Degradation of newly synthesized collagen
Explants of rabbit lung parenchyma maintained in serum-free medium incorporate [14C]proline and [14C]lysine into collagen as [14C]hydroxyproline and [14C]hydroxylysine at a constant rate for at least 24 h. Evaluation of the size distribution of the [14C]hydroxyproline and [14C]hydroxylysine containing peptides within the explants demonstrated that 20 to 40% of the α chains of newly synthesized collagen were destroyed within minutes of being synthesized. Since the percentage of newly synthesized collagen destroyed did not increase with time of incubation, the mechanisms which mediate this process operate on the collagen molecule during a short interval in the sequence of collagen synthesis, secretion, and maturation. The degradation of newly synthesized collagen was not due to an extracellular collagenase; none could be detected in the culture medium and intact collagen added to the cultures was not degraded. It occurred so rapidly (30% of the labeled collagen was destroyed 8 min after the addition of [14C]proline to the cultures) that it is unlikely to be the result of phagocytosis and digestion within phagolysosomes. The relative amount of degraded collagen was also independent of the addition of serum to the cultures and the use of inhibitors of proteolysis during the postincubation analytic procedures. In addition, this degradative process was age-invariant; the relative amount of newly synthesized collagen destroyed by explants of lung from old rabbits was identical to that destroyed by explants from young rabbits. Degradation of newly synthesized collagen is probably a normal process, independent of collagenase or phagocytosis, by which tissues modulate the quantity or quality, or both, of collagen being secreted from the cell.