The effect of extremolytes ectoine and hydroxyectoine on the heat-induced protein aggregation: The case of growth hormone

The extremolytes ectoine and hydroxyectoine are osmolytes found in extremophilic microorganisms. They are stabilisers of proteins and other macromolecules, including DNA and lipids. The aim of the study was to investigate the effect of the additives on the heat-induced aggregation of mink growth hormone as a model protein. The first-order rate constants of protein aggregation were determined at 60 °C depending on the additive concentration and pH of the solution. The onset temperature of aggregation was also recorded using a circular dichroism spectropolarimeter. The study showed that the effect of the additives depended on the pH of the solution. The first-order rate constants of aggregation were lower when the protein molecule had a negative charge. The effect also depended on the structure of the extremolyte itself. When the protein molecule was positively charged, hydroxyectoine destabilised the mink growth hormone molecule and promoted the aggregation. The different effects of the additives were determined by the different interactions with the protein molecules, as shown by circular dichroism measurements and previously by fluorescence spectroscopy. Therefore, when using ectoine or hydroxyectoine for protein formulation, the effect of the additive should be carefully analysed for each protein individually.