Collagen turnover time

One further difference between the tissues should be noted briefly—that of turnover—which holds implications for the nature of the isotopic signal recorded and its interpretation. Bone is constantly resorbed and reformed during life, i.e., it turns over , whereas enamel and dentine do not, although secondary dentine can be later accreted. Enamel and dentine form during a discrete period in the individual s life. This means that carbon isotope dietary signals in bone, for both collagen and apatite, reflect diet integrated over years, whereas those in enamel and dentine increments reflect diet at time of formation. 

The major sources of carbon for possible isotopic analysis in bone are collagen, which contains about 45% carbon and hydroxyapatite which contains a few percent of carbon present as carbonate ion substituted in the apatite crystals These two carbon sources are derived from different pathways and different parts of the diet Turnover of bone tissues is the slowest of all tissues in the body that have been studied the mean residence time of carbon in collagen being at least 30 years (14) ... 

Proteins differ significantly in their turnover rates, which are measured in half-lives. (A half-life is the time required for 50% of a specified amount of a protein to be degraded.) Proteins that play structural roles typically have long half-lives. For example, some connective tissue proteins (e.g., the collagens) often have half-lives that are measured in years. In contrast, the half-lives of regulatory enzymes are typically measured in minutes. Several selected examples are listed in Table 15.1. 

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