Liquid chromatography of free hydroxyproline in serum

We report a "high-performance" liquid-chromatographic method for measuring free hydroxyproline in serum. After separating the free hydroxyproline from the protein- and peptide-bound forms by centrifugal ultrafiltration, we derivatize the free form with 4-dimethylaminoazobenzene-4'-sulfonyl ("dabsyl") chloride and separate and quantify the derivative on a C18 chromatographic column. The mobile phase is sodium acetate buffer and acetonitrile, in a multilinear solvent gradient. The derivative is detected at 436 nm. Mean analytical recovery was 73%. The assay curve is linear from 14 to 213 pmol of hydroxyproline per 5-microL injection. The within- and interassay CVs were 8% at 10.7 mumol/L hydroxyproline and 10% at 13.5 mumol/L hydroxyproline, respectively.