444 TRANSACTIONS OF SECTION B. 
The terpenes and essential oils are mostly C,, derivatives; the alkaloids 
have complex formule 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 proportion of the 
changes which occur in living organisms—which constitute vital meta- 
bolism—are directed changes. What is the nature of the directive 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 pre- 
cisely 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 substance 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 alteration 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 methylglucosides, which differ merely in 
the manner displayed by the following formule:—  . 
H OH HS OE 
| | | Le? 
C—_C C—=¢ 
I 
A On ase ee 4 OR: A \ tate 
CH,0.CH 1 aie —C—OH HC.OCH, HC-——C—C—OH 
\ ! \ / 
tw OH H \ Jo OR 
ve NA 
Sieg / 
‘O O 
a—-Methylglucoside: B-Methylglucoside. 
The relative positions of the simple hydrogen atom and of the hydroxyl 
group attached to the carbon atom are merely interchanged, yet this is 
sufficient to render the one (the a) proof against the action of emulsin— 
the enzyme of the almond—the other (the,8) proof against that of maltase 
—the enzyme present in yeast. 
The enzyme may be pictured as attaching itself to a surface of the mole- 
cule 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, 
In the case of the albuminoids, the action is probably more local in 
character, in so far as the resolution of their polypeptide section is con- 
cerned, 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; 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 
