THE CONVERSION OF THE PRODUCTS OF ASSIMILATION. I 151 



Temperatur-C 



ra 



Fig. 30. Dependence of the activity of malt 

 diastase on temperature. After Kjeldahl 



be a body of quite unknown composition, not as yet isolated from the proteid. 

 Since very small quantities of diastase possess great hydrolytic powers, the 

 actual diastase itself may form merely a trifling impurity in the proteid 

 obtained by alcoholic precipitation from the malt extract. 



What has especially attracted the attention of physiologists to diastase is, 

 on the one hand, its behaviour at different temperatures, and on the other, its 

 action on certain substances. At 0° C. the dissolving action of diastase on 

 starch is scarcely noticeable ; an increase in temperature is followed by a rapid 

 increase in its activity, until at 50° C. it reaches a maximum maintained till 

 63° C. is reached ; if it be heated still further, the activity of diastase again de- 

 creases, until finally, at about 85° C, its power becomes destroyed (Kjeldahl, 

 1879). If we construct a graphic curve (Fig. 30) whose abscissae indicate 

 degrees in temperature and whose ordinates show the amount of starch dissolu- 

 tion effected by the diastase, it will be found that the curve bears a strong 

 likeness to those other curves which express the dependence of various functions 

 of the living plasma on temperature, such as we have still to study in growth 

 and movement, and which we have already seen in the case of carbon assimilation. 

 The maximum, minimum, and optimum points 

 of this curve given by different diastases 

 are not always coincident (Lintner and Eck- 

 HARDT, 1890). The diastatic curves differ 

 from other physiological curves chiefly in the 

 fact that the optimum point always stands 

 very high ; indeed, it stands so high that it is 

 never reached in the plant, since at 5o°-6o°C. 

 carbon assimilation is impossible and gene- 

 rally the limits of life itself are reached or ex- 

 ceeded before the optimum effect is obtained. 



In considering the influence on diastase ^' ^^ ' 

 of certain substances we will begin with those which tend to retard its activity, 

 and which act on diastase just as poisons do on protoplasm. According to Bokor- 

 NY (1901), formaldehyde is to be considered in this light, since, even in a con- 

 centration of o-oi per cent., it affects both protoplasm and diastase injuriously, 

 after it has operated for a certain time. Diastase, however, responds to the 

 majority of poisons in a different manner from protoplasm, viz. in being 

 much less sensitive to them. While the latter is destroyed by very minute 

 quantities of corrosive sublimate (0-00005 percent.) and silver nitrate (o.oooooi 

 per cent.), a concentration of o-oi per cent, in both cases is required to produce 

 a poisonous effect on diastase. We know also that by introducing certain 

 poisons in sufficient quantity it is possible to kill micro-organisms, and yet 

 the enzyme remains fully active. Since, however, micro-organisms, as we 

 shall see by and by, can greatly influence experiments with malt extract, their 

 exclusion becomes of very great significance. Thymol or chloroform is usually 

 employed for this purpose, not corrosive sublimate. 



In contrast to these inhibitory poisons, other substances are known which 

 act in the highest degree as accelerating agents. Generally speaking, all addi- 

 tions of free mineral acids act in this way, if they be present in traces only ; so 

 also do neutral salts (e. g. sodium chloride) in somewhat larger doses, and 

 finally, salts of aluminium, phosphoric acid compounds, and asparagin in suffi- 

 ciently high concentrations. Thus Effront (cited by Green, 1901) found 

 that in a certain time, the following unequal amounts of maltose were pro- 

 duced by a malt extract from starch paste : — 



1. Without any addition 



2. With addition of 0-5 % calcium phosphate 



3. „ „ 0-25 % ammonium alum 



4. ,, ,, 0.25 % aluminium acetate 



5. „ „ 0.05% asparagin . 



8-63 maltose 

 46.12 „ 

 5630 „ 

 62.40 ,. 



6l-20 „ 



