The effect of carbon dioxide,aeration rate and sodium chloride on the secretion of proteins from growing bakers' yeast (Saccharomyces cerevisiae)

Since yeast may be an important microorganism for industrial use when its genes are modified by recombinant DNA techniques to overproduce certain proteins, (particularly those which are glycosylated), it is desirable to study how environmental variables affect its protein secretion ability. It is also of interest to understand how proteins such as proteases, lipases and amylases are excreted in solid matrices to develop a basis for learning more about solid fermentations. With these two applications in mind, the total protein excreted by both aerated and non-aerated Saccharomyces cerevisiae growing in a liquid batch culture (with varying levels of CO₂ and NaCl) was tracked. Using a modified Bradford method (Coomassie Blue dye-binding assay) for the concentration of total proteins in the extracellular fermentation broth, it has been determined that by 24 h of the run, excreted proteins rose to levels of about 10% of the total cell protein (500 μg ml^?1 protein from about 10 g of yeast, containing about 5 g total protein). No cell lysis was observed during the 24 h run. The highest protein levels at the top of the fermentor seemed to be those achieved in response to CO₂ alone. Additions of NaCl did not seem to enhance the secreted protein level. Large inconsistencies in replicating anaerobic runs for protein concentration appeared to be explained by noting that rising CO₂ bubbles may cause ‘foam fractionation’ of the proteins in the broth.