IN VITRO DECOMPOSITION OF BONE COLLAGEN BY SOIL BACTERIA: THE IMPLICATIONS FOR STABLE ISOTOPE ANALYSIS IN ARCHAEOMETRY*

To understand biogenic collagen type I decomposition and to establish how diagenesis may bias archaeometric data, modern mammalian bone was inoculated with a selection of ubiquitous soil bacteria. The presence of exogenous microbial biomass in the inoculated specimens was then checked microscopically prior to collagen extraction. The experimentally degraded bone collagen showed altered amino acid compositions, attributable to the selective breakdown of certain amino acids by the bacteria. While both the bulk collagen extract and the single amino acids exhibited shifts to more negative δ¹³ C-values, enrichment was recorded for general δ¹³ N, and a depletion trend relative to unaltered collagen was observed for individual amino acid δ¹³ N. One explanation for the enrichment of the global δ¹⁵N-values is cleavage of peptide bonds, which leaves ¹⁵ N within the substrate, while the change of ¹³C is mostly due to the altered amino acid composition. On the other hand, possible repolymerization of cleavage products under experimental conditions may also be responsible for the depletion trend of individual amino acid δ¹³C- and δ¹⁵ N-values. This paper discusses the results as a basis for the development of a method for the reconstruction of the isotopic abundance of the original collagen, using the amino acid composition of the degraded product, the contribution of individual amino acids to its global δ-values and of isotope discriminations implied in the microbial decomposition.     

Serum Proteins in Archaeological Human Bone

Research on the decomposition of bone collagen offers the key to a wealth of hidden information, for instance, to understanding palaeoclimatic conditions; habitat-specific parameters such as altitude; and for the reconstruction of palaeodiet and subsistence patterns. Radiocarbon dating, one of the most commonly used geochronological techniques is also preferentially carried out on bone collagen. Because negatively charged ions, and especially phosphate groups, are responsible for the tight bonding between organic molecules and bone, soluble serum proteins need not necessarily be leached from buried bone. The authors' hypothesis is that intruding minerals, aided by recrystallization of the bone mineral matrix, and colloid formation, aided by humic substances, form protective layers that preserve serum proteins in bone even after long periods of burial. We have used electrophoresis and Western blotting to recover protein fractions from about 150 archaeological bones from various sites and epochs (up to 5500 BC ) and have succeeded in recovering, purifying all, and identifying some of the proteins. Molecular weight bands corresponding to albumin, transferrin and a-2HS-glycoprotein (A2HS) were frequently recovered. This paper presents the method for separating serum proteins from archaeological bone and for their identification. Twenty-five per cent of samples with the respective molecular weight band still gave a positive immunological reaction with antibodies. We conclude that non-collagenous proteins, especially serum proteins, may be well preserved in bone.