but still very low compared with that of the glycine of muscle or liver. There was a slow increase of activity with time. With the very young rats initial collagen activity was very high, only slightly lower than the corresponding value for muscle and about half that of liver. The activities found in the young rats decreased with time in marked contrast to the young adults. The results indicate that tendon collagen in the old rats is met-abolically almost completely inert. In young rats interpretation is complicated by growth, but when correction is made for this factor, it appears that collagen is relatively inert even in young actively growing animals, though the possibility cannot be excluded that the turnover rate of collagen is somewhat faster than in older animals. Radioactive compounds may be deposited in the extracellular spaces of young subjects where they may remain for long periods.
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