Heat-Induced Aggregation of Hen Ovalbumin Suggests a Key Factor Responsible for Serpin Polymerization

Although ovalbumin (OVA), a main component of hen egg white and a non-inhibitory serpin superfamily protein, has been reported to form fibrillar aggregates, its relationship with amyloid fibrils associated with various degenerative diseases is unclear. We studied the heat-induced aggregation of intact OVA using an amyloid-specific thioflavin T assay with a fluorometer or direct imaging with a light-emitting diode lamp and several physicochemical approaches, and the results confirmed that intact OVA forms aggregates with a small part of amyloid cores and dominantly amorphous aggregates. We isolated the amyloidogenic core peptide by proteolysis with trypsin. The isolated 23-residue peptide, pOVA, with marked amyloidogenicity, corresponded to one (β-strand 3A) of the key regions involved in serpin latency transition and domain-swap polymerization leading to serpinopathies. Although the strong amyloidogenicity of pOVA was suppressed in a mixture of tryptic digests, it was observed under acidic conditions in the presence of various salts, with which pOVA has a positive charge. Cytotoxicity measurements suggested that, although heat-treated OVA aggregates exhibited the strongest toxicity, it was attributed to a general property of amorphous aggregates rather than amyloid toxicity. Predictions indicated that the high amyloidogenicity of the β-strand 3A region is common to various serpins. This suggests that the high amyloidogenicity of β-strand 3A that is important for serpin latency transition and domain-swap polymerization is retained in OVA and constitutes β-spine amyloid cores upon heat aggregation.