INHIBITION OF ENZYMES 819 



the substrate. Reaction of a mercurial with a vicinal SH group could by 

 introducing a new side chain break down this water structure. 



(G) The mercurial reduces the binding of an inhibitory 'product. L-Gluta- 

 mate dehydrogenase is stimulated by PM at pH 8.5 and this is reduced by 

 substrate, NAD, and glutarate, a competitive inhibitor (Greville and Mild- 

 van, 1962). Thus PM must combine at or near the active center. The prod- 

 uct of the reaction, a-ketoglutarate, is inhibitory. PM increases the K^ for 

 a-ketoglutarate 8-fold and for glutarate more than 20-fold. Part of the stim- 

 ulation by PM can be due to reduction of the effects of a-ketoglutarate. 

 It would seem that such behavior would be reflected in the rate curves, 

 little stimulation being expected initially. 



(H) The mercurial dissociates the e^izyme into active subunits. Some of the 

 active centers may be more accessible when the enzyme is disaggregated, 

 and it is known that mercurials can sometimes split enzymes into subunits 

 (page 788). GreviUe and Mildvan (1962) observed that PM dissociates glu- 

 tamate dehydrogenase, and Rogers et al. (1962) also noted effects on the 

 sedimentation properties. The possibility of such dissociation playing a role 

 in the mercurial activation was studied by Rogers et al. (1963), who found 

 no change in molecular weight upon treatment with MM when the enzyme 

 is in high concentration. However, when low enzyme concentrations were 

 used, a disaggregation sensitive to the mercurial was detected, but it is not 

 certain if this is related directly to the stimulation. 



(I) The mercurial reacts primarily with an inhibitory SH groups. The stim- 

 ulation of ATPase by mercurials has generally been explained since the 

 report of Kielley and Bradley (1956) in terms of differently located SH 

 groups around the active center. An SH group, for example, might bind a 

 group on ATP and interfere with the optimal orientation on the enzyme. 

 This SH group has been postulated to react with the 6-amino group of ATP; 

 when mercaptide formation occurs, this discouraging action on ATP is abol- 

 ished (Gilmour, 1960; Greville and Tapley, 1960). In essence, the mercurial 

 prevents the excessive occupancy of the active center by disoriented ATP, 

 allowing ATP to proceed directly to hydrolysis. A somewhat different view 

 has been voiced by Blum (1960): ATP induces a configurational change in 

 the active center, this involving the SH groups, and mercurials at low con- 

 centrations tend to prevent this change. ITP does not so alter the structure 

 and its hydrolysis is inhibited only by mercurials. Mercurials would thus 

 maintain the active center in the configuration binding ITP, a state con- 

 ducive to rapid hydrolysis of ATP. 



(J) The mercurial inhibits a second enzyme which suppresses the reaction 

 rate. A number of possibilities for stimulation were discussed in Chapter 

 1-7. In a monolinear chain: 



E, Es 



