Reversible transformation of fibrous collagen to a soluble state in vivo.

Metabolic pathways of pre-existing and newly synthesized collagen fractions from skin and induced connective tissue were studied in rats that had been labeled with (3)H-L-proline 6 and 20 weeks previously. The distribution of specific radioactivity was determined for soluble and insoluble collagens. Multiple extractions of neutral salt-soluble and citrate-soluble collagen demonstrated constant specific radioactivities which indicated the achievement of an isotopic steady state. Citrate-soluble collagen was four times more radioactive than neutral salt-soluble collagen, which indicated a large difference in biological age (months) between fractions. Different levels of specific radioactivity in collagen fractions from implanted sponge indicated that both pre-existing (citrate-soluble) and newly synthesized collagen (salt-soluble) contributed to the insoluble collagen of induced connective tissue in normal and hypophysectomized rats. The isotopic data indicated that pre-existing fibrous collagen could be solubilized in vivo and translocated from skin to sponge via citrate-soluble collagen.