The metabolism of collagen from liver, bone, skin and tendon in the normal rat

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Abstract
Alpha-C14- glycine was injd. into rats of varying ages. The animals were killed at different intervals and samples of mixed proteins of liver and muscle and of collagen from skin, tendon, bone and liver were obtained. Dinitrophenylglycine was isolated from the hydrolysate of these proteins and its radioactivity determined. In rats more than 14 raos. old the activities of the glycine samples obtained from all the collagen fractions were very low, bone collagen showing the highest values. In young adults which were still growing slowly, low activity was observed with skin and tendon collagen, but appreciably higher values were obtained with collagen from bone and liver. With young, rapidly growing rats initial activities were high with all collagen fractions. The difficulties inherent in the interpretation of isotope data in relation to protein metabolism are discussed, and it is concluded that only such changes can be detected as involve participation of free amino acids in protein turnover. The criteria of purity of collagen are also examined. Subject to the limitations mentioned the results suggest that, even in the adult rat, collagen fibers are continually, but very slowly, replaced, the turnover rates being highest for collagen from bone. It is likely that in young rats this change is somewhat faster. However, in all age groups all collagens showed turnover rates which are low compared with those of intracellular proteins.

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