Effects of Cysteine and Other Thiols on the Growth of a Brewer's Yeast

L-Cysteine at low concentrations (up to 25 mg. cysteine-S per litre) in an other wise sulphur-deficient medium will support limited growth of the brewer's yeast under study. At higher concentrations (25–250 mg. cysteine-S per litre) cysteine is toxic and produces diminished growth. This effect is also observed when the yeast is grown in presence of sulphate, glutathione or L-methionine as sources of sulphur. Cysteine acts by prolonging the lag phase of growth without affecting cell size. Mercaptoacetate is also growth-inhibitory, but DL-homocysteine considerably less so. Cysteamine inhibits growth only when the sulphate concentration is suboptimal. Reduced glutathione, mercaptosuccinate and DL-penicillamine are not growth-inhibitory. Cysteine and to a lesser extent mercaptoacetate also depress the respiration of the yeast.