140 1. lODOACETATE AND lODOACETAMIDE 



EFFECTS ON LIPID METABOLISM 



Our knowledge of the effects of iodoacetate on lipid metabolