The purity of the analysed elastin samples
If biochemical age estimation is to be performed on purified proteins, the purity of the samples is essential for the quality of age estimation. Contaminating proteins will have negative effects on the results as the kinetics of AAR depends on the molecular structure of a protein (see [24]). Thus, the absence of collagen is crucial for a correct analysis of AAR in elastin. Elastin contains 1.5–5.2 Asx/1,000 residues (dependent on splice variant) compared to 42 Asx/ 1,000 residues in arterial alpha 1 (III) collagen [7, 28]. The slower racemisation kinetics of Asx in collagen will result in an underestimation of the AAR in collagen-contaminated elastin samples. Histological analysis did not indicate any collagen contamination. With the purification protocol applied here, an elastin preparation with a reproducible amino acid composition was obtained, which was in close agreement with the composition of the elastin standard and with the predicted amino acid composition of human elastin from sequence data (Fig. 2). Amino acid compositions displayed a low amount of hydroxyproline and biochemical- ly confirmed the absence of contaminating collagen in the elastin specimens. However, some prolyl-residues in elastin are transformed through post-translational hydroxylation [18] but to a considerably lower amount as compared with collagen. Thus, the detected low amount of hydroxyprolin residues in combination with the absence of methionine is a strong indicator for elastin purity. Tryptophan, theoretically an additional marker for the presence of collagen, is destroyed during hydrolysis and, thus, was not detected.