Hydroxylation of peptide-bound proline and lysine before and after chain completion of the polypeptide chains of procollagen

The synthesis of procollagen hydroxyproline and hydroxylysine was examined in matrix-free cells which were isolated from embryonic tendon by controlled enzymic digestion and then incubated in suspension. After the cells were labeled with ¹⁴C]proline for 2 min, or about one-third the synthesis time for a Pro-α chain, ¹⁴C]hydroxyproline was found in short peptides considerably smaller than the Pro-α chains of procollagen. The results, therefore, confirmed previous reports indicating that the hydroxylation of proline can begin on nascent chains. In similar experiments in which the cells were labeled with ¹⁴C]lysine, ¹⁴C]hydroxylysine was found in short, newly synthesized peptides, providing the first evidence that the hydroxylation of lysine can also begin on nascent peptides. However, further experiments demonstrated that the synthesis of hydroxyproline and hydroxylysine continues until some time after assembly of the polypeptide chains is completed.