118 1. MALONATE 



It would be convenient if one could specify a malonate concentration, 

 or range of concentrations, which would most likely be specific, but there 

 are too many factors involved to do this with any confidence. Some of the 

 factors may be listed: (a) the species, tissue, or preparation used, (b) the 

 enzymes or metabolic pathways involved in what is measured, (c) the 

 conditions of the experiment, e.g., the pH or the Mg++ concentration, (d) 

 the degree to which succinate can accumulate and antagonize the inhibi- 

 tion, (e) the effective concentration of malonate within cells, and (f) the 

 possibility of nonenzyme effects on cell membranes or other structures. 

 Specificity for an inhibitor is not a constant to which can be given a value 

 for all cases, but a characteristic that must be evaluated for each experi- 

 ment. Aside from the direct actions of malonate on enzymes, there are the 

 problems of metal cation depletion, the possible effects of Na+ or K+ added 

 with the malonate, and the inactivation of the coenzyme A in some pre- 

 parations through the formation of malonyl-CoA. 



Bearing in mind these difficulties, a few general remarks may be made. 

 It is probable that malonate usually does not inhibit any cycle enzyme more 

 strongly than succinate dehydrogenase, so that the major effect will be re- 

 lated to the inhibition at this site, but it will be recalled that in certain 

 species the succinate dehydrogenase is not very susceptible to malonate. It 

 is impossible to achieve a nearly complete block of succinate oxidation 

 without affecting other cycle reactions; if one wishes a specific effect, one 

 must be satisfied with a moderate inhibition of succinate oxidation. If a 

 single malonate concentration for general usefulness had to be chosen, 5 mM 

 might be taken provisionally. Although in some cases rather incomplete inhi- 

 bition will be obtained, this concentration will probably not inhibit other 

 cycle reactions significantly. This applies to noncellular preparations where 

 penetration is not a factor but otherwise higher external concentrations 

 may have to be used. In any case, good evidence for a specific action should 

 be obtained under the conditions of the investigation, and reliance should 

 not be based on generalities. 



EFFECTS OF MALONATE 

 ON OXIDATIVE PHOSPHORYLATION 



An uncoupling action on oxidative phosphorylation has been claimed for 

 malonate several times and it is important to determine if this is actually 

 so. This is quite difficult because pertinent or reliable data are generally 

 lacking. There are four important ways in which malonate could alter the 

 P : ratio: (1) a direct effect on the coupling between oxidation and phos- 

 phorylation, (2) an alteration of the pattern of substrate oxidation, since 

 different substrates may have different P : ratios, (3) a differential inhi- 

 bition of electron transport pathways for a single substrate but with differ- 



