SUMMARYThe flocculation of a strain of brewers' yeast was absolutely dependent upon the presence of calcium; a concentration of 200 mM-CaC1, was sufficient to ensure almost complete flocculation. No other metal could replace calcium ; several metals aggregated potentially flocculent cells but also aggregated non-flocculent cells. Sodium ions antagonized the action of the calcium. The effects of pH value and esterification suggested that carboxyl groups were involved. The flocs had a ' melting temperature ' of 50-60" and were dispersed by urea, suggesting that hydrogen bonds were also present.Non-flocculent yeast was aggregated when the dielectric constant of the medium was decreased by the addition of organic solvents, but this aggregation was also dependent on the presence of traces of calcium. Conversely, increase of the dielectric constant of the medium, by adding formamide, dispersed flocculent yeast. Certain specific sugars also dispersed flocculent yeast. It is suggested that flocculent yeast cells are linked by salt bridges formed by calcium atoms joined with two carboxyl groups in the surfaces of different cells and that this structure is stabilized by hydrogen bonds formed between complementary patterns of carbohydrate hydrogens and hydroxyls in the cell surfaces.
SUMMARYPreparations of flocculent and of non-flocculent cell walls were obtained from flocculent and non-flocculent cells of a strain of Saccharomyces cerevisiae. Flocculent walls contained 46 yo glucan, 43 yo mannan, 0.4 Yo P, 1-1 yo hexosamine, 0.79 yo non-hexosamine N; non-flocculent walls contained 47 % glucan, 44 yo mannan, 0.3 % P, 1.2 % hexosamine and 0.98 7;non-hexosamine N.Mannose-6-phosphate was identified as the principal phosphorus compound present. The mannose residue formed part of the cell-wall mannan and the phosphate was also linked by a second, labile, ester bond to an unidentified site. The degree of phosphorylation of the mannan varied from 1 phosphate to 19 mannose residues in non-flocculent cells to 1 phosphate to 13 residues in flocculent cells.
SUMMARYThe point in the growth cycle at which a strain of brewer's yeast became potentially flocculent could be delayed by supplementing the medium with ammonia, basic amino acids, glutamine, asparagine, y-aminobutyric acid or urea. Other amino acids were ineffective. ,8-Alanine and 2-chloro-4-aminobenzoic acid led to an abnormally early appearance of potential flocculence. No development of flocculence occurred in the absence of glucose. It is suggested that the maintenance of non-flocculence is dependent upon the presence in the cell wall of a nitrogenous compound; potential flocculence will develop when this compound is not synthesized a t a rate sufficient to maintainits concentration in the wall. The nitrogenous nutrients which delay flocculation would then act by enhancing this rate of synthesis.Brewer's yeast cells are freely dispersed in the early stage of logarithmic growth, but as this continues they tend to adhere to each other, forming clumps or flocs. The tendency to flocculate varies with different strains and is, to some extent a t least, under genetic control (Gilliland, 1951 ;Thorne, 1951). Cells which have become flocculent may be reversibly separated by a variety of treatments, even washing in saline will suffice. But such redispersed cells differ from the disperse cells of the early logarithmic phase, in that when the former are transferred to a suitable medium containing calcium, they will flocculate a t once, whereas the latter will not. Clearly the two types of cells must have structural differences. Those cells which have been reversibly separated by simple changes of environment may be called potentially flocculent, in distinction from those which are dispersed as a consequence of their structure, and which may truly be called non-flocculent.Three distinct areas of study may be recognized in approaching the phenomenon of flocculation and it is necessary that information obtained in one field should not be confused with that from another: (i), there is the inherited nature of the cell; (ii) there is the nature of the change from non-flocculence to potential flocculence, together with the stimulus which provokes this change; (iii) there is the nature of the interactions of the cells with each other and with the environment, which manifest potential flocculation in the actual formation of flocs.The present paper is concerned with the second of these fields of study, more especially with the nature of the stimulus which leads to a change to potential flocculence. It will be shown that this change is connected with a specialized part of the nitrogen metabolism of the organism.
pigeon-liver homogenates stimulated the aerobic and anaerobic rates of reduction of oxidized glutathione to the same degree. At concentrations of oxidized triphosphopyridine nucleotide above 0-11 mm the anaerobic reduction of oxidized glutathione was lower than the aerobic reduction. 4. Citrate, fumarate and glucose 6-phosphate stimulated oxidized glutathione reduction in pigeon-liver homogenates. The addition of oxidized triphosphopyridine nucleotide (0.04 mM) further stimulated the reduction.5. In pigeon-liver homogenates incubated under 5 %carbon dioxide, 5nIm-pyruvate inhibited the anaerobic reduction of oxidized glutathione by about 50% but had no effect aerobically. Higher concentrations (25 mM) of pyruvate inhibited both the aerobic and anaerobic reduction of oxidized glutathione by about 70 %. Added fumarate and oxidized triphosphopyridine nucleotide both partially counteracted the inhibition of oxidized glutathione reduction due to pyruvate. Fumarate was more effective in aerobic conditions. 6. The significance of these findings is discussed in relation to the mechanism of maintenance of glutathione in the reduced form in intact tissue.
obtained with an inoculum size which does not give maximum mycelial dry weight. The pectic enzymes show a marked increase during the third day of growth, and this point is marked also by the virtually complete utilization of the sucrose originally present in the culture and by a discontinuity in the curve of mycelial dry weight. SUMMARY 1. A method is described for the preparation of a mixture of pectic enzymes with cultures of A8pergillu8 niger. 2. The cultures were produced in stirred fermenters with a medium consisting of 005 % of Na%S04, 02 % of NH4NO,, 2 % of sucrose and 2 % of pectin in a boiled extract of groundnut flour. Fermentation was continued for 5-6 days at 30°. The pH of the culture remained between 3 and 4. 3. The preparations induced virtually complete hydrolysis of pectin. 4. Enzymic preparations obtained from the culture fluid by precipitation with ethanol were more active, on a weight for weight basis, than commercially available materials. We wish to acknowledge the award of research fellowships by the Istituto Superiore di Sanit&. We are grateful to the Direction and staff of the pilot plant of the Institute for the provision of facilities and technical assistance. We would especially like to recognize the encouraging guidance and help of Professor E. B. Chain, F.R.S.
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