SUMMARY The effect of biotin deficiency on the osmotic behaviour of was studied by following the changes in the extinction of dilute suspensions of the yeast in phosphate buffer containing different concentrations of NaCl. The extinction of suspensions of biotin-optimal yeast increased as the NaCl concentration was increased to 1.5M, but with yeast grown in unsupplemented biotin-deficient medium the readings decreased with increasing NaCl concentration. Yeast grown in biotin-deficient media supplemented with aspartate, or with oleate + aspartate, showed the same type of osmotic behaviour as yeast grown in unsupplemented biotin-deficient medium. But with yeast grown in biotin-deficient medium supplemented with oleate alone, the extinction changes in buffer containing up to 0.7M-NaCl resembled those of suspensions of biotin-optimal yeast. These differences in osmotic behaviour were not due to breakage of the osmotic barrier by the osmotic pressure differences across the cytoplasmic membrane, but to an increased permeability of the membrane to Cl. This was accompanied by an increased permeability to HPO to higher molecular weight solutes, including bovine plasma albumin. The increase in permeability of biotin-deficient yeast was not as great as that of biotin-optimal yeast in which the osmotic barrier had been broken with 5% (v/v) -butanol.


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