METABOLISM OF MALONATE 227 



The first reaction appears to be an oxidative deamination. Since methyl- 

 malonyl-CoA is a common intermediate in many tissues, methylmalonate 

 probably arises from any substance forming the coenzyme A derivative. 

 (This will be discussed in greater detail in the following sections). 



General Occurrence and Nature of Malonate Metabolism 



The metabolism of malonate by many organisms and tissues has been 

 conclusively demonstrated by a variety of techniques. The ideal method is 

 the determination of C^^Og or other labeled products formed from labeled 

 malonate, but in some instances good evidence is provided by studies of 

 oxygen uptake, especially when the endogenous respiration is very small, 

 or by growth in media containing only malonate as a utilizable substrate. 

 In other cases, the evidence is more indirect. For example, a stimulation of 

 growth rate or a rise in respiration in the presence of other substrates may 

 suggest the utilization of malonate but does not prove it. In the tabulation 

 on page 228 of organisms in which malonate metabolism has been claimed 

 to occur, those that are probable and based on indirect evidence are des- 

 ignated by (P). It may be noted in addition that Shannon et al. (1959) found 

 malonate to be metabolized by the excised leaves of 15 different common 

 plants (such as fig, peach, eucalyptus, azalea, and lantana) and 30 other 

 plants representing 27 families, from which it must be concluded that plants 

 are generally capable of utilizing malonate. 



The bacterial oxidation of malonate occasionally shows a lag period, first 

 observed by Lineweaver (1933). The rate of oxidation by Azotobacter is 

 very low for 2-3 hr, rises to a maximum around 6-8 hr, and falls off by 

 10 hr. The malonate is eventually 99% metabolized with an R.Q. of 1.6. 

 The theoretical R.Q. for complete oxidation: 



CHaiCOOH), + 2 O2 -» 3 CO2 + 2 H,0 



is 1.5. Lineweaver postulated that two separate reactions are involved: the 

 decarboxylation of malonate to acetate, and the oxidation of the acetate. 

 He attributed the lag phase to the slow decarboxylation, which was sup- 

 ported by the progressive decrease in the R.Q. with time. This interpreta- 

 tion was criticized by Karlsson (1950) because Lineweaver had not used 

 cells adapted to malonate. Malonate-grown Azotobacter does not decarbox- 

 ylate malonate appreciably under anaerobic conditions, so Karlsson con- 

 cluded that oxygen is required for the initial attack on malonate, either 

 because malonate must be oxidized before decarboxylation or because 

 oxygen may be required for some activation of malonate (for example, by 

 a phosphorylative mechanism). A lag phase was also demonstrated for 

 Aerobacter by Barron and Ghiretti (1953), the maximal oxidative rate oc- 

 curring around 2-3 hr after malonate addition. Only 40% of the malonate 



