Procollagen secretion meets the minimum requirements for the rate-controlling step in the ascorbate induction of procollagen synthesis.

Ascorbate addition to primary avian tendon cells has been shown previously to cause a approximately 6-fold increase in procollagen translation that is first observable after 4 h and reaches a maximum level after 48 h. Similarly, procollagen mRNA has been shown to increase after ascorbate addition by approximately 6-fold starting at 12 h and reaching a maximum level by 72 h. The rate constant for procollagen secretion is now shown to also react to ascorbate by a 6-fold change. This results in a drop in the half-life of procollagen within the cell from 120 to 20 min. In sharp contrast to the other steps in the procollagen pathway, the change in the secretion rate constant is extremely fast occurring in less than 30 min. Moreover, after ascorbate addition, greater than 80% of the internal procollagen can be secreted at the fast rate. Since this change results from an increase in hydroxylation of proline residues and since the hydroxylation reaction has been localized to the endoplasmic reticulum, this evidence strongly supports the model that the slow step in the secretion pathway is transport out of the endoplasmic reticulum. Further support for this comes from electron microscope autoradiography of [3H]proline-labeled cells where the labeled procollagen pool within the cells was highly localized to the endoplasmic reticulum.