804 7. MERCURIALS 



Carbamyl-P synthetase — frog liver (Marshall et ah, 1961) 



Cytochrome c reductase — calf liver (P. Strittmatter, 1959) 



Malate dehydrogenase — beef heart (mitochondria) (Siegel and Englard, 1962). 



Type E 



Lactate dehydrogenase — pig muscle (Jecsai and Elodi, 1963) 

 Phosphoglucomutase — rabbit muscle (Milstein, 1961) 

 Phosphorylase — potato (Lee, 1960 b). 



No clear-cut example of type F has been reported, but presumably the 

 inhibition of D-amino acid oxidase by p-MB, which is competitive with 

 respect to the benzoate portion of the inhibitor, would fall into this category. 

 Certain enzymes have been shown to have a single SH group at the active 

 center and necessary for activity, namely, papain (Kimmel and Smith, 1957; 

 Finkle and Smith, 1958; Sanner and Pihl, 1963), ficin (Liener, 1961), and 

 glycerol-P dehydrogenase (van Eys et al., 1959). They seem generally to 

 belong to type A. 



A few selected titration curves may further illustrate the relations be- 

 tween SH reaction and inhibition. The titration of 3-phosphoglyceraldehyde 

 dehydrogenase with p-MPS has been discussed (Fig. 7-12) and is seen to 

 follow type A behavior (deviating toward type C), although the release of 

 NAD is not exactly parallel to the disappearance of SH groups. Yeast alcohol 

 dehydrogenase contains 10-12 SH groups per molecule but some inhibition 

 occurs when only one is reacted, although the curve (Fig. 7-21) shows the 

 inhibition at first to lag behind; it is difficult to know if this is type C or D. 

 ATPase presents a more complex situation (Fig. 7-22) since reaction of the 

 first 4 SH groups seems to produce only some stimulation of the activity, 

 reaction of the next 2 SH groups causing complete inhibition. Other prop- 

 erties of myosin, e.g., the ability to complex with actin and the viscosity 

 response of actomyosin to ATP, are more directly dependent on the reac- 

 tion of the first SH groups. ITPase activity conforms more to type A be- 

 havior. The relationship of ATPase activity to SH reaction depends on the 

 state of the enzyme (Fig. 7-23), no initial stimulation being observed when 

 EDTA is the activator instead of Ca++. The titration of muscle phosphor- 

 ylase a gives partial type C behavior, but over most of the range there is a 

 linear relationship between SH reaction and inhibition (Fig. 7-24). Since 

 the inhibition may be completely reversed by cysteine, it is unlikely that 

 a secondary inactivation is involved. Microsomal cytochrome c reductase 

 demonstrates typical type D behavior, one SH group being closely related 

 to the enzyme activity, as shown by the extrapolation of the inhibition 

 curve to complete inhibition (Fig. 7-25), from which it may be estimated 

 that K2IK1 is around 25-50. 



Accurate, reliable, and directly interpretable titrations of enzymes are 

 not easy to perform in some cases. Some of the possible difficulties which 

 may arise will be summarized. (1) There is a failure to reach equilibrium, 



