446 2. ANALOGS OF ENZYME REACTION COMPONENTS 



was found by Moos et al. (1960) to be unable to replace ATP in contracting 

 glycerinated muscle and not to be hydrolyzed by myosin ATPase. However, 

 some inhibition on ATPase is exerted (the affinities of the enzyme for ATP 

 and its analog appear to be roughly the same), although this is not com- 

 petitive. Mg++ is able to overcome the inhibition at a concentration lower 

 than that of the analog, indicating that Mg++ is not simply complexing 

 with and removing free analog. The mechanism of the inhibition was re- 

 presented by the following reactions: 



EMg 



where E is low-activity enzyme and EMg is high-activity enzyme. The inhi- 

 bition has a dual basis: (1) removal of Mg++, thus decreasing the fraction 

 of the enzyme in the high-activity form, and (2) reaction of the low-activity 

 enzyme directly with the analog. This situation may be fairly common in 

 inhibitions on enzymes with activating metal ions. 



A few other nucleotidase inhibitions may be mentioned. ITPase is inhib- 

 ited by IDP and ADP (Blum, 1955; Kielley and Kielley, 1953). Indeed, 

 ADP inhibits ITPase more strongly than ATPase. The ITPase of fly muscle 

 is strongly inhibited by ADP (K^ = 0.0165 mM) and much less readily by 

 IDP (Kj = 1.59 mM), the inhibition being competitive at low but noncom- 

 petitive at higher concentrations (Sacktor and Cochran, 1957). GTPase is 

 likewise inhibited but UTPase is unaffected by either ADP or IDP. In 

 phage-infected E. coli the hydrolysis of deoxycytidine diphosphate (deoxy- 

 CDP) is inhibited by deoxyCMP and deoxyCTP, and the hydrolysis of 

 deoxyCTP is inhibited by deoxyCMP and deoxyCDP, in both cases the 

 deoxyCMP being relatively less active (Zimmerman and Romberg, 1961). 



AMP- ATP transphosphorylase (myokinase) from rabbit muscle is inhi- 

 bited by ADP {K^ = 0.33 mM) and this is competitive with respect to 

 both AMP and ATP (Noda, 1958). The reverse reaction from 2ADP->AMP 

 -f ATP is inhibited by AMP {K, = 0.5 mM) and ATP (A', = 0.32 mM), 

 the K/s being the same as the K„^s for these substances (Callaghan and 

 Weber, 1959). A much more effective analog is adenosine monosulfate 

 {K^ = 0.0186 mM). Creatine kinase is inhibited competitively by ADP 

 (Ki = 0.27 mM), AMP {K^ = 7 mM), adenosine (Z, = 7 mM), tripoly- 

 phosphate {K, = 8 mM), orthophosphate (A, = 13 mM), sulfate (A, = 

 6 mM), and nitrate {K^ = 22 mM) (Noda et al, 1960). The substrate here 

 is MgATP= and it is possible that the most effective inhibitors form Mg 

 complexes. Most of the anions inhibit the forward reaction competitively 

 with respect to MgATP= and the reverse reaction competitively with re- 



