A protein prominent in guanidine hydrochloride extracts of adult bovine and equine
digital flexor tendons was confirmed to be Cartilage Oligomeric Matrix Protein (COMP)
by non-reducing and reducing SDS-PAGE, reaction with rabbit anti-COMP polyclonal antiserum
on Western blots, trypsin digestion followed by HPLC on a C2/C18 column, and identification
of COMP mRNA from tendon on Northern blots. Immunohistochemistry and Western blots
of extracts showed COMP to be present in all regions of digital flexor tendons. Equine
tendon COMP was purified by ion exchange chromatography and gel filtration and used
in a heterologous inhibition ELISA to quantify COMP in equine digital flexor tendons
at different ages, and in other tendons and ligaments. Mean COMP levels in digital
flexor tendon were approximately 2-5mg/g wet weight, but they showed a large variation.
Levels were low in neonatal tendon but rose rapidly during growth, with the metacarpal
(tensional) superficial digital flexor tendon having the highest levels (approximately
10mg/g wet weight). Levels subsequently declined in this region, while in areas which
experience a variable amount of compression, levels increased less but then remained
constant. Extensor tendons and collateral ligaments, which experience less loading
in vivo, had levels similar to those in neonatal tendon. COMP was identified in scarred
skin and granulation tissue but not in normal skin, chronic fibrosis, or a fibrosarcomatous
skin growth. A unilateral non-weight-bearing growing animal contained three to six
times more COMP in the weight-bearing digital flexor tendons compared to the paralyzed
limb, while the extensor tendons had similar amounts in both limbs. With the recent
discovery of a COMP gene mutation causing pseudoachondroplasia (Hecht et al., 1995),
in which lax tendons and ligaments are a feature, the present data suggest that COMP
is synthesized in response to, and is necessary for tendon to resist, load.