Identification of phosphopeptides andγ-carboxyglutamic acid-containing peptides in epiphyseal growth plate cartilage

Uncalcified cartilage from the epiphyseal portion of bovine scapulae, both distant and adjacent to the epiphyseal growth plate, and the calcified cartilage of the epiphyseal growth plate itself were analyzed for the presence of O-phosphoserine [Ser(P)], O-phosphothreonine [Thr(P)] and γ-carboxyglutamic acid (Gla). Only trace amounts of these Ca2+-binding amino acids or the peptides containing them were found in the unmineralized tissues. In contrast, whole calcified cartilage, and especially the most mineralized fraction obtained by density centrifugation, contained considerable amounts of all three amino acids. Essentially all of the Gla and the majority of the Ser(P) and Thr(P) were present in non-collagenous, non-diffusible proteins extractable in EDTA at near-neutral pH.