METABOLISM OF MALEATE 313 



acetone powders of pig tissues and various alcohols, but it is unlikely 

 that this could be important in the intact tissues. 



The most obvious reactions by which maleate could be metabolized are: 

 (1) isomerization to fumarate, (2) hydration to malate, (3) reduction to 

 succinate, (4) oxidation to meso-tartrate, and (5) decarboxylation to acry- 

 late. Enzymes for only the first two reactions have been reported, but the 

 other pathways cannot be neglected, although they are less likely to occur. 

 A cis-trans isomerase that converts maleate to fumarate was isolated from 

 Pseudomonas sp. by Otsuka (1961). This enzyme is fairly active and suf- 

 ficient reaction occurs in several minutes to make readings. An enzyme 

 converting maleate to malate was obtained from ground corn by Sacks 

 and Jensen (1951) and called malease. The activity seems to be quite low, 

 since when maleate was 1 mM initially, 2% reacted in 1.5 hr, 6% in 7 hr, 

 18% in 12 hr, and 95% in 24 hr. Unfortunately, the distribution of these 

 enzymes is not known and hence their importance generally in the me- 

 tabolism of maleate cannot be assessed. 



Let us turn to studies purporting to demonstrate maleate utilization in 

 whole animals in order to determine which pathways may be operative. 

 Orten and Smith (1937) injected the sodium salts of different acids intra- 

 venously at a dose of 4.35 millimoles/kg in dogs, and determined the urinary 

 citrate over a 24-hr period. The results are given in Table II-1-22 along 

 with data from higher doses in rats provided by Simola and Kosunen 

 (1938) and Krusius (1940). There is no doubt that maleate can markedly 

 increase the excretion of citrate, usually more so than most of the other 

 organic acids. The question is whether maleate serves as a precursor for 

 citrate, presumably thorough fumarate or malate, or produces an accu- 

 mulation of citrate as a result of some inhibition in the cycle. Krebs et al. 

 (1938) showed that in rabbits not only urinary citrate is elevated by mal- 

 eate, but also a-ketoglutarate and succinate (7.5 and 2.5 times, respectively). 

 They pointed out that maleate acts similarly to malonate, but that metab- 

 olism of maleate can not be excluded. It seems to me that these results 

 cannot be taken as evidence for maleate metabolism, but are more important 

 for demonstrating inhibitory effects on the cycle in vivo. If maleate goes 

 to citrate through fumarate or malate, it is odd that maleate is usually 

 somewhat more effective than fumarate or malate in elevating the urinary 

 citrate. 



A clear demonstration of maleate metabolism in dogs and man was made 

 by Sacks (1958), who administered maleate-2-Ci* and determined the blood 

 levels of C^^Oa at intervals. In man the specific activity of the COg reaches 

 a maximum in around 60-70 min, so the metabolism occurs fairly rapidly, 

 although not nearly as rapidly as the metabolism of fumarate. No data on 

 the extent of the conversion were given. Since it was thought that the first 

 step might be the conversion of maleate to fumarate, labeled fumarate in 



