Assessment of the Inhibitory Effect of Rifampicin on Amyloid Formation of Hen Egg White Lysozyme: Thioflavin T Fluorescence Assay versus FTIR Difference Spectroscopy

The inhibitory effect of rifampicin on the amyloid formation of hen egg white lysozyme was assessed with both Thioflavin T (ThT) fluorescence assay and Fourier transform infrared (FTIR) difference spectroscopy. We reveal that ThT fluorescence assay gives a false positive result due to rifampicin interference, while FTIR difference spectroscopy provides a reliable assessment. With FTIR, we show that rifampicin only has marginally inhibitory effect. We then propose that FTIR difference spectroscopy can potentially be a convenient method for inhibitor screening in amyloid study. 1. Introduction Amyloid aggregates are insoluble protein aggregates with the characteristic cross-β structure and fibrous morphology [1]. In vivo, amyloid deposition is a pathological hallmark of more than 40 neurodegenerative, systemic, and nonsystemic diseases [2]. These human diseases, collectively termed as amyloid diseases, include some well-known disorders such as Alzheimer’s disease, Parkinson’s disease, and Type II diabetes. Each of these diseases is associated with amyloid fibrillation of one unique type of protein or peptide [2]. The apparent linkage between amyloid formation and amyloid disease makes designing and developing molecular interventions into amyloid formation an attractive strategy to prevent and treat amyloid diseases [3]. Amyloid formation is a rather complex protein aggregation process. The aggregation kinetic curve usually follows a typical sigmoidal function featuring a lag phase, a growth phase, and an equilibrium phase [4]. Furthermore, there can be oligomeric intermediates in diverse sizes and morphologies. These intermediate oligomers have been referred to in numerous ways as amorphous aggregates, protofibrils, prefibrillar aggregates, globulomers, and annular protofibrils [5]. Both mature fibril and nonfibrillar oligomers have been hypothesized to be the possible pathogenic agents in amyloid diseases [3]. As amyloid fibril has been hypothesized to be one of the pathogenic agents in amyloid diseases [3], searching molecules that can inhibit amyloid fibrillation has been one of the major research efforts in the field of amyloid research. Rifampicin is an antibiotic which has been found to be able to inhibit amyloid formation of numerous proteins such as amyloid-β, -synuclein, islet amyloid polypeptide (IAPP), and hen egg white lysozyme (HEWL) [6–9]. However, the general picture of the inhibitory effect of rifampicin on amyloid formation has been challenged in recent years. Meng et al. reported that rifampicin in fact had no inhibitory effect on the