444 2. ANALOGS OF ENZYME REACTION COMPONENTS 



involved in the binding of these substances. The inhibition by these organic 

 sulfates was stated to be noncompetitive but since some are slowly hydro- 

 lyzed substrates, the inhibition may be mixed. Phosphate seems generally 

 to be a more potent inhibitor than sulfate; this has been observed on the 

 arylsulfatases of rabbit liver (Maengwyn-Davies and Friedenwald, 1954), 

 ox liver (Webb and Morrow, 1959), Helix pomatia (Dodgson and Powell, 

 1959), Charonia lampas (Takahashi, 1960 a), and beef and rabbit cornea 

 (Wortman, 1962). The inhibitions, where examined, are competitive. Weak 

 inhibitions by xylenesulfonate (Maengwyn-Davies and Friedenwald, 1954) 

 and benzenesulfonate (Dodgson et al., 1955; Dodgson and Powell, 1959) 

 have been observed. 



The potent inhibition by sulfite is interesting but the mechanism is not 

 yet understood, although it would appear to be competitive (Roy, 1953). 

 For all the ox liver arylsulfatases, sulfite is bound 3-4 kcal/mole more tightly 

 than is sulfate; indeed, sulfatase C, although resistant to sulfate, is inhibited 

 55% by 0.1 mM sulfite (Roy, 1956). Sulfite is also more inhibitory than 

 sulfate on sulfatases from bacteria and molluscs (Dodgson et al., 1955; Dodg- 

 son and Powell, 1959; Dodgson, 1959), although in most cases the concen- 

 trations used were too high to evaluate the potency readily. 



Choline sulfatase of Pseitdomonas nitroredvcences is inhibited very strongly 

 by sulfite (33% by 0.01 mM and 83% by 0.1 mM when choline sulfate is 

 10 mM), but is not affected or somewhat stimulated by sulfate and phos- 

 phate (Takebe, 1961). The steroid sulfatase and glucosulfatase of Patella 

 vulgata are inhibited by sulfate and even more potently by phosphate 

 (Roy, 1954 a). Since the sulfatases comprise so heterogeneous a group of 

 enzymes and relatively few have been adequately studied, it is difficult to 

 draw general conclusions or make valid correlations, but it is at least evident 

 that analogs are occasionally effective inhibitors. It is also likely that phos- 

 phate must exert a regulatory action on sulfatase activity in vivo. 



ADENOSINETRIPHOSPHATASES 

 AND TRANSPHOSPHORYLASES 



Enzymes hydrolyzing ATP or transferring its terminal phosphate to 

 various acceptors are frequently inhibited by other nucleotides. Competi- 

 tive product inhibition by ADP has been noted for ATPases from several 

 sources; the inhibition is never marked, since ADP is usually bound some- 

 what less tightly than ATP to the enzyme, but is sufficient to slow progres- 

 sively the rate of ATPase reactions. The ATP-P^ and ATP- ADP exchange 

 reactions catalyzed by mitochondria and digitonin particles are also inhib- 

 ited by ADP (Low et al., 1958; Cooper and Kulka, 1961). Some results with 

 ADP and other related substances are shown in Table 2-26. The inhibitions 

 however, depend on the pH and the concentrations of Ca++ and Mg++, as 



