806 



7. MERCURIALS 



mercurial may alter the rate and extent of reaction with the SH groups. 

 The titrations of yeast alcohol dehydrogenase by p-MB in phosphate and 

 in THAM buffers at pH 7.5 are quite different (Hoch and Vallee, 1960). 

 It is probably advisable to reduce the buffer concentration as far as possible. 

 (7) The number of reactive SH groups on an enzyme and the titration of 

 these groups vary with several experimental conditions, such as pH, temper- 

 ature, and absence or presence of substrate, and the question often arises 



100 



80 



60 



40 



S 20 



-20 



SENSITIVrTY OF 



ACTOMYOSIN TO 



ATP 



8 , 6 



SH (M/IO G MYOSIN) 



Fig. 7-22. Titration of myosin with mersalyl, showing 

 the effects on ATPase activity, the ability to form 

 actomyosin, and the sensitivity of the actomyosin 

 to ATP (measured by viscosity changes). (From 

 Barany, 1959.) 



as to what conditions are optimal. Titrations are often done at unphysi- 

 ological pH's because reaction is faster or more complete, but it must be 

 remembered that the results do not necessarily apply to the enzyme under 

 normal conditions. Boyer and Segal (1954) showed definite difference in 

 the titration of 3-phosphoglyceraldehyde dehydrogenase spectrophotome- 

 trically at pH 4.6 and 7, and this is probably a general phenomenon. The 

 effect of temperature is weU illustrated by the study of yeast hexokinase, 



