Soil may contribute to the horizontal transmission of the prion diseases sheep scrapie and chronic wasting disease of deer, elk and moose by serving as an environmental reservoir for the infectious agent. We previously demonstrated that the disease-associated form of the prion protein (PrPTSE) binds to soil particles and that PrPTSE interaction with montmorillonite (Mte) is remarkably avid. Here, we investigate the role of organic carbon on the interaction of pathogenic prion protein with soil particles by studying PrPTSE sorption to humic acid-montmorillonite (HA-Mte) complexes. We found that HA-Mte complexes had a lower affinity for PrPTSE than Mte alone and that the binding capacity for the protein decreased with increasing HA content. Extraction of humic acid from HA-Mte complexes after PrPTSE sorption suggests that the pathogenic prion protein associates primarily with Mte surfaces and that organic carbon blocks PrPTSE binding sites on Mte. Sorption of prions to Mte prior to oral exposure dramatically enhances disease transmission. The influence of organic carbon on oral disease transmission by soil particle-bound prions warrants investigation.
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