56 ENZYMES OR SOLUBLE FERMENTS. 



SPECIAL DESCRIPTION OF THE MORE IMPORTANT ENZYMES *. 

 Ptyalin. 



While occurring chiefly and characteristically in saliva, a similar 

 enzyme may be obtained in minute amount, but fairly constantly from 

 almost any tissue or fluid of the body, more particularly in the case of 

 the pig. It was first separated out from saliva, but in an impure 

 condition, by Mialhe, who precipitated the saliva with an excess of 

 absolute alcohol 2 . It has been prepared in the purest (?) form by 

 Cohnheim 3 . His method consists in the addition of phosphoric acid to 

 the saliva until it is strongly acid ; the mixture is then neutralised by 

 the careful addition of lime-water, whereupon a copious precipitate 

 of phosphate of lime is formed. This carries down with it a large 

 proportion of the proteids which are present together with all the 

 ptyalin. On extraction of the precipitate with a volume of water 

 equal to that of the saliva originally employed, the enzyme passes 

 chiefly into solution since it is less firmly adherent to the precipitate 

 than are the proteids ; it may now be purified still further by repeat- 

 ing the above process and finally precipitating with absolute alcohol. 

 Prepared in this way, the enzyme is obtained as a fine white amorphous 

 powder. Dissolved in water it is extremely active in hydrolysing 

 starch and the solution yields none of the reactions most typically 

 characteristic of proteids. On these grounds it is asserted that ptyalin 

 is not a proteid, but the evidence is not conclusive. More recently this 

 enzyme has been prepared as follows 4 . Saliva is diluted with an equal 

 volume of water and saturated with neutral ammonium sulphate. The 

 precipitate thus formed is treated on the filter for five minutes with 

 strong alcohol, removed from the filter and further treated with absolute 

 alcohol for one or two days. It is now dried at 30 and yields, on 

 extraction with a volume of water equal to that of the original saliva, 

 a solution which is actively zymolytic and is stated to be free from 

 all proteid reactions. The hydrolytic activity of ptyalin is most 

 marked in neutral or nearly neutral solutions 5 . 



An amylolytic enzyme is found in urine 6 . 



No experiments have as yet established the existence of any 

 zymogen of ptyalin (ptyalinogen) 7 . 



1 Consult the article ' Fermente ' by Emmerling in Ladenburg's Handworterluch 

 d. Chem. Bd. iv. 1887, S. 95. 



2 Compt. Rend. T. xx. (1845), pp. 954, 1485. 



3 Virchow's Arch. Bd. xxvni. (1863), S. 241. 



4 Krawkow, loc. cit. 



5 Langley and Eves, Jl. of Physiol. Vol. iv. (1882), p. 18. 



6 For litt. see ref. 1, sub Pepsin, p. 61. 



7 Langley, Jl. of Physiol. Vol. in. (1881), p. 288. 



