Structural determinants of stability to proteolysis, processing and impact on allergenic potential of non-specific lipid transfer proteins
Lipid transfer proteins (LTPs) are a class of low molecular weight hydrophobic conserved proteins comprising four intramolecular disulphide bonds making the structure very resistant to proteolysis and harsh food processing conditions. These proteins are identified as strong allergens sensitizing through the gut and share epitopes with LTPs from closely related species. Peach LTP, Pru p 3 is the primary sensitizer in the Mediterranean area being the most frequent food allergen. Wheat LTP, Tri a 14 is a relatively weak allergen with a very low prevalence. The study here compares the structural properties of these proteins and their resistance to various digestive and processing processes. Ligand binding experiments showed that Pru p 3 binds to ligands more strongly than Tri a 14. The gastroduodenal digestion of these LTPs revealed that both are stable to gastric digestion and while Pru p 3 is susceptible to duodenal digestion, Tri a 14 digestion is negligible. Ligand binding did not affect the digestibility of Pru p 3 but improved the duodenal digestibility of Tri a 14. The IgE binding studies using sera from peach allergic individuals confirmed that both Pru p 3 and its digestion fragments in the presence and absence of ligand were IgE reactive. Model processing conditions were employed to treat these LTPs. It was found that heat treatment destroys the secondary structure of Pru p 3 at 121°C and slightly affects that of Tri a 14. Heat treatment also increased the susceptibility of Pru p 3 to gastric digestion while Tri a 14 was less affected. The IgE binding studies showed that heat treatment of Pru p 3 appeared to reduce its IgE recognition while its digestion fragments lost all of their IgE reactivity. To investigate the effect of the food matrix on the digestibility of these LTPs, peach peel containing Pru p 3 and wheat flour containing Tri a 14 were digested under simulated conditions. It was found that they were resistant to proteolysis in their native matrices. Effect of heat treatment to the food matrix again confirmed that both of these proteins were more stable to heat in the matrix and were less digestible. In conclusion, this study shows that there are factors in food matrices which enhance structural stability of LTPs to both processing and digestion. Thus factors such as the effect of food matrix and effect of processing should be taken into account in assessing the allergenic risk posed by foods and not simply rely on data from purified proteins.
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