Utilization of Peptides by a Lager Brewing Yeast

Abstract

The nutritional quality of selected peptides [(Ala)2, (Asp)2, and (Leu)2] and their constituent amino acids were compared by examining the growth response of the commercial lager brewing strain, Saccharomyces cerevisiae NCYC 1324 (formerly Saccharomyces uvavum NCYC 1324), to these nitrogen sources in a defined medium. Single amino acids were better sources of nitrogen than were single sources of each homodipeptide. Peptides [(Ala)2 and (Leu)2] and nonpeptide nitrogen sources (ammonium sulfate, allantoin, arginine, leucine, proline, and urea) were combined to increase the nutritional complexity of the defined medium. A synergistic effect on growth resulted in faster growth rates and shorter growth cycles. The HPLC analysis of these culture supernatants demonstrated that S. cerevisiae NCYC 1324 utilized amino acids and peptides in a specific order. In binary mixtures of nitrogen sources, amino acids, ammonia, allantoin, and urea were preferred and appeared to inhibit the utilization of dipeptide from culture medium. In complex nitrogen mixtures (three amino acids and three dipeptides), S. cerevisiae NCYC 1324 simultaneously used both amino acids and peptides as sources of nitrogen. This research shows that these model dipeptides are definitely used as additional sources of nitrogen for continued yeast growth in a defined medium. Physiological conditions that influenced nitrogen repression of peptide utilization were also defined. It appears that the presence of ammonium ion in a defined culture medium inhibited peptide utilization inside the yeast cells, whereas leucine enhanced the ability of the yeast to utilize peptides.