INHIBITION OF ENZYMES 789 



loss of activity during the titration. These results indicate that the SH 

 groups are not directly involved in the catalysis, but function to stabilize 

 the enzyme in the active configuration. 



Another type of structural change is depolymerization of the enzynae 

 into subunits following mercurialization. Muscle phosphorylase is progres- 

 sively inhibited by p-MB until around 18 equivalents of the mercurial are 

 combined, and this is accompanied by the appearance of a new molecular 

 species in the ultracentrifuge, the sedimentation constant being lower than 

 that for either phosphorylase a or b (Madsen and Cori, 1955): 



Phosphorylase a: S = 13.2 



Phosphorylase b: S = 8.2 



Inactive enzyme: S = 5.6 



Both phosphorylase a and b form this new species with p-MB and it was 

 suggested that the former is split into 4 subunits, the latter into 2 subunits. 

 Light scattering studies are consistent with this interpretation (Madsen, 

 1956). The inhibition develops more rapidly than the depolymerization, 

 however, so the relationship between them is not clear. Removal of the 

 p-MB with cysteine restores both activity and the normal dimer or tetra- 

 mer (Madsen and Cori, 1956). The extent of the conversion of the phos- 

 phorylase tetramer to the monomer is proportional to the number of SH 

 groups reacted and an all-or-none dissociation of the units is likely (Madsen 

 and Gurd, 1956). The sedimentation constant of yeast alcohol dehydroge- 

 nase is reduced from 7.2 to 3.3 by p-MB, this being secondary to the inhi- 

 bition of the enzyme, so that here dissociation into subunits apparently 

 occurs (Snodgrass et al., 1960). Reaction of myosin ATPase with MM also 

 causes the appearance of a small subunit, but this is not related to the 

 binding to the SH groups responsible for the activity (Kominz, 1961). In 

 addition there is some aggregation to a faster sedimenting species and this 

 is perhaps correlated with reaction of SH groups at the active center. The 

 inhibition of rabbit muscle enolase by p-MB was considered to be secondary 

 to denaturation and not directly due to SH group reaction, on the basis of 

 the variation of activity with the equivalents of mercurial present and the 

 appearance of turbidity (Malmstrom, 1962). The sedimentation constant of 

 liver glutamate dehydrogenase is reduced by MM and PM and again the 

 most likely explanation is a splitting into subunits (Rogers et al., 1962, 

 1963; Greville and Mildvan, 1962). The relationship of the disaggregation 

 to the unique changes in enzyme activity is not clear. 



There is no doubt that mercurials can induce structural changes in cer- 

 tain enzymes, and cause aggregation or fractionation into subunits in others, 

 but the significance for primary inhibition has not been clarified. Is the 

 inhibition due to the blocking of functional SH groups or secondarily to 

 the structural changes? Does the denaturation result from general SH group 



