1 J. Amer. Chem. Soc, 1913,35, 1617-1G23. 2 Biochem. Zeitsch., 1917, 80, 211.

Cytase (cellulase) is another enzyme found in germinating barley. It attacks the cellulose walls of the cells which enclose the starch, softening or dissolving them, and thus allowing the diastase to obtain access to the starch and exert its solvent action upon the latter.

Sucrase (invertase) is an enzyme present in yeast, and also in malt. It hydrolyses cane-sugar, converting it into "invert-sugar," a mixture of dextrose and lævulose. The optimum temperature for this action is 55-60°, and the enzyme is rendered inactive at 75°. It can be obtained from yeast by drying the latter at 105° and extracting it with cold water, the solution being then precipitated by adding an excess of alcohol. The dried enzyme may be heated for some time at 140-150° without destroying its properties.

Maltase is another enzyme which occurs in malt, and also in many yeasts. Croft Hill obtained it from ordinary brewery yeast by extracting the dried material with a dilute solution of sodium hydroxide and toluene.1 It attacks maltose, transforming it into the simpler sugar dextrose, the optimum temperature for this effect being 40°. At about 50-55°, it is rendered inactive. Maltase is not very soluble in water. It exerts its maximum activity in faintly acid solutions, but is very sensitive towards alkalinity or excess of acid.

Zymase is the enzyme, secreted in the yeast-cells,which directly produces alcohol from dextrose and from lævulose. It will be seen that from the point of view of alcohol production the action of all the foregoing enzymes - cytase, diastase, maltase, and sucrase - is, as it were, a preparatory action, bringing the material into a form in which it can be attacked by the zymase. Cytase facilitates the action of diastase upon starch up to a certain point; diastase transforms starch into maltose, and maltase converts this into dextrose; sucrase changes cane-sugar into dextrose and lævulose; and it is the two final products, dextrose and lævulose, which are attacked by zymase and converted into alcohol. Zymase does not itself transform either starch or cane-sugar directly.

The enzyme is contained in the liquid contents of the yeast-cells, but it cannot pass through the cell wall, and hence can only be obtained by rupturing the cells. A method of doing this was published by E. Buchner in 1897,1 and it marks a great step forward in our knowledge of the fermentation processes. In separating the cell-sap or "yeast juice," fresh brewing-yeast, freed from adherent water by pressure, was ground up with quartz sand and kieselguhr in order to rupture the cells. The resulting plastic mass was then mixed with water and squeezed through cloth in a hydraulic press, at a pressure of 400 to 500 atmospheres. The juice thus expressed was a slightly opalescent, yellow liquid, which when mixed with solutions of dextrose, grape-sugar, cane-sugar, or maltose rapidly fermented them, although no organism visible under a high magnifying power was present, nor could such be detected even when the solutions had been fermenting for some days. Furthermore, the fermenting power was still retained after the juice had been filtered through a Berkefeld filter, which would retain any organisms of the size of yeast-cells; and also after being dialysed through parchment-paper. Thus the fermentation was not due directly to the action of yeast-cells. Moreover, the yeast juice could induce the fermentation even when powerful protoplasmic poisons such as chloroform and sodium arsenite were present, so that it was not a case of the fermentative activity being due to particles of living protoplasm. In fact, the fermentative power was shown to be due, not directly to any vital process, but to a chemical or enzyme action of some constituent of the yeast juice, and this enzyme Buchner termed " zymase."

1 Trans. Chem. Soc, 1898, 73, 634.

Harden and Young subsequently found that by submitting yeast juice to filtration through a Chamberland filter coated with a film of gelatin a residue was left upon the filter. But - a very remarkable fact - neither this residue by itself, nor the filtrate by itself, was capable of setting up alcoholic fermentation. When, however, the two portions were reunited, the mixture produced almost as active a fermentation as the original juice.2 The active constituent of the filtrate is designated the " co-enzyme "; the enzyme is contained in the colloidal residue left on the filter. There is evidence in favour of the view that the co-enzyme is probably acetaldehyde, though at present this cannot be stated with certainty.3

Zymase, or at least a solid preparation containing active zymase, may be obtained as an amorphous powder by precipitating yeast juice with alcohol, or with a mixture of alcohol and ether, or with acetone. One volume of juice, for example, is poured into ten volumes of acetone, the mixture rapidly centrifuged, and the liquid decanted; the precipitate is washed with acetone and with ether, and then dried over sulphuric acid. The resulting amorphous, white powder does not dissolve completely in water, but is nearly all soluble in a mixture of water and glycerol containing from 2 5 to 20 per cent, of the latter: the solution thus obtained is practically as active as the original juice in effecting alcoholic fermentation.

1 Ber., 1897, 30, 117; also vols. 31, 32, and 33.

2 A. Harden, " Alcoholic Fermentation," Chap. IV.

3 Harden, Biochem. J., 1917, 11, 64.

Zymase appears capable of withstanding a temperature of 100° for six hours at least, since dried yeast which has been thus treated, and so deprived of the power of growth and reproduction, is still able to ferment sugar and to furnish an active juice.

Endotryptase is a proteolytic enzyme which occurs in yeast Like zymase, it is formed within the cells, and cannot be extracted until the cell is ruptured; it is therefore found in yeast juice. It converts yeast albumin into simpler products - tyrosin, leucin, etc. - and is of interest because it is believed to have a digestive action upon the zymase of the fresh juice. This would explain the otherwise puzzling fact that the juice quickly loses its fermentative power - in the course of a day or more, according to the temperature at which it is kept.

Hexosephosphatase is the term applied by Harden to a hydrolytic enzyme which is considered to be present in yeast. Its role in fermentation is to decompose hexosephosphate into the hexose sugar and free phosphate.1

Lactase, an enzyme which hydrolyses milk-sugar, occurs in kephir ferment and in a few culture yeasts, but not in ordinary yeast, which therefore is unable to ferment milk-sugar. It converts lactose into d-glucose and d-galactose.