288 



NA TURE 



[September 2, 1909 



of compounds. Moreover, not only the sugars and most 

 of the other products of the disintegration of the albu- 

 minoids but also the amino-acids, in like manner, are 

 derivatives of compounds containing at most six atoms of 

 carbon ; the fats alone are of a considerably higher degree 

 of complexity but they are probably collocations of the 

 simpler units. 



The terpenes and essential oils are mostly C,„ deriva- 

 tives ; the alkaloids have complex formulae but the units 

 of which they are composed are simple ; as all of them 

 are optically active, it is clear that only some of the 

 possible enantiomorphous combinations are present. 



We are bound, therefore, to assume that a large pro- 

 portion of the changes which occur in living organisms — 

 which constitute vital metabolism — are directed changes. 

 What is the nature of the diVective power? We are already 

 able to go far in explaining this, although our knowledge 

 is mainly of analytical changes, the nature of synthetic 

 changes being, at present, only inferentially disclosed to us. 



It has long been known that under natural conditions 

 many complex compounds such as starch, cane sugar, and 

 other similar substances are broken down hydrolytically, 

 not by the unassisted action of water but by the co- 

 operation of enzymes ; the effect produced by these enzymes 

 is precisely similar to that of acids, except that all acids 

 produce the effect, acting only with different degrees of 

 readiness, whilst enzymes are strictly selective, a given 

 enzyme acting only, as a rule, either on a single sub- 

 stance or on a series of substances similar in structure. 

 Indisputable evidence has been obtained that the enzymes 

 which act on the carbohydrates are intimately related in 

 structure to the compounds which they attack, fitting 

 them — to use the apt simile introduced by E. Fischer — 

 much as a key fits into a good lock : the slightest altera- 

 tion in the structure of the carbohydrate is sufficient to 

 throw the enzyme out of action. The closeness of the 

 association is well illustrated by the case of the two methyl- 

 glucosides, which differ merely in the manner displa\'ed 

 bv the following formulae : — 



H 



I 



OH 



CH^O.CH 



OH 



I 



H 



HC- 



H H 



I I 

 -C— C— OH 

 I I 

 OH H 



o-Methylglucoside. 

 H OH 



IICOCH 



H H 



-OH 



OH H 



/3-Methylglucoside. 



The relative positions of the single hydrogen atom and 

 of the hydroxj'l group attached to the carbon atom are 

 merely interchanged, but this is sufficient to render the 

 one (the a) proof against the action of emulsin, the enzyme 

 of the almond ; the other (the ff) proof against that of 

 maltase, the enzyme present in yeast. 



The enzyme may be pictured as attaching itself to a 

 surface of the molecule, and at the same time as associated 

 with hydrone in such a manner that this is brought to 

 bear at the junction which undergoes disruption. The 

 action of acids, although similar, is simpler in that the 

 attachment is not to the molecule as a whole but only 

 at, or near to, the junction which is resolved. 



NO. 2079, VOL. Si] 



In the case of the albuminoids, the action is probably 

 more local in character, in so far as the resolution of their 

 polypeptide <-°ction is concerned, the same enzyme being 

 able to effect the resolution of a considerable number of 

 compounds. 



All the peptolytes have in common the junction C.CO.N ; 

 the peptoclasts by which such substances are gradually 

 resolved probably fit this group alone ; but other enzymes 

 are of more complex organisation, akin to that of the 

 sucroclasts — such as arginase, for example. In principle, 

 however, the enzymes are to be regarded as all acting 

 alike, each as fitting some particular asymmetric centre 

 if not the whole of the molecule which undergoes hydro- 

 lylic disruption under its influence, the asymmetric centre 

 being that at wdiich the cleavage is effected. 



In synthetic changes the operation is reversed. It may 

 be supposed that the separation of hydrone is determined 

 by the circumstance that water can be formed by the 

 interaction of this hydrone as it separates with that which 

 is attached to the hydrolated enzyme ; the formation of 

 water, in fact, plays a great part in such changes. ^ 



The action of oxydases may be regarded from a similar 

 point of view. The early observations of Adrian Brown 

 on the oxidising activity of Bactcriiiui Xyliniiin, coupled 

 with the later work of Bertrand, afford clear proof that 

 these enzyines are possessed of selective powers. It is 

 conceivable that such enzymes become attached to a mole- 

 cule at some one centre, and that they then deliver their 

 attack at some more or less distant point by presenting 

 the oxygen with which they are loosely associated at this 

 point. It is easy, on such an assumption, to understand 

 how ethenoid linkages may be developed in various posi- 

 tions in the molecule of a fatty acid. 



Rosenthaler's recent observations on the formation of 

 optically active phenyl hydroxycyano methane, 

 PhCH(OH)CN, 



from hydrogen cyanide and benzaldehyde on shaking the 

 solution with enuilsin are, however, lunong the most 

 significant yet made. I have myself confirmed his state- 

 ments. The ease with which the change takes place — the 

 manner in which the change is accelerated by the enzyme — 

 is altogether remarkable. 



Although there can be little doubt, in the case of plants 

 and animals, that the synthetic processes do not occur 

 spontaneously and directly between the inleragents, but 

 are for the most part at some stage or other directed or 

 controlled, it cannot well be supposed that every asym- 

 metric compound is the direct outcome of a controlled 

 process ; nor is it necessary to assume that such is the 

 case. Not a few asymmetric compounds are probably but 

 secondary products formed by the breakdown of com- 

 pounds which are the products of directed synthesis. 



Ehrlich's observations on the formation of the amylalcohols 

 from the isomeric aminocaproic acids (leucines) may be re- 

 ferred to in this connection. Taking into account the 

 maimer in which the vegetable organism is provided with 

 conservative powers and its tendency to retain nitrogen, 

 in view of the peculiar structure of the members of the 

 terpene group — especially the presence of the isopropyl 

 group and of methyl in association with the ring in such 

 hydrocarbons — it is highly probable that the terpenes are 

 derived from amino-acids. A molecule of leucine, a mole- 

 cule of alanine and a molecule of formaldehyde obviously 

 provide the materials for the production of methyliso- 

 propyldihydrobenzene ; it is not difficult to picture the 

 series of changes which would lead to the formation of 

 the hydrocarbon from such a conjunction. 



The general impression produced by facts such as have 

 been referred to is that directive influences are the para- 

 mount influences at work in building up living tissues. 

 These come into operation, it is to be supposed, at a very 

 early stage in the case of the plant. The initial step 

 probably involves the electrolysis of w^ater under the in- 

 fluence of solar energy and the reduction by the hydrogen 

 thus liberated of the carbon dioxide, which is eventually 

 converted into formaldehyde, either directly or, it may be, 

 through the intermediation of oxalic and formic acids. 

 The part which chlorophyll plays in this process can only 

 be surmised : it is not improbable that reduced chlorophyll 

 is the active reducing-agent : that chlorophyll itself is active 



