Collagen Turnover in Normal and Degenerate Human Intervertebral Discs as Determined by the Racemization of Aspartic Acid

Nico Sakkee,Frits Van Der Ham,Sarit-Sara Sivan,Eve Tsitron,Sally Roberts,Alice Maroudas,Ellen Wachtel,Jeroen Degroot

doi:10.1074/jbc.m709885200

Abstract

Knowledge of rates of protein turnover is important for a quantitative understanding of tissue synthesis and catabolism. In this work, we have used the racemization of aspartic acid as a marker for the turnover of collagen obtained from healthy and pathological human intervertebral disc matrices. We measured the ratio of the d- and l-isomers in collagen extracted from these tissues as a function of age between 16 and 77 years. For collagen taken from healthy discs, the fractional increase of d-Asp was found to be 6.74 x 10(-4)/year; for degenerate discs, the corresponding rate was 5.18 x 10(-4)/year. Using the racemization rate found previously for the stable population of collagen molecules in dentin, we found that the rate of collagen turnover (k(T)) in discs is not constant but rather a decreasing function of age. The average turnover rate in normal disc between the ages of 20 and 40 is 0.00728 +/- 0.00275/year, and that between the ages of 50 and 80 is 0.00323 +/- 0.000947/year, which correspond to average half-lives of 95 and 215 years, respectively. Turnover of collagen from degenerate discs may be more rapid than that found for normal discs; however, statistical analysis leaves this point uncertain. The finding of a similar correlation between the accumulation of d-Asp and that of pentosidine for three normal collagenous tissues further supports the idea that the accumulation of pentosidine in a particular tissue can, along with the racemization of aspartic acid, be used as a reliable measure of protein turnover.

Highlights

The intervertebral disc (IVD),2 the largest avascular cartilaginous structure [1, 2], plays a primary mechanical role in transmitting loads through the spine and providing flexibility to the spinal column
When a balance is maintained, damaged tissue can be restored by cellular repair responses; otherwise, the matrix composition and organization are altered, and the cellular repair responses become inadequate
In which, as defined above, D/L is the ratio of D- to L-Asp, t is the time of heating, ki* is the in vitro rate constant for aspartic acid racemization, and the logarithmic term evaluated at t ϭ 0 describes the amount of D-Asp isomer measured in unheated collagen

Summary

Introduction

The intervertebral disc (IVD),2 the largest avascular cartilaginous structure [1, 2], plays a primary mechanical role in transmitting loads through the spine and providing flexibility to the spinal column. In which, as defined above, D/L is the ratio of D- to L-Asp, t is the time of heating, ki* is the in vitro rate constant for aspartic acid racemization, and the logarithmic term evaluated at t ϭ 0 describes the amount of D-Asp isomer measured in unheated collagen.

Objectives

Results

Conclusion