Measurement of free and free-plus-peptide hydroxyproline fractions in plasma

Plasma hydroxyproline fractions--free-plus-peptide and free--were determined by a method involving centrifugal ultrafiltration and the procedure of Kivirikko et al. [Anal. Biochem. 19, 249 (1967)]. Within- and between-day precision (CV) for the combined fractions was 4--5% and 2.8--6%, respectively. Mean analytical recoveries of free hydroxyproline were 95.2% (87--109%) in the normal range, and 97% (91--108%) in the above-normal range. Normal ranges for plasma hydroxyproline fractions, based on data for 11 men and 19 women, were free-plus-peptide, 7.5--25.3; free, 4.9--22.7; and peptide (calculated), 0.1--5.1 mumol of hydroxyproline per litre, respectively. These ranges showed no sex-related differences and were gaussian in distribution. A factor was derived to transform data on concentrations in ultrafiltrate into plasma concentrations.