Chapter 23 - Products of Bone Collagen Metabolism

Abstract

It has been a challenge for researchers to assess the balance between bone collagen synthesis and degradation by the simultaneous measurement of metabolic products related to these two processes. There are two possible analytes related to the synthesis of type I collagen, the carboxyterminal (PICP) and the amino-terminal (PINP) propeptides of type I procollagen; and the respective assay methods are already quite well established. There is a great deal of clinical experience on their performance in different physiological and pathological situations. Although in general both propeptides give relevant information of type I collagen synthesis, in some situations the assay for the intact PINP seems to be superior to that for PICP. The general picture of the degradation of bone type I collagen has been cleared by recent research, with the clarification of the roles of the cathepsin K and matrix metalloproteinase pathways. However, a number of more specific questions remain, related to the further metabolic fates of the primary degradation products, to the breakdown products of the helical domain, or to the origins of the free forms of cross-links. Much clinical experience has been gained using urine samples, but the present trend is towards serum assays.