Patricia M. Masters

Aspartic acid racemisation in the human lens during ageing and in cataract formation

D-ASPARTIC acid has been shown to accumulate with age in human tooth enamel1 and dentine2 at a rate of about 0.1% yr−1. We have predicted that racemisation should take place in any metabolically stable protein in long-lived mammals and that, as a consequence of racemisation, these proteins will have altered conformations which would probably produce changes in their biological activities or chemical properties3. We have extended these studies to soft tissue proteins, in particular those in the human lens. The proteins in the central portion of the lens are among the most stable in the human body4. Numerous changes in the properties of the lens proteins occur with age and cataract formation—denaturation5, increasing pigmentation6, cross linking5,6 insolubility6–9, and fluorescence10. Pirie11 has suggested that physicochemical processes may be responsible for these changes. We report here the results of D/L enantiomeric analyses of normal human lenses and cataracts: aspartic acid racemisation was seen during ageing and cataract formation.

Preferential preservation of noncollagenous protein during bone diagenesis: Implications for chronometric and stable isotopic measurements

Abstract Preferential preservation of noncollagenous proteins (NCP) in diagenetically altered bone will affect amino acid compositions, inflate D L aspartic acid ratios, and increase C N ratios. Human skeletal remains representing both well preserved (collagenous) and diagenetically altered (noncollagenous) bones were selected from several southern California coastal archaeological sites that date from 8400 to 4100 years B.P. Amino acid compositions of the poorly preserved samples resembled NCP, which are probably retained by adsorption to the hydroxyapatite mineral phase of bone whereas collagen is degraded and lost to the environment over time. Since the racemization rate of aspartic acid in NCP is an order of magnitude faster than in collagen, the conservation of NCP in diagenetically altered bone can explain the high D L aspartic acid ratios, and the erroneous Upper Pleistocene racemization ages calculated from these ratios, for several California Indian burials. Amino acid compositional analyses also indicated a non-amino acid source of nitrogen in the poorly preserved samples, which may account for their lower C N ratios despite the acidic amino acid profiles typical of NCP. Preservation of NCP rather than collagen also precludes the extraction of a gelatin residue for radiocarbon dating and stable isotope analyses, but remnant NCP can yield apparently accurate radiocarbon dates. As collagen and phosphoprotein purified from a sample of modern human dentin have the same δ13C and δ15N values, remnant NCP may also be useful for paleodiet reconstructions based on stable carbon and nitrogen isotope compositions. Dentin collagen appears to be more resistant to diagenetic changes than does bone collagen. Consequently, dentin promises to be a more reliable material than bone for chronometric and stable isotope measurements.

Stereochemically Altered Noncollagenous Protein from Human Dentin

SummaryHighly phosphorylated noncollagenous proteins (NCP) with molecular weights of ∼70–100,000 daltons have been purified from rat and bovine dentin. Efforts to isolate phosphoprotein from human teeth have not yielded consistent results, and failures have been attributed to proteolysis due to preparative techniques. Diagenetic reactions affecting metabolically stable proteinsin vivo also can interfere in protein purification. Racemization is one of the reactions known to take place in human dentin. EDTA extraction of dentin from an age-graded series of human teeth has yielded an EDTA-soluble NCP fraction having an aspartic acid racemization rate 3 X that in unfractionated dentin and 8 X the rate in EDTA-insoluble protein. D-Aspartic acid is accumulating in EDTA-S protein at a rate of 0.22% yr−1. For humans, more than 13% of the aspartyl residues in NCP will be the D-enantiomer by 60 years of age. While racemization presents no problem for shorter lived mammals, such as rats, it could be partly responsible for purification difficulties with human dentin.

Racemization of isoleucine in fossil molluscs from Indian middens and interglacial terraces in southern California

The amino acid racemization dating technique is applied to the West Coast mollusc Chione from interglacial terrace deposits and Indian midden sites. The racemization kinetics of isoleucine were studied in Total hydrolysates and in the non-dialyzable Protein fraction from Chione shell. The reliability of the isoleucine dates was evaluated by (1) comparison with ages determined by radiocarbon and uranium disequilibrium dating, (2) species effects, and (3) reproducibility. Isoleucine dates for the interglacial terrace shell were found to be more reliable than the midden shell dates. A radiocarbon analysis of amino acids extracted from Del Mar midden shell was performed in order to resolve the discrepancies between the racemization ages and the 14 C dates on shell carbonate.

