Improved determination of small amounts of free hydroxyproline in biological fluids

A fast, reliable, and sensitive (< 20 pmol) method for the quantification of 4-hydroxyproline is described. It ensures good separation of imino acid peaks, eliminates interference by primary amino acids, and guarantees full (96-105%) recovery of hydroxyproline (HYP). Interfering primary amino acids are derivatized by o-phthaldialdehyde and removed from the sample by use of a discardable C18 column. HYP is measured photometrically at 254 nm after a second derivatization with phenylisothiocyanate and isocratic separation on a reversed-phase HPLC column.