Fractionation and characterization of beer proteins

Abstract

A combination of large scale gel filtration with preparative isoelectric focusing and ion exchange chromatography has been used for fractionation of beer proteins. From gel filtration on Sephadex G-150, a high molecular weight fraction, eluting close to the void volume of the column, a fraction appearing corresponding to a molecular weight of 44,000 and a fraction containing rather low-molecular weight (about 10,000) components were obtained and used for preparative isoelectric focusing in the pH-range 3.5-10. The high-molecular weight fraction was rich in carbohydrate and cross-reacted with yeast antibodies. After preparative isoelectric focusing the amino acid composition of the isolated subfractions resembled that of yeast cell wall components. Preparative isoelectric focusing of the two fractions with molecular weight about 44,000 and 10,000 revealed the presence of two classes of components based on their amino acid composition. One class, which seemed to be present in low concentration in nearly all isolated subfraction, had an amino acid composition resembling that of barley glutelins. In the fraction with molecular weight about 44,000 another class of components-having an amino acid composition resembling barley albumins and globulins-were observed in the region with isoelectric points of about pH 4-5. They cross-reacted immunologically with antibodies against soluble barley proteins. This class of components could also be separated from the glutelin-like constituents by ion exchange chromatography. However, they could not be completely separated from carbohydrate by the fractionation procedures employed. Carbamylation expents demonstrated that approx, half of the ε-amino groups of lysine residues of these proteins were blocked. Furthermore, partial amino acid sequence determination by the Edman procedure revealed a strong heterogeneity with respect to N-terminal amino acid residues. These observations are consistent with a suggestion that a large part of the beer proteins might be polypeptide chains crosslinked by carbohydrates. © 1978 Carlsberg Laboratory.