A Hydroxyproline-containing, Collagen-like Protein in Plasma and a Procedure for Its Assay.

The presence of dialyzable hydroxyproline-containing peptides in plasma (1) and urine (2-4) has been known for some t,ime. Although their exact significance is not clear, the levels of these peptides in the absence of collagen or gelatin in the diet are a reflection of endogenous collagen metabolism (l-5). In a previous publication (6) it was shown that, in addition to free hydroxyproline and dialyzable hydroxyproline peptides, there is also present in human plasma a form of hydroxyproline which is nondialyzable, precipitable with protein precipitants, and released by acid or alkaline hydrolysis. The presence in plasma protein hydrolysates of material which interferes with the procedure for hydroxyproline assay made it difficult to quantify or characterize this protein. It. has now been possible to modify the existing procedure (7) for hydroxyproline assay to make it sufficiently specific and sensitive for this purpose. With the modified procedure it has been found that plasma from laboratory animals and humans contains 6 to 14 Fg per ml of proteinbound hydroxyproline. The behavior of this protein (hyproprotein)’ to ammonium sulfate precipitation and its migration on Sephadex columns suggest that it is a large molecule. Its resistance to digestion by chymotrypsin and Pronase suggests further that the protein may be a form of collagen.