The determination of pyridinium crosslinks in urine and serum as a possible marker of cartilage degradation in rheumatoid arthritis

The determination of the collagen crosslinks pyridinoline (Pyd) and deoxypyridinoline (Dpyd) seems to be a successful way of characterizing topic destructive processes in rheumatoid arthritis (RA). Dpyd is a specific marker of collagen I resorption in bone, whereas Pyd is released from types I and II collagen in bone and cartilage. Both crosslinks were examined in 38 RA patients concurrently by RP-gradient-HPLC in urine and serum. A positive correlation was found between the inflammatory activity (measured by CrP) and. the level of collagen crosslinks in urine. A correlation between serum and urine concentrations was demonstrable for Pyd, but not for Dpyd. Different elimination kinetics for fragments containing either Pyd or Dpyd are a possible explanation for this observation. The ratio of Pyd/Dpyd is known to be a useful marker to distinguish between destruction of cartilage and bone collagen. Because the Pyd/Dpyd ratio in urine does not necessarily correspond to that in serum, probably as a result of metabolic or elimination processes, the usefulness of the relationship between the crosslinks in urine as a method of differentiating between cartilage and bone degradation must be questioned.