importance. When a suitable quantity of a soluble phosphate best in the form of a solution of disodium phosphate saturated with carbon dioxide is added to a slowly fermenting mixture of the juice with glucose, a rapid rise is observed in the rate of fer- mentation, as measured by the amount of carbon dioxide evolved. As change proceeds, the amount of free phosphate in solution diminishes up to the point at which the rate of change begins to diminish; the diminution has been traced to the formation of a phosphoric glucoside. Apparently, action takes place as ex- pressed in the equation:
2C 6 Hi 2 O 6 +2PO 4 HR2=2CO2+2C 2 H 6 O-|-2H 2 O-|-C6H 1 o0 4 (PO,R 2 )2; that is to say, while one molecule of sugar is fermented a second is fixed as phosphate. Apparently, however, all the sugar passes through the phosphate stage on its way to fermentation; as this slackens and finally ceases the amount of free phosphate in solu- tion steadily rises, the action being reversed the while:
The formation and destruction of the phosphate are changes due to the action of an enzyme, hexosephosphatase.
The point of importance to be noted is, that whatever sugar be fermented glucose, mannose or fructose the hydrolytic product is fructose: one function, at least, of the hexosephospha- tase would seem to be the presentation of the sugar to the resolv- ing mechanism in the form most sensitive to rearrangement.
The resolving mechanism has several components. It contains one or more enzymes easily destroyed by heat, together with a so-called co-enzyme which survives when the liquid is boiled. These may be separated by mere nitration, under pressure, through a film of gelatin supported in a Chamberland filter- candle: neither residue nor filtrate alone will condition fermenta- tion, but when they are reunited a mixture is obtained which is almost as active as the original fluid. Little, if any, light has been thrown on the nature of the resistant constituent: the most sug- gestive observation made is that it disappears from boiled yeast juice when this is digested with castor-oil lipase, an enzyme which hydrolyses fats and similar substances.
As to the course of change at some stage apparently the hexose molecule is resolved into two " halves,"- but whether before or after rearrangement is uncertain. There is, however, reason to suppose that the production of alcohol involves the prior pro- duction of aldehyde and the ultimate reduction of this latter. The formation of aldehyde is attributed to that of pyruvic acid, CH 3 CO.CO 2 H, which is resolved into carbonic acid and alde- hyde by the action of carboxylase, an enzyme normally present in yeast:
CH 3 CO.CO 2 H+OH 2 = CH 8 .COH+CO 8 H 2 .
Not only has this acid been obtained as a product of fermentation, but when fermentation is effected in presence of an excess of alkaline sulphite an amount of aldehyde is produced approach- ing that to be expected on these assumptions, if one half the molecule were so affected; at the same time, glycerol is produced in almost corresponding amount.
It seems probable, therefore, that in the ordinary fermentation process the hexose is normally resolved into a mixture of glyceral- dose and glyceroketose, which became rearranged into pyruvic aldehyde, by enolisation and rehydration. The oxidation of these two molecules of aldehyde to pyruvic acid might then conceiv- ably be the consequence of the reduction of two molecules of ordinary aldehyde to alcohol the reduction of these must in some way be accounted for, if acetaldehyde be an intermediate product of fermentation. As a matter of fact, the function of an ordinary hydrolytic enzyme is nearly of this order, involving as it does either the separate presentation of the H and OH of water at two contiguous regions in a molecule or their withdrawal from two contiguous molecules, according as its action is either hydro- lytic or synthetic. A directed interaction of the character con- templated is therefore not improbable. Not only is yeast known to contain the enzyme carboxylase which fits pyruvic acid, but also another enzyme, glyoxalase, by which pyruvic aldehyde is converted into lactic acid, an operation involving (i) hydration, (2) enolisation, (3) reversed rehydration, starting from the
aldehydrol CH 3 .CO.CH(OH) 2 : CH 3 .C(OH) 2 .CH(OH) 2 CH 3 .C(OH) = C(OH) 2 CH 3 .CH(OH).CH(OH) 2 .
That the yeast complex may do all the things suggested is, therefore, by no means improbable. Glyoxalase, it may be added, occurs in various animal tissues, and the lactic acid formed as the result of muscular action may well be produced under its di- rective action. A striking observation made with yeast juice is that the action stops on adding hydrogen cyanide but re- commences when this is removed. Yeast ceases to decompose hy- drogen peroxide when the cyanide is added. Maybe, in both cases either an oxidase or a peroxydase is held in check which is effective in the pyruvic change.
A discovery of great significance, as throwing light on the re- ductive stage, is that recently made by Gowland Hopkins, of a minute constituent of yeast juice, liver substance and muscular tissue, glutathione, a neutral derivative (dipeptide) formed by the condensation of the two amino-acids, cysteine and glutaminic acid. It is a powerful reducing agent and acts as a carrier of hydrogen; cysteine is a sulphur derivative of alanine and is read- ily converted into cystin, by oxidizing agents; moreover, the change is reversible.
,SH
o
-2H +
H.CH..SH
Cysteine Cystin
Glutathione apparently is but cysteine weighted by glutaminic acid, and its activity is doubtless the consequence of a similar change. Possibly the hitherto unidentified co-enzyme of yeast juice may prove to be this substance.
General Synthetic Activity. That the plant exercises its syn- thetic activity with the aid merely of the simple cleavage prod- ucts derived from carbohydrate material, by processes similar to those involved in alcoholic fermentation, is clear. The ad- juncts are merely atmospheric oxygen and various materials ob- tained from the soil especially ammonia phosphoric acid, mag- nesium and silicon; these are all of structural significance; in ad- dition, iron and manganese, calcium and potassium, appear also to be indispensable, but are mainly, if not entirely, of value as functional agents. Although it is established that potassium is essential to the formation of starch, if not of other carbohydrates, no clue has yet been found to the office it exercises. Sodium, be- ing there, is taken into the plant; whether it be in any way neces- sary, as it is to the animal, we do not know.
Whilst many compounds are undoubtedly formed under en- zymic influences, others are products of the direct spontaneous interaction of materials which happen to meet. The precise manner in which even the simple benzene derivatives met with in plants are formed is not yet clear. That even substances so complex as the opium and other alkaloids may be formed, without difficulty, is shown by R. Robinson's remarkable obser- vation that tropinone, a compound closely related to one group of these alkaloids, is produced when the aldehyde of succinic acid, methylamine and acetone, or still better its dicarboxylic acid, are merely brought together, in aqueous solution, at the ordinary temperature:
CH,-CH-CH,
N.CH,c!o
CH,.COH I CHi.COH
CO CH 3
CHj.NHa
CHi-CH
CH,
Succinic Acetone Methylamine Tropinone aldehyde
Plant Colours. Considerable diversity in character may be the outcome of small differences in chemical structure: this is weD illustrated in the colouring matters of flowers which, it is well known, vary over a considerable range. The yellows, however, appear all to be derivatives of a simple compound, not itself coloured, flavone, which occurs as a mealy deposit on the leaves and flower stalks of a large number of Primulaceae. It is resolved,
