CHAP. I.] CIRCUMSTANCES AFFECTING ENZYMES. 9 



It appears almost certain that amongst catalytic phenomena, employing 

 the term in the sense in which J. R. Mayer employed it, ferment pheno- 

 mena resemble those in which there is apparently a periodic synthesis and 

 dissociation of the catalysing agent, which acts in a similar manner to the 

 agent which explodes a train of gunpowder. Amongst chemical phenomena 

 in which one body acts apparently as a go-between, and leads to an almost 

 indefinite series of exchanges of matter and energy, two at once suggest 

 themselves to the mind of the thinker as affording a clue to the probable 

 action of ferments, to wit: (1) the function of haemoglobin as an oxygen 

 carrier, as a go-between the atmospheric oxygen, on 'the one hand, and 

 the organic molecules which are oxidised by it in the organism, on the 

 other, and (2) the function of sulphovinic acid in the process of setheri- 

 fication. In the latter case, however, the results of the process are the 

 reverse of those which follow the normal action of ferments, in so far that 

 whilst the latter as their primary function lead to the decomposition of 

 complex into simpler compounds, the action of sulphovinic acid leads to the 

 synthesis of a more complex out of simpler molecules, to a conversion of 

 kinetic into potential energy. 



Enzymes The principal enzymes of the alimentary canal 



differ in their belong either to the group of ' proteolytic,' or to that of 

 actions. < amylolytic ' ferments. The enzymes of the first group 



(pepsin and trypsin), dissolve proteids and effect their more or less 

 profound decomposition. The enzymes of the second class (as the so- 

 called ' ptyalin,' the diastatic enzyme of the salivary glands, and the 

 powerful diastatic enzyme of the pancreas) liquify boiled, gelatinous 

 starch, breaking down the complex starch molecule into molecules of 

 greater and greater simplicity, of which the final representatives are 

 a dextrin, maltose and dextrose. 



In addition to the two groups of ferments referred to above, there 

 occur in the alimentary canal ' curdling/ ' inverting,' and ' fat-decom- 

 posing ' or ' piolytic ' ferments. These enzymes will be considered 

 in detail in the sequel. 



Circum- ^ Temperature. All enzymes exert a more ener- 



stances which getic action at a moderately high than at a low tempe- 

 influence the rature, though the influence of a rise in temperature is 



more marked in some cases than in others, ii. Reaction. 



The reaction of the medium in which they are placed, 

 influences remarkably the activity of certain enzymes ; thus the 

 proteolytic enzyme of the stomach, pepsin, is inactive in neutral or 

 alkaline solutions, the presence of a free acid being essential to its 

 activity ; whilst the proteolytic ferment of the pancreas, trypsin, acts 

 with feebleness in solutions which are neutral or feebly acid and needs 

 a decidedly alkaline medium for the full exercise of its powers, iii. 

 Presence or absence of excess of certain salts. The influence exerted 

 by salts upon certain reactions induced by ferments is illustrated, 

 (a) by the impossibility of inducing the curdling of casein in the 

 absence of calcium salts. (6) by the hindering action exerted by 

 certain neutral salts on the coagulation of the blood, and of certain 



