Collagen propeptides as indicators of collagen assembly

It is old knowledge that bones are for about 70% made up of calcium salts and for about 30% of collagen. More specifically, almost all of the collagen in the collagen fibres of mineralized tissues is type I collagen, while most of the other almost 20 genetically distinct collagen types are essentially proteins of soft tissues. Since type I collagen is also the most abundant collagen type in soft tissues, it forms the vast majority of all the collagenous proteins in the body. It seems reasonable to assume that 50-70% of the type I collagen in the body is in the skeleton. As bone is a metabolically more active tissue throughout life than most soft tissues, the contribution of bones to the biochemical markers reflecting type I collagen metabolism is even larger than is the proportion of this collagen type in the body.