Aspartic acid racemization variability in ancient human remains: implications in the prediction of ancient DNA recovery

摘要

The extent of racemization of aspartic acid (Asp) - expressed as D/L ratio - has been used as a marker ofudbiomolecular degradation in ancient remains. However, Asp racemization rate is highly variable, anduddepends on biochemical and geochemical factors. In this paper we aim to determine to which extent theudfraction analyzed and the kind of sample used may influence the D/L Asp ratios. Other factors, such asudburial site and sample preservation conditions, are also considered.udD/L Asp ratios were obtained in 38 ancient human samples from 30 different individuals in which RealudTime PCR quantification and amplification of short mtDNA fragments had been previously achieved. Fourudsamples were taken from bones, 16 from whole teeth and 18 from dentine. In 7 cases whole tooth anduddentine fractions from the same individual were analyzed. The samples belonged to 8 archaeologicaludsites from Pre-pottery Neolithic B (PPNB), Cardial Neolithic and Chalcolithic time periods. Results showudsignificant differences between the D/L Asp ratios obtained in dentine and whole tooth fractions from theudsame tooth and individual, as well as among dentine samples from the same archaeological site andudamong samples of the same age from different, though nearby, archaeological sites. Ancient DNA (aDNA)udcould be characterized in the majority of the samples, independently from their racemization values,udwhich suggests that other factors, apart from the sample preservation stage, are more significantlyudaffecting the racemization rate. A generalized use of the Asp racemization ratio as a threshold value forudancient DNA preservation needs to be questioned until further methodological standardization isudconsidered.