Intracellular Hydroxylation of Non‐Helical Protocollagen to Form Triple‐Helical Procollagen and Subsequent Secretion of the Molecule

Cells liberated from the tendons of chick embryos by controlled enzymic digestion were used to examine the synthesis of the polypeptide chains of collagen separately from the secretion of collagen in the precursor form known as procollagen. When the cells were incubated under anaerobic conditions, the synthesis of collagen hydroxyproline was completely inhibited but the rate of protein synthesis remained about the same as the control rate for about 30 min. Under these conditions the cells synthesized the non‐hydroxylated precursor known as protocollagen and the protocollagen was retained in the cells. By incubating the cells with [ 14 C]proline under anaerobic conditions and then exposing the system to air, it was possible to follow the intracellular hydroxylation of [ 14 C]protocollagen to [ 14 C]procollagen. The conformation of the intracellular [ 14 C]protocollagen and [ 14 C]procollagen was examined by proteolytic digestion under conditions in which the triple‐helical portion of collagen is resistant to digestion. When digestion with either pepsin or α‐chymotrypsin was carried out at either 30 ° C or 37 °C, all of the [ 14 C]protocollagen was digested. After the same protein was hydroxylated to [ 14 C]procollagen by exposing the cells to O 2 a large fraction of the protein became resistant to proteolysis and was secreted. The results suggested that the intracellular hydroxylation of collagen polypeptides converts them from a non‐helical to a helical form and that the helical conformation probably is required for the protein to be secreted at an optimal rate.