Total and free deoxypyridinoline after acute osteoclast activity inhibition

Background: Deoxypyridinoline (Dpd) is one of the two pyridinium cross-link s that provide structural rigidity to type I collagen in bone. During osteo clastic resorption, Dpd is released into circulation and is excreted in the urine in free and peptide-bound forms. Free and total Dpd are highly corre lated, but whether the free-to-total cross-link ratio is constant in both n ormal and high bone turnover states remains controversial. To compare free and total Dpd performance in a physiological condition, urinary free and to tal Dpd were measured after a short-term inhibition of osteoclast activity such as that induced by an oral calcium load. Methods: Total and free Dpd were measured by HPLC and by immunosorbent assa y, respectively, in two groups of subjects, one (calcium-treated; n = 16) t aking calcium and the other not (control; n = 9). Results: The urinary excretion of total Dpd at 2 and 4 h after oral calcium loading was decreased compared with controls. By contrast, changes in free Dpd were similar in the calcium-treated and control groups, reflecting onl y circadian rhythm. Conclusions: Total and free Dpd do not show comparable sensitivity in detec ting short-term inhibition of osteoclast activity. The degradation process of peptide-bound to free Dpd could render free Dpd insensitive to acute cha nges of osteoclast activity. (C) 1999 American Association for Clinical Che mistry.