PEPTIDASE 417 



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range of 10-12 mm. with one complete revolution giving a movement 

 of 1 mm. Since the circumference is divided into 100 parts, measure- 

 ments may be made with an accm'acy of 0.01-0.02 mm. The appara- 

 tus is mounted on a triangular base fitted with leveling screws. 



The position of a drop with reference to a standard drop is deter- 

 mined by turning the micrometer screw to zero, setting the reference 

 mark in the microscope ocular (cross hair or scale division) at the 

 lower edge of the standard drop by means of rack C and clamp E, 

 moving the microscope horizontally so that it will be in line with the 

 drop to be observed, and bringing the reference mark to the lower 

 edge of the latter drop by adjustment of the micrometer screw. 



Measurement of Reaction Rate. When the gradient tube con- 

 tains a series of standard drops embracing the range of specific 

 gravity within which the reaction mixture falls, measurements of 

 reaction rate may be carried out by determination of positions at 

 suitable intervals. Readings are begun after 15-20 min. from the time 

 the drop of reaction mixture is introduced into the tube. It has been 

 shown that this is an adequate time to allow the drop to assume its 

 proper position for observation. A plot of the densities of the drop as 

 a function of time represents the course of the reaction. 



PEPTIDASE 



The use of the gradient tube for dilatometric determination of 

 peptidase activity was described by Linderstr0m-Lang and Lanz 

 (1938). They employed DL-alanylglycine as substrate, of which only 

 the D form is hydrolyzed enzymatically by pig stomach and intestinal 

 extracts, which were used as the source of the enzyme. The change 

 of density with time was found to be a linear function. The contrac- 

 tion constant for the scission of the peptide bond was found to vary 

 with both pH and the concentration of the phosphate buffer em- 

 ployed. The magnitude of this variation, as determined by both 

 direct macro dilatometry and the micro method, is shown in 

 Table IX. These measurements were made at 30° on reaction mix- 

 tures consisting of equal volumes of substrate and enzyme solutions. 

 The substrate solutions had the composition given in Table X; the 

 pH values were calculated on the assumption that the pK value for 

 DL-alanylglycine at 30° is 8.09. The enzyme solutions were prepared 



