Mechanism of Nitrogen Assimilation from Amino-Acids by Yeast

Abstract

EHRLICH¹ showed, in general, that an α-amino-acid if supplied to yeast as its sole source of nitrogen undergoes deamination, since the corresponding higher alcohol can be recovered from the fermented medium ; he inferred that the ammonia simultaneously liberated is utilized for the synthesis of cell proteins. Coupled with the known fact that fusel oil (a mixture of these higher alcohols) is produced during the fermentation by yeast of natural substrates such, as beer wort, the primary source of nitrogen of which consists of a complex mixture of α-amino-acids, this, work of Ehrlich initiated the widespread view² that deamination of α-amino-acids is the normal mechanism of nitrogen assimilation by yeast. It is a corollary of this view that no amino-acid could be superior to ammonia as a nitrogen nutrient for yeast. While this is generally true when single amino-acids are used as the nitrogen source³ (deamination being obligatory in such cases), it is demonstrably false when the nitrogen is supplied as a mixture of many amino-acids. I have shown that approximately half the assimilable nitrogen of beer wort is assimilated by yeast fully five times faster than ammonia nitrogen.