Proline recycling during collagen metabolism as determined by concurrent 18O2-and 3H-labeling

In a previous study where rat skin collagen was labeled with 180 in the hydroxyl group of the collagen hydroxyproline we noticed that the decat rate of this label was much faster than had been observed when the skin collagen hydroxyproline was labeled with oH in the prolyl ring. In this study a ratwas labeled concurrently with [1802] and [3H] proline and the rate of decline of both labels was determined in rat skin collagen hydroxyproline. After correction for growth dilution of the skin collagen the [180] hydroxyproline was found to have a half-life of 27 days while the [3H] hydroxyproline had a half-life of 53 days. The decay rate of the [180] hydroxyproline represents the true turnover rate of collagen since there is no possibility of recycling this label. Hence, the difference between this and the [3H] hydroxyproline decay rate is due to recycling of L-[3H] proline into new collagen. The efficiency of recycling of proline from catabolized collagen into new collagen was about 93%.