TRICARBOXYLATE CYCLE 877 



Stimulation of glycolysis by the mercurials is not confined to yeast and 

 muscle. Hg++ below 0.11 mM stimulates glycolysis in guinea pig blood and 

 inhibits in higher concentration (Fuentes and Rubino, 1923), while in hu- 

 man blood Hg++ stimulates anaerobic glycolysis from 0.0185 to 1.85 vaM 

 although at 18.5 mM there is almost complete inhibition (Rubino and 

 Varela, 1923). Glucose utilization, COg release, and lactate formation in 

 human erythrocytes are all stimulated by p-MB up to 5 //moles/ml of 

 erythrocytes (Jacob and Jandl, 1962). No explanation for these results 

 is immediately evident. 



TRICARBOXYLATE CYCLE 



Despite the fact that no analysis of the effects of mercurials on the cycle 

 or on the operation of mitochondria has been made, one would predict 

 quite potent inhibition on the basis of the sensitivities of the individual 

 enz\Tnes (Table 7-13). Mercurial concentrations in the neighborhood of 

 0.01 mM should depress several enzjTnes very significantly (e.g., pjTuvate 

 oxidase, isocitrate dehydrogenase, or-ketoglutarate oxidase, succinate de- 

 hydrogenase, malate dehydrogenase, and some ancillary enzymes, such as 

 acetate kinase), and concentrations of the order of 0.1 mM should block 

 completely. However, since we have already noted that glycolysis is often 

 not inhibited as much as one would expect from studies of the individual 

 enzymes, we must be very careful in considering inhibitions of the cycle 

 in cellular preparations. The utilization of pyruvate and acetate by a va- 

 riety of cellular and subcellular preparations has been shown to be readily 

 inhibited by mercurials (Table 7-16), but in no case was the operation of 

 the entire cycle tested, so that the entire inhibition, as far as one knows, 

 might be on the initial enzyme reaction (p>Tuvate oxidase or acetate kinase). 

 If the cycle is operating by regenerating oxalacetate, much stronger inhi- 

 bition would undoubtedly be observed. In work with mitochondria, homo- 

 genates, or cell suspensions, however, one must always remember the role 

 of nonenzyme protein in reducing the mercurial available for inhibition, 

 so that concentrations such as those in Table 7-16 are not of much quanti- 

 tative significance, but show definite interference with cycle activity. 



In order to answer some of these questions relative to the action of the 

 mercurials on the cycle, Dr. Yang kindly consented to examine the effects 

 of Hg++ on the Oj uptake of rabbit heart mitochondria by the same tech- 

 niques used in a previous study of iodoacetate (Yang, 1957). The changes 

 over a 60 min period obtained from results on three preparations are shown 

 in the accompanying tabulation. These data show clearly that several steps 

 in the cycle are inhibited rather strongly as the concentration is raised from 

 0.003 mM to 0.01 mM, and that at 0.1 mM the cycle activity is essentially 

 completely blocked. The stimulation observed with a-ketoglutarate as the 



