122 1. MALONATE 



rat heart (1954) that malonate increases the P:0 ratio with a-ketoglutarate 

 as the substrate as the malonate concentration is increased up to 20 mM; 

 at 40 mM the P:0 ratio decreases somewhat, so that at this high concen- 

 tration a small degree of uncoupling may occur. Azzone and Carafoli (1960) 

 found the same in pigeon muscle. Evidence against uncoupling by malonate 

 intracellularly is provided by the study on ascites carcinoma cells by 

 Greaser and Scholefield (1960). Comparison of the changes brought about 

 by malonate (20 mM) in the respiration and the sum of the concentrations 

 of ADP and ATP led to the estimation of a 15% increase in the P:0 ratio, 

 whereas the classic uncoupler, 2,4-dinitrophenol, depressed the P:0 ratio 

 70%. Malonate may actually stimulate the esterification of phosphate. In 

 addition to the two examples in Table 1-23, Jackson et al. (1962) reported 

 an 8% elevation in phosphate uptake by 0.1 mM malonate in barley root 

 mitochondria oxidizing succinate, and Rosa and Zalik (1963) found such a 

 stimulation in pea seedling mitochondria, which is maximal around 0.01 

 mM malonate, the oxygen uptake not being altered. Above this concentra- 

 tion, both phosphorylation and oxidation are depressed in a parallel fashion; 

 respiration is hence always depressed somewhat more than is phosphate 

 esterification, and no uncoupling is seen at any malonate concentration. 

 The transfer of phosphate from phosphorylated coenzyme Q to ADP to 

 form ATP in mitochondrial preparations is not altered by even 10 milf 

 malonate (Gruber et al., 1963), whereas dinitrophenol inhibits this readily. 

 It would therefore appear to be legitimate to conclude that malonate 

 does not exhibit uncoupling activity except possibly at high concentrations 

 (above 30 mM). The evidence for uncoupling at concentrations commonly 

 used is considered to be inadequate and outweighed by the mass of indirect 

 evidence that P:0 ratios are not reduced in the oxidations of citrate, a- 

 ketoglutarate, and succinate. 



EFFECTS OF MALONATE ON GLUCOSE METABOLISM 



The actions of malonate on carbohydrate, lipid, amino acid, porphyrin, 

 and other types of metabolism will now be considered, after which it will be 

 possible to evaluate the specificity of malonate more broadly. The effects 

 on glucose metabolism are important but difficult to analyze. In the first 

 place, the interrelationships between the cycle and the glycolytic pathways 

 are complex and secondary effects on glucose utilization must be expected. 

 In the second place, much of the work on the alteration of glucose utiliza- 

 tion by malonate has not been adequate for the determination of mecha- 

 nisms nor do the results even provide useful information in many cases, and 

 for this reason only certain reports will be discussed. There are three basic 

 ways by which malonate, might alter glucose oxidation. (1) Malonate will 

 usually cause a depression of the oxygen uptake related to glucose metabo- 



