242 1. MALONATE 



inward transport. It is not known if such an effect is observed with other 

 dicarboxylate anions. 



Sulfonate, Phosphonate, and Arsenate Analogs of Succinate 



Succinate and malonate are bound to the active center of succinate de- 

 hydrogenase in part through electrostatic forces involving the negatively 

 charged carboxylate groups. It might be expected that substances in which 

 the carboxylate groups are replaced by other anionic groups would also be 

 inhibitory. The structures for some of the compounds and the intercharge 

 distances are given in Table 1-1. Klotz and Tietze (1947) first demonstrated 

 the inhibition of succinate dehydrogenase (rat liver) by 1,2-ethanedisulfo- 

 na'te and /5-sulfopropionate, 50% inhibition being observed in both cases 

 when the inhibitor is 13 mM and succinate is 20 mM, these inhibitions 

 being approximately equivalent to those of malonate. Intact E. coli cells 

 are not affected by 1,2-ethanedisuIfonate but the oxidation of succinate 

 by cell-free preparations is well inhibited, corresponding to the results with 

 malonate (Ajl and Werkman, 1948). Some inhibitor constants for these 

 substances are shown in Table 1-29. 



There appears to be definite differences in the susceptibility of the suc- 

 cinate dehydrogenases from different sources. Seaman (1952) noted that 

 /?-phosphonopropionate inhibits the Tetrahymena enzyme more strongly 

 than does malonate, and yet no inhibition is observed on the enzymes 

 from rat heart, liver, brain, or muscle. The inhibitions are competitive 

 wherever they have been tested and there is no evidence that the mecha- 

 nism is in any way different from that of malonate. Since these groups are 

 larger than the carboxylate group, the interchange distances are greater 

 than in malonate or succinate, and this must play some role in determining 

 their binding to the enzyme. However, the distances for the malonate ana- 

 logs, methanedisuffonate and arsonoacetate, are between those for malon- 

 ate and succinate, so that binding should be as tight as for these latter 

 substances if this were the only factor. Another factor of importance is the 

 total net charge on these inhibitors, inasmuch as each of the sulfonate, 

 phosphonate, and arsonate groups can ionize more than once. In other 

 words, a disulfonate can exist in five different forms with charges 0,-1, 

 — 2, —3, and —4. Furthermore, the third and fourth ionization constants 

 are in the physiological range of pH (see accompanying tabulation). The 



vK„ vK„ 



