164 PHYSIOLOGY OF NUTRITION 



velocity of the decomposition of hydrogen peroxide by alkalies is distinctly in- 

 creased by a very small amount of platinum or other metals. In both cases 

 platinum plays the part of the inorganic catalyzer. 1 The catalytic activity, of 

 both organic and inorganic catalyzers, depends upon the amount of catalyzer 

 present, upon the temperature and upon the properties of the surrounding 

 medium. Chemical reactions can not only be accelerated by foreign substances, 

 but they can also be retarded. As an instance of this, the catalytic effect of 

 platinum upon the decomposition of hydrogen peroxide by alkalies is greatly 

 reduced by the presence of a trace of hydrocyanic acid, arsenic acid, hydrogen 

 sulphide or other poisons. 



Diastase is the most widely distributed of the plant enzymes. It causes the 

 transformation of starch into glucose. A very slight amount of the enzyme is 

 able to hydrolyze large amounts of starch; one part by weight of the powder 

 called diastase decomposed 2000 parts of starch. 



Diastase, according to investigations carried out by Baranetskii, 2 is very 

 widely distributed in plants. It is found in especially large amounts during 

 the germination of starchy seeds. The approximate isolation of diastase is best 

 effected from barley malt. The malt is first digested with water, the extract 

 is then filtered, and the enzyme is finally precipitated in the filtrate by the addi- 

 tion of alcohol. The white precipitate obtained in this way is purified by being 

 repeatedly dissolved in water and reprecipitated with alcohol. The precipitate 

 from alcohol is soluble in water, and when thus dissolved, possesses the ability 

 to hydrolyze starch. The chemical composition of diastase appears to be very 

 similar to that of the proteins. 



The first stage of starch hydrolysis is indicated by the fact that addition of 

 iodine to the mixture fails to give the usual blue color of starch with this reagent. 

 In a somewhat early stage of the process, the color produced by addition of 

 iodine is violet, but at a later stage of the hydrolysis a brown color is produced. 

 Finally, there is no color change at all with addition of iodine. The reaction is 

 hastened by higher temperature, up to 45 or 5o°C. The decomposition of undis- 

 solved starch (starch grains) occurs only in the presence of acids (hydrochloric, 

 formic, acetic, and citric acid), formic acid being especially active, according to 

 Baranetskii's results. The action of diastase on starch grains in vitro, shows the 

 same peculiarities as appear when the starch grains are dissolved in the seed dur- 

 ing germination. The diastase attacks only portions of each grain first; these 

 portions becoming transparent, glassy and giving no color with iodine; the whole 

 starch grain becomes transparent at length, showing only a sort of framework, 

 and finally even this is dissolved. Much information is now available upon the 

 formation and distribution of diastase in germinating barley. The following 

 table shows the relative amounts and distribution of diastase in Hordeum 

 (barley) seedlings four days old. 3 



' Bredig, Georg, Anorganische Fermente. Leipzig, 1901. Idem, 1902. [See note 1, p. xxx.] 

 - Baranetzky, J., Die Starkeumbildenden Fermente in den Pflanzen. Leipzig, 1878. 

 : Moritz., E. R., and Morris, G. H., Handbuch der Brauwissenschaft (German transl. by Windisch). 

 P. 142. Berlin, 1893.* [Idem, Textbook of the science of brewing. London, 1891.] 



