Tendon proteoglycans and proteins (other than Type I collagen) were solubilized by sequential extraction of powdered adult bovine deep flexor tendon. Only the proximal (tensile) region of this tendon was used. The experiments involved 24-hour extraction with phosphate buffered saline (to remove components that are readily soluble), followed by repeated extraction with 4 mol/L guanidine (to break noncovalent bonds holding components into the tissue), and then extraction with 4 mol/L guanidine containing dithiothreitol (to dissociate components held by disulfide bonds). Proteins accounting for approximately 5% of the tissue dry weight could be removed by the extraction protocol. Proteins were identified by Western blotting and correlation with stained gels. The major extracted components were identified as decorin, Type VI collagen, fibromodulin and a member of the leucine rich repeat protein family (PRELP). In addition an oligomeric matrix protein initially identified in cartilage (COMP), aggrecan, and biglycan were present. Most of these proteins were entirely extracted in cold 4 mol/L guanidine. However, some Type VI collagen and cartilage oligomeric matrix protein could not be removed unless the tissue was reduced with dithiothreitol. Many of these proteins have been considered as molecules that are primarily or exclusively components of cartilage. Cartilage and tendon are tissues with different histologic appearance and different function. However, the fact that the same biochemical components are found in cartilage and tendon shows the relatedness of these connective tissues and the cells that produce them. Tissue engineering attempts to reconstruct tendon using only its major or unique components may omit significant aspects of the tissue’s structure.

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