Effectiveness of isoascorbate versus ascorbate as an inducer of collagen synthesis in primary avian tendon cells

In contrast to most biologically active molecules, the isomeric form of ascorbate retains significant biological activity. Moreover, in studies in vitro the isomer was found to be an equally effective cofactor in the enzymatic proline hydroxylation reaction. This raises questions about whether the lower biological activity in vivo results from selective transport into the cell, greater instability of the molecule, or stereospecificity by certain enzyme complexes. Distinguishing these possibilities can be accomplished most directly using a cell culture model. In this study primary avian tendon (PAT) cells were used. With PAT cells isoascorbate was shown to be three- to fivefold less active at inducing procollagen production than ascorbate. Isoascorbate was also internalized by the cell at about one-fifth the ascorbate level. In addition, isoascorbate was degraded in the medium at a slightly higher rate (half-life of 1.6 h) than ascorbate (2.1 h). The data are consistent with a model that postulates that once inside the cell isoascorbate is equally effective at inducing procollagen production but selectivity at the transport step restricts the percentage that is actually internalized. In addition, both ascorbate and isoascorbate were found to degrade very quickly inside the cell in the highly oxygenated environment of cell culture (approximately 2 h half-life). When ascorbate was added to the medium (100 micrograms/mL) the level inside the cell quickly reached a maximum (less than 2 h) and declined rapidly.(ABSTRACT TRUNCATED AT 250 WORDS)