Brazilin Inhibits Prostatic Acidic Phosphatase Fibrillogenesis and Decreases its Cytotoxicity

A 39-amino acid peptide fragment that is derived from prostatic acidic phosphatase (PAP), PAP(248-286), is secreted in large amounts in human semen and forms amyloid fibrils. These fibrils can capture HIV virions and increase the attachment of virions to target cells; as such, they are called a "semen-derived enhancer of virus infection" (SEVI). Therefore, the inhibition of the formation of PAP(248-286) amyloid fibrils is of great significance. Herein, we demonstrate that brazilin effectively inhibits PAP(248-286) aggregation. The inhibitory effect increases with increasing brazilin concentration. Thioflavin T fluorescence assays and TEM observations confirmed that a few fibrils formed when brazilin was present with PAP(248-286) in an equimolar concentration. Circular dichroism spectroscopy indicated that brazilin inhibited the secondary structural transitions from a-helices and random coils into beta-sheets. Cytotoxicity assays showed that brazilin significantly decreased the cytotoxicity of the fibrils at 0.01 mu mol L-1. Isothermal titration calorimetry revealed that hydrophobic interactions were the main driving force for the binding of brazilin to the PAP(248-286) monomer (dissociation constant, 4.03 mu mol L-1), and that the binding affinity of brazilin for the fibrils was at least three orders of magnitude lower than that for the monomer. These results indicate that brazilin holds great potential as a small-molecule agent against SEVIs.