Abstract
Little is known about the rate at which protein turnover occurs in living tendon and whether the rate differs between tendons with different physiological roles. In this study, we have quantified the racemization of aspartic acid to calculate the age of the collagenous and non-collagenous components of the high strain injury-prone superficial digital flexor tendon (SDFT) and low strain rarely injured common digital extensor tendon (CDET) in a group of horses with a wide age range. In addition, the turnover of collagen was assessed indirectly by measuring the levels of collagen degradation markers (collagenase-generated neoepitope and cross-linked telopeptide of type I collagen). The fractional increase in D-Asp was similar (p = 0.7) in the SDFT (5.87 x 10(-4)/year) and CDET (5.82 x 10(-4)/year) tissue, and D/L-Asp ratios showed a good correlation with pentosidine levels. We calculated a mean (+/-S.E.) collagen half-life of 197.53 (+/-18.23) years for the SDFT, which increased significantly with horse age (p = 0.03) and was significantly (p < 0.001) higher than that for the CDET (34.03 (+/-3.39) years). Using similar calculations, the half-life of non-collagenous protein was 2.18 (+/-0.41) years in the SDFT and was significantly (p = 0.04) lower than the value of 3.51 (+/-0.51) years for the CDET. Collagen degradation markers were higher in the CDET and suggested an accumulation of partially degraded collagen within the matrix with aging in the SDFT. We propose that increased susceptibility to injury in older individuals results from an inability to remove partially degraded collagen from the matrix leading to reduced mechanical competence.
Highlights
Tendons play a key role in locomotion by providing the mechanical link between muscle and bone
We aimed to assess molecular matrix age by calculating protein half-life in equine tendons using the racemization of aspartic acid as a probe
Matrix turnover rate gives an indication of the ability of the tenocytes to repair any micro-damage and is of particular relevance to the accumulation of damage with aging in specific tendons
Summary
Tendons play a key role in locomotion by providing the mechanical link between muscle and bone. An appropriate compliance is required by the energystoring tendons to allow stretching and recoil to occur at a rate in keeping with the gait cycle These energy-storing tendons have a higher non-collagenous protein content, predominately proteoglycan [3], which is thought to allow sliding movement between collagen fibrils [4]. Unexpected findings from our previous work [3], suggest that the matrix of the high strain equine SDFT is turned over more slowly than in the low strain positional common digital extensor tendon (CDET) This conclusion was based upon a simple measurement of tissue-associated fluorescence; longlived proteins such as collagen are subjected to age-related glycation and subsequent spontaneous formation of advanced glycation end-products (AGEs), some of which fluoresce naturally. A more widely accepted and robust method of assessing molecular age is to measure the rate of amino acid racemiza-
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