Phosphopeptides and gamma-carboxyglutamic acid-containing peptides in calcified turkey tendon: their absence in uncalcified tendon.

Uncalcified samples of turkey tendon obtained prior to calcification, and other samples from areas of tendon that never calcify, contain little or no O-phosphoserine [Ser(P)], O-phosphothreonine [Thr(P)] and gamma-carboxyglutamic acid (Gla). Significant amounts of all three of these Ca2+-binding amino acids, which are found in EDTA-extractable, non-collagenous proteins, are detected coincident with the onset of mineralization of a tendon and increase in concentration as mineralization proceeds.