engleski

Unusual neutral lipid composition of the plasma membrane and mitochondria of the recycled bottom-fermenting brewer’s yeast as adaptation to growth conditions

Introduction Strain of Saccharomyces cerevisiae yeast used for the production of lager beer is known as the bottom-fermenting brewer's yeast. In the industrial process of beer production it undergoes propagation and fermentation during which it is exposed to various types of stresses, among which anaerobic conditions and ethanol toxicity have the strongest effect on the cells. Plasma membrane (PM) largely determines cell growth and survival ability, and it is known that membrane lipids make a part of an effective adaptation mechanism. The mitochondria (MI) are essential for brewing yeast physiology and fermentation performance, and ethanol tolerance is closely related to their lipid composition. The aim of this study was to investigate how brewer’s yeast copes with the shift from aerobic to anaerobic conditions when ergosterol and unsaturated fatty acids cannot be synthesized. We have determined the composition of neutral lipids and fatty acids in PM and MI of the pure culture (zero generation) grown aerobically and 1st recycled generation, grown anaerobically. Results The highly purified plasma membranes and crude mitochondria were isolated after enzymatic disruption of the cell wall. Lipids were extracted by Folch method. Neutral lipids were separated by two-step TLC of total lipid extract and quantified by densitometry, while fatty acids were determined by GC of the corresponding methyl esters. In PM unsaturated acids accounted for 39% of total identified fatty acids in the zero and 54% in the 1st generation, respectively, while in MI they prevailed in both generations (51% and 56%, respectively). C16-acids prevailed in both generations of PM and MI, as well, accounting between 64 and 70%. Palmitic acid was the main one in all preparations except in the MI of the 1st generation. Among neutral lipids of PM and MI only ergosterol and squalene were present in significant amounts. Ergosterol accounted for 18.4 µg/mg proteins in the zero and 11.0 in the 1st generation of the PM, respectively. The concentration of ergosterol was higher in the zero than in the 1st generation of MI, as well (9.7 and 7.0 µg/mg proteins, respectively). Interesting finding was that the PM and MI of both generations, the one grown anaerobically, but also the one grown aerobically, contained substantial amounts of squalene, PM 10.4 and 19.4 µg/mg proteins and MI 4.0 and 7.1 µg/mg proteins, respectively. Conclusions Fatty acid and neutral lipid composition of PM and MI of the zero and 1st recycled generation of the bottom-fermenting brewer’s yeast differ significantly. We assume that the accumulation of squalene in these membranous systems is not just a way of storing it, but that it has an assigned function in the regulation of their fluidity and permeability as a part of adaptive mechanism.

Brewer's yeast; lipids; plasma membrane; mitochondria

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