Food Additive Hexametaphosphate Promotes Amyloid Formation in Human Serum Albumin: A Molecular Insight

This study investigates the aggregation behavior of human serum albumin (HSA) in its cationic (pH 2.0) and anionic (pH 8.0) states upon exposure to hexametaphosphate (HMP), a polyanionic compound. UV-Vis turbidity measurements revealed that cationic HSA aggregated in a concentration-dependent manner starting at 0.01 mM HMP and plateaued beyond 0.05 mM, while anionic HSA remained soluble even at 15 mM HMP. Intrinsic fluorescence analysis showed a blue shift in the emission maximum of cationic HSA, indicating conformational changes associated with aggregation, whereas no shift was observed in anionic HSA. Far-UV circular dichroism (CD) spectroscopy demonstrated that cationic HSA lost its alpha-helical structure and adopted cross-beta sheet conformations at HMP concentrations ≥ 0.05 mM, consistent with amyloid formation, which was further supported by increased Thioflavin T (ThT) fluorescence. Rayleigh light scattering (RLS) and ThT kinetic studies confirmed rapid, saturation-limited aggregation without a lag phase. Transmission electron microscopy (TEM) further verified the presence of amyloid-like fibrils in cationic HSA treated with HMP. In contrast, anionic HSA showed no structural or aggregation changes under identical conditions. These findings highlight the pH-dependent, amyloidogenic potential of HSA in the presence of HMP and underscore the role of electrostatic interactions in protein aggregation.