Issue Date: June 25, 2012
Mummy Sugar Shock
Glycans—the sugars that decorate the backbones and ends of many proteins—can survive thousands of years in the preserved flesh of natural mummies, scientists reported last month at the American Society for Mass Spectrometry (ASMS) annual meeting. Natural mummies, unlike the more familiar Egyptian ones, form when a corpse is trapped in an extreme environment that arrests the degradation process.
The work has implications for understanding the evolution of such characteristics as human blood types, which are determined by sugars in the blood.
In the study, the scientists—Rudolf B. Grimm of Agilent Technologies, who is also an adjunct professor at the University of California, Davis; chemistry professor Carlito Lebrilla and graduate student Sureyya Ozcan, also of UC Davis; and professor Hyun Joo An, of Chungnam National University, in South Korea—were looking for biological clues about how each person died. They started out by looking for proteins in the mummies, but they quickly turned their attention to the glycans attached to those proteins.
“One of the main roles of glycosylation is to protect the protein backbone. If that’s the case, glycans should be one of the first things to be degraded,” Lebrilla said. “We wanted to know how long glycosylation would persist.”
The team analyzed samples from four natural mummies—the 5,300-year-old Ötzi, also called the Iceman, found in the Italian Alps; a 2,500-year-old Scythian warrior, found in the Mongolian Altai Mountains; a 2,500-year-old Scythian female, also found in the Altai Mountains; and a 50-year-old contemporary mummy, found in an apartment in Austria.
The contemporary mummy was an elderly man who lived alone. When he died, the bed linens absorbed fluids, and cold temperatures helped preserve the body. His mummified body was found 50 years later.
Before the researchers used any of the tiny samples available to them from each mummy, they needed to figure out how to retain the greatest amount of protein. They optimized the method using beef jerky as a stand-in for desiccated mummy flesh. In the case of the Iceman, the small sample size allowed them to characterize only the glycans, not complete glycoproteins.
By using a combination of chromatographic retention time and accurate mass from mass spectrometric analyses, they were able to determine the glycans present in samples from each of the mummies. The number of glycans depended on both the age and the preservation of the body. The contemporary mummy had many more glycans than the older ones. The number of glycans also depended on how well the body had been preserved.
In addition to the numerical differences, the researchers also found compositional differences between the older mummies and the younger mummy. For example, they found sialic acid in the younger mummy but not in the older ones. Sialic acid, which is usually found at the end of glycans, is a binding site for bacteria and viruses. “It looks like there’s a lot of bacterial degradation on these mummies,” Lebrilla said, which might account for the sialic acid loss.
The older mummies contained primarily glycans with a high proportion of the sugar mannose. “High-mannose glycans are actually a very small component of serum,” Lebrilla told C&EN. “The fact that we saw only high-mannose glycans in the Iceman indicates that they’re actually quite stable.” Many bacteria focus preferentially on sialic acid, fucose, and N-acetylgalactose instead of mannose, he noted, so the results make sense from the standpoint of bacterial degradation.
“This was a fascinating application of glycan profiling methods that have been perfected in the Lebrilla group over the last 20 years,” said I. Jonathan Amster, a professor at the University of Georgia, who heard Ozcan’s talk at the ASMS meeting. “It was quite remarkable to find that glycans persist in naturally mummified human tissue that is over 5,000 years old.”
Grimm and Lebrilla hope to use mummy glycan analysis to study the evolution of blood types, which is dictated by the sugar molecules on blood proteins. “The last big evolutionary step of humans happened about 1,000 years ago, when the blood group AB formed,” Grimm said. “We’re looking at what glycan structures we can find in mummies of different ages.”
So far, they’ve skipped past the point that will give them the evolutionary information they seek. They hope to soon gain access to mummies of intermediate ages—500, 750, and 1,000 years—to fill in the gaps and to find trends.
In future work, the collaborators hope to compare the glycans in natural mummies with those in mummies formed as part of burial rituals, such as chemically preserved Egyptian mummies.
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