Age at death determinations for autopsied remains based on aspartic acid racemization in tooth dentin: Importance of postmortem conditions

The extent of aspartic acid racemization in human dentin can be used to assess chronological age of the individual. This novel chemical method of aging human remains has potential applications in forensic sciences. Six autopsy cases have been analyzed for D/L aspartic acid contents, and their ages at death calculated. Independent age information came from known identity, tooth maturation, or tooth wear estimations. The six cases represented a range of postmortem fates: recent demise, burial, and ground surface exposure. Five samples, four recently deceased and one a burial, showed racemization ages in close agreement with other age data. One specimen, subjected to 51 days of open air exposure, exhibited a vastly inflated racemization age. Postmortem conditions are crucial to the accuracy of racemization age at death measurements.

Long-term tectonic control on Holocene shelf sedimentation offshore La Jolla, California

A high-resolution Compressed High-Intensity Radar Pulse (CHIRP) survey reveals shore-parallel variations in the Holocene sediment thickness offshore La Jolla, California. Sediment thicknesses decrease from >20 m in the south near Scripps Canyon to zero in the north approaching Torrey Pines. In addition to the south-to-north variation in sediment thickness, the transgressive surface observed in seismic lines shoals from Scripps Canyon to the north. Despite these dramatic shore-parallel subsurface changes, the nearshore bathymetry exhibits little to no change along strike. A left jog (i.e., a constraining bend) along the Rose Canyon fault causes local uplift in the region and appears to explain the northward shoaling of the transgressive surface, the decrease in relief on the transgressive surface away from the left jog, and the Holocene sediment thickness variation. This tectonic deformation is shore parallel, and thus the accommodation can be separated into its tectonic and eustatic components.

Aspartic Acid Racemization and Radiocarbon Dating of an Early Milling Stone Horizon Burial in California

Clark, D. E., C. G. Pantano, Jr., and L. L. Hench 1978 Glass corrosion. Glass Industry, New York. Clark, David E., and Barbara A. Purdy 1978 Surface characterization of weathered Florida cherts. Symposium on Archaeometry and Archaeological Prospection, Bonn, Germany. n.d. Weathering of thermally altered prehistoric stone implements (in preparation). Dragoo, Don W. 1976 Some aspects of eastern North American prehistory: a review 1975. American Antiquity 41:3-27. Friedman, I., and R. L. Smith 1960 A new dating method using obsidian: Part 1, the development of the method. American Antiquity 25: 476-522.

Unexamined bodies of evidence.

In his News & Analysis story “Do island sites suggest a coastal route to the Americas?” (4 March, p. [1122][1]), M. Balter discusses the implications of evidence that more than 10,000 years ago, people used marine resources and specialized technology on Californias Channel Islands. He mentions

Comment on Jonathan Benjamin's ‘Submerged Prehistoric Landscapes and Underwater Site Discovery: Reevaluating the ‘Danish Model’ for International Practice’

Jonathan Benjamin (2010) offers a wellreasoned and tested procedure to maximize thediscoveryofprehistoricunderwater sites, building on the work of Fischer along the coastof Denmark, and then extends the focus to the northeast Adriatic and the lee of the Outer Hebrides off northwest Scotland. In addition to the regions discussed by Benjamin, additional marginal sea coasts that lie in temperate and subtropical zones with limited fetch and resultant low wave energy are attractive areas for prehistoric site exploration. These include the Gulf of Mexico, Black Sea, Mediterranean, and the seas bordering southeast Asia and China. All of these areas provide opportunities to apply the approach proposed by Benjamin, and some already have confirmed sites. For a truly international “systematic search strategy,” however, other types of coasts—see Inman (2005) for a classification of the world’s coasts—must be considered.

Facing the Coastal Challenge: Modeling Coastal Erosion in Southern California

Erosion due to natural and human activities poses a challenge to the future of California’s coast. A process-based coastal evolution model is being developed to evaluate the past, present, and future rates of erosion of the southern California coast and present this dynamic environment in a visual format. The model consists of a mobile sediment transport component and a bedrock cutting component, both coupled and operating in varying time and space domains determined by sea level and boundaries of the littoral cell. We will utilize retrospective data from geomorphology, tectonics, sea level, climate, and paleoecology to investigate erosional and depositional processes and rates of change. Correlating the earlier shorelines with past climate conditions and time-stepping the ancient coastlines forward to the modern coastline will serve to validate the model. The model then will project the future evolution of the coastline using three scenarios: a most likely change, a minimum change, and a maximum change based on climate projections and possible human interventions. Our goals are to make this modeling technology and 3D visualization accessible to coastal planners and to advance public understanding of coastal evolution.