Prediction of the absolute aggregation rates of amyloidogenic polypeptide chains

Protein aggregation is associated with a variety of pathological conditions, including Alzheimer's and Creutzfeldt-Jakob diseases and type II diabetes. Such degenerative disorders result from the conversion of the normal soluble state of specific proteins into aggregated states that can ultimately form the characteristic amyloid fibrils found in diseased tissue. Under appropriate conditions it appears that many, perhaps all, proteins can be converted in vitro into amyloid fibrils. The aggregation propensities of different polypeptide chains have, however, been observed to vary substantially. Here, we describe an approach that uses the knowledge of the amino acid sequence and of the experimental conditions to reproduce, with a correlation coefficient of 0.92 and over five orders of magnitude, the in vitro aggregation rates of a wide range of unstructured peptides and proteins. These results indicate that the formation of protein aggregates can be rationalised to a considerable extent in terms of simple physico-chemical parameters that describe the properties of polypeptide chains and their environment.

Antimalarial drugs influence the pH dependent solubility of heme via apparent nucleation phenomena

Recently, we measured a more acid digestive vacuolar pH for drug resistant Plasmodium falciparum [Dzekunov S, Ursos LMB, Roepe PD. Mol Biochem Parasitol 2000;in press; Ursos LMB, Dzekunov S, Roepe PD. Mol Biochem Parasitol 2000;in press]. We suggested this acidification contributes to drug resistance via the profound effects that pH has on the solubility of unpolymerized heme found in the vacuole (ferriprotoporphyrin IX mu oxo dimers). In this report, we measure how FPIX concentration, time, NaCl concentration, and several antimalarial drugs affect FPIX pH dependent solubility. Aggregation is essentially instantaneous below pH 5.3, but at vacuolar pH previously measured for HB3 parasites [Dzekunov S, Ursos LMB, Roepe PD. Mol Biochem Parasitol 2000;in press] can increase to several minutes as NaCl is lowered. As FPIX is decreased, the midpoint of the pH dependent solubility curve shifts to higher values. Addition of antimalarial drugs also increases the midpoint of the pH dependent FPIX solubility curve, with the net shift proportional to the relative affinity of the drug for FPIX. Surprisingly, however, for all drugs tested shifts of essentially identical magnitude are found at all drug: FPIX molar ratios inspected, spanning eight orders of magnitude (to as low as 0.0000001:1). This suggests that changes in pH dependent FPIX solubility by addition of antimalarial drugs is via previously unrecognized drug/FPIX nucleation phenomena. These data could have important implications for understanding the role of previously observed changes in pH(vac) [Dzekunov S, Ursos LMB, Roepe PD. Mol Biochem Parasitol 2000;in press; Ursos LMB, Dzekunov S, Roepe PD. Mol Biochem Parasitol 2000;in press] upon development of antimalarial drug resistance.