RuBisCO from alfalfa – native subunits preservation through sodium sulfite addition and reduced solubility after acid precipitation followed by freeze-drying

Abstract

Alfalfa (Medicago sativa L.) is a potential source for food protein, being rich in the soluble protein ribulose-1,5-biphosphate carboxylase/oxygenase (RuBisCO). Redox enzymes hinder the purification of plant proteins by oxidizing polyphenols, instigating enzymatic browning, and forming quinoproteins. In this study, it was hypothesized that such reactions can be inhibited by addition of sodium sulfite during purification. The addition of sodium sulfite at concentrations of 10 and 25 mmol/L improved the color and total phenolic content of the alfalfa juice, and decreased quinoprotein formation in both alfalfa juice and concentrates. Furthermore, the sulfite-treated juice samples showed darker RuBisCO bands in both SDS-PAGE (55 and 15 kDa) and Blue Native-PAGE (∼550 kDa) analyses, compared to control samples, indicating higher solubility of the native protein. However, the addition of sodium sulfite decreased the solubility of the protein concentrate at neutral pH, as assessed by both total nitrogen and Lowry assays. The reduced solubility was attributed to the insolubility of RuBisCO subunits after processing, which is likely due to the formation of disulfide bridges and hydrophobic interaction of the proteins. This work demonstrates the importance of controlling polyphenol oxidation and aggregation behavior of RuBisCO during purification, in order to provide high quality protein ingredients.