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The turnover of prolyl hydroxylase and an immunoreactive protein that corresponds in size to the smaller subunit of the enzyme was studied in vivo after injection of [(3)H]leucine into 11-day chick embryos. The specific radioactivity and total radioactivity of the monomer-size protein were much higher than those of the enzyme tetramers in the cartilaginous bone at 3h and 12h after the radioisotope injection, indicating that the monomer-size protein represents precursors rather than degradation products of the enzyme tetramers. Between 24 and 144h after the injection the specific radioactivity and total radioactivity of the two forms of the enzyme protein showed essentially identical decay rates, the observed specific radioactivity of the monomer-size protein being about 120-130% and total radioactivity about 80% of that of the enzyme tetramers. The true half-life, when corrected for dilution caused by tissue growth and re-utilization of the [(3)H]leucine, was 37.9h for the monomer-size protein and 39.0h for the tetramers. The results obtained in the lung were less reliable owing to high blank radioactivity values in the immunoprecipitation, but even so some definite differences were found between this tissue and the cartilaginous bone. The specific radioactivity of both forms of the enzyme protein at 24h was only about 20-25% of that in the cartilaginous bone. The total radioactivity of the monomer-size protein in the lung remained about 5 times that of the enzyme tetramers, whereas it was only about 0.8 times that of the tetramers in the cartilaginous bone. As in the cartilaginous bone, the decay rates of both forms of the enzyme protein were essentially identical in the lung, with a true half-life of about 46h. The results suggest that the rate of prolyl hydroxylase synthesis is slower in the lung than in the cartilaginous bone, whereas the degradation rates are fairly similar in these two tissues. The data further suggest that, in the lung at least, a large part of the monomer-size protein became degraded without being converted into enzyme tetramers.

Turnover of prolyl hydroxylase tetramers and the monomer-size protein in chick-embryo cartilaginous bone and lung in vivo