The urinary excretion of pyridinolines either in the free form or linked to different peptide fragments of type I collagen are intensively studied as new biochemical markers of bone resorption. In the present study we compared the urinary excretion of free pyridinoline (F-Pyr) determined by enzyme-linked immunosorbent assay (ELISA) (Collagen CrosslinksTM Kit, Metra Biosystems) to pyridinoline (Pyr), and deoxypyridinoline (D-Pyr) determined by high performance liquid chromatography (HPLC) in early postmenopausal women treated with either hormone replacement therapy or placebo and in healthy age-matched premenopausal women. Other markers of bone metabolism were included for comparison. Compared with the premenopausal women, the postmenopausal women had significantly increased values of the biochemical parameters. F-Pyr, Pyr, D-Pyr, and T-Pyr (=Pyr+D-Pyr) decreased during hormone therapy. D-Pyr correlated with the rate of bone loss, whereas this was not the case for F-Pyr. The correlations between the markers yielded r values of 0.71 (F-Pyr vs Pyr), 0.67 (F-Pyr vs D-Pyr), and 0.71 (F-Pyr vs T-Pyr). In conclusion, the present study shows that the newly introduced ELISA for determination of the free pyridinolines is less sensitive than pyridinium crosslinks measured by high performance liquid chromatography (HPLC) in hydrolyzed urine for the changes in calcium metabolism that occur at menopause and during hormone replacement therapy. Whether this limitation will be balanced out by avoiding the inconvenience of the complicated, expensive, and timeconsuming HPLC procedure is still being debated.
PyridinolineFree pyridinolineDeoxypyridinolineUrinary excretionPre- and postmenopausalOsteoporosis