292 2. MALE ATE 



(Block et al., 1955). Detection in simpler mixtures by polarography is also 

 possible (Kolthoff and Lingane, 1952). 



General Reactions of Maleate 



Most of the biologically important reactions of maleate involve addition 

 to the double bond. (1) Addition of water to form malate occurs very slowly 

 at physiological temperatures and the uncatalyzed reaction need not be 

 considered in work with maleate. Even at 140°, only 25% of maleate is 

 converted to malate in 10 hr (Weiss and Downs, 1922), and a 2.1 M solution 

 of maleic acid allowed to stand for 10 hr at 130^ contains 30% fumarate and 

 10% malate (Davies and Evans, 1956). The possibility of enzymic con- 

 version to malate will be discussed later (page 313). (2) Addition of amines 

 to form substituted aspartate does not occur readily and the reaction: 



CH— COO- R— NH— CH— COO- 



R-NH, + 11 -> I 



CH— COO- CHj— coo- 



is very slow even at elevated temperatures. (3) Addition of pyrroles may 

 proceed readily, the reaction with a-methylpyrrole 



CH— COO" 



n n + II" — _ ^ M |i cH-coo- 



H3C'^N^ ^"-^^ H3C^N-^iH-COO- 



H H 



occurring at room temperature, especially in acid solutions, to form 2- 

 methyl-5-pyrrolesuccinate. Such a reaction might be of importance biologi- 

 cally if it occurs with other pyrroles or heterocyclic compounds. (4) Hydro- 

 genation usually is rather difficult; e. g., with Raney nickel catalyst it requires 

 100° and 2500 lb/square inch pressure, but with platinum catalyst it may 

 occur measurably at ordinary temperatures. There is no evidence that 

 succinate is formed directly from maleate under physiological conditions. 

 Maleate can be oxidized by permanganate and other strong oxidants to 

 meso-tartrate, and possibly an oxidation of this type with the introduction 

 of two hydroxyl groups must be considered biologically. Neuberg and 

 Rubin (1914) believed acetaldehyde to be an important metabolic product 

 and showed that it could be formed from maleate and fumarate by treat- 

 ment with hydrogen peroxide and Fe++. Decarboxylation of maleate is 

 not common under ordinary conditions, but ultraviolet irradiation for 

 even short periods produces significant amounts of acrylate and COg. 

 The book by Flett and Gardner (1952) should be consulted for the chemistry 

 of maleate and its derivatives. 



