Proline

Proline is a non-essential cyclic amino acid and, together with hydroxyproline, the defining structural constituent of collagen, the principal protein of tendon, bone, and skeletal-muscle extracellular matrix. Salivary proline reflects connective-tissue amino-acid turnover, rising when collagen synthesis and degradation are accelerated by mechanical loading. Langberg et al. (1999) showed that exercise markedly increases peritendinous type I collagen turnover, the matrix in which proline predominates.