226 1. MALONATE 



values in legumes (2-3% of the plant dry weight) and that in kidney-bean 

 and clover leaves malonate represents 45% of the total di- and tricarbox- 

 ylates present. Bush-bean {Phaseolus vulgaris) leaves often contain as 

 much as 10 mg/g dried tissue and malonate is more concentrated than fu- 

 marate or succinate, although less than malate and citrate (Young and 

 Shannon, 1959). In man malonate is excreted in the urine at an average 

 rate of 0.0047 mg/kg/day and in the rat at 10 times this rate (Stalder, 1958). 

 This amounts to only 0.32 mg/day in man (only two individuals were tested 

 so these averages are not accurate). Since malonate is apparently metabol- 

 ized in mammals, the tissue concentration or rate of excretion will reflect a 

 balance between formation and destruction. In other words, these excretion 

 values do not necessarily represent the rates of malonate formation. 



Relatively little is known about the pathways for the formation of mal- 

 onate, but the miscellaneous observations make it likely that different reac- 

 tions are involved in various organisms. Malonate can arise from many 

 different substrates but in most cases the pathways are complex and the 

 immediate precursors are not known. Malonate can be formed from pyri- 

 midines and barbiturates in the mycobacteria (Hayaishi and Kornberg, 

 1952), from pyrimidines in Nocardia (Lara, 1952), from acetate in Hevea 

 hrasiliensis (Fournier et al., 1961) and avocado (Mudd and Stumpf, 1961), 

 from citrate in Aspergillus niger (Challenger et al., 1927), from succinate 

 in Aspergillus niger (Subramanian et al., 1929), from asparagine in rats 

 (Thomas and Stalder, 1959), from oxalacetate in pig heart extracts catalyzed 

 by metmyoglobin and Mn++ (Vennesland and Evans, 1944; Vennesland 

 et al., 1946), and from malonyl-CoA in Penicillium cyclopium, (Bentley and 

 Keil, 1961). The high concentrations of malonate in bush-bean plants led 

 Huffaker and Wallace (1961) to study the mechanism of the accumulation. 

 They found that the malonate synthesis is related to the dark COg fixation 

 in the roots, phosphoenolpyruvate being carboxylated to oxalacetate and 

 this going to malonate with the help of one or more enzymes. The addition 

 of phosphoenolpyruvate and Mg++ to root homogenate leads to the for- 

 mation of labeled malonate from 0^*02- It was also found that any other 

 reactions utilizing oxalacetate decrease the yield of malonate. It is very 

 interesting that frogs accumulate malonate-C^* from C^Oa, along with other 

 dicarboxylates (Cohen, 1963). In normal strains the malonate accounts for 

 only 0.3-0.5% of the total incorporation but in hybrids {R. pipiens X R. 

 sylvatica) the value is 6-23%. This increased accumulation in the hybrids 

 was attributed to some defect in the metabolism of malonate. 



Methylmalonate can be formed from propionate (Flavin et al., 1955) in a 

 variety of tissues, and in rat liver the pathway has been shown to go through 

 succinate (Katz and Chaikoff, 1955). The feeding of isobutyrate and valine 

 to rats leads to the formation of methylmalonate (Thomas and Stalder, 

 1958) and the feeding of isoleucine leads to ethylmalonate (Stalder, 1959). 



