Collagen
Proteins

4-hydroxyproline, the end-product of the reaction catalized by prolyl-hydroxylase, is required for the thermal stability of the folded triple helix at physiological temperatures. An additional function of prolyl 4-hydroxylase (EC 1.14.11.2) is to act as a chaperone by retaining unfolded procollagen chains in the ER, releasing them for secretion only when they have folded correctly. Hydroxylysine is essential in collagen cross-link formation, and in its absence the collagen would be structurally unstable. Ascorbic acid is a fundamental signal for collagen production which appears to be independent of its cofactor function for hydroxylation of proline and lysine.
Collagen polypeptide synthesis, posttranslational hydroxylations, and activities of the two hydroxylases are independently regulated by ascorbate.
Lysyl hydroxylase activity increased 3-fold in response to Lascorbate administration.
A significant proportion of newly synthesized collagen is degraded before it ever gets out of the cell.
It has been theorized that the mechanism constitutes a quality control step for ridding the organism of defectively synthesized collagen. Since ascorbic acid serves as a cofactor for the hydroxylation of proline, an argument could be formulated that underhydroxylated collagen synthesized in the absence of ascorbic acid is defective and more likely to be degraded while still in the cell.
The relative increase in collagen production seen in the presence of ascorbic acid could be a result of synthesis of fully hydroxylated collagen, which is less susceptible to intracellular degradation.

Several analogs of ascorbic acid have been studied; these include dehydroascorbic acid, D-ascorbic acid, and D-isoascorbic acid. Each of these analogs was capable of stimulating relative collagen production, but none was effective at the low concentration demonstrated for L-ascorbic acid. In general, a tenfold increase in concentration was required for these compounds.
It should be noted that D-ascorbic acid and D-isoascorbic acid are able to substitute for the cofactor effect of ascorbic acid on the actions of lysyl and prolyl hydroxylase at equimolar concentration. Thus a stereospecificity for ascorbic acid is present, although stereoisomers and dehydroascorbic acid were able to support the reaction at tenfold higher concentrations.
Collagen synthesis continued to increase even after prolyl hydroxylation had been maximally stimulated, suggesting that ascorbate acts at a level other than hydroxylation. mRNA specific for collagen was stimulated twofold in the presence of ascorbic acid.
The data demonstrate that collagen-specific mRNA is stimulated in the presence of ascorbic acid, but whether or not this specific effect related to stimulated transcription, stability of mRNA, or mRNA processing is not distinguished.

After prolonged exposure to ascorbate, collagen synthesis in cultured human skin fibroblasts increased approximately 8-fold with no significant change in synthesis of noncollagen protein.

An alternative mechanism for ascorbate action may involve the protein synthesis machinery itself: ultrastructural examination of normal and scorbutic tissues revealed a considerable reduction in rough endoplasmic reticulum in ascorbate deficiency, consistent with this effect of ascorbate.

Under certain conditions, including ascorbic acid deficiency it can be demonstrated the formation in vitro of a hydroxyproline- and hydroxylysine-deficient collagen, termed protocollagen. Studies in vivo on the biosynthesis of collagen and elastin, 1969

Ascorbic acid deficiency in vivo gave rise to impaired hydroxylation of collagen proline with the concomitant formation of a proline-rich collagen.
It is believed that protocollagen is of a size at least comparable with that of the a-chains of tropocollagen.
Protocollagen remains essentially intracellular, but it is extruded from the cell and is able to aggregate or cross-link or both in a similar manner to tropocollagen. If protocollagen remains essentially in a soluble form, it seems possible that it could be readily removed by proteolytic digestion.
Increased urinary excretion of hydroxyproline had been seen in human patients fed on a scorbutogenic diet. This might arise as a result of degradation of partially hydroxylated collagen precursors formed as a consequence of the vitamin deficiency and rapidly eliminated as a consequence of their abnormality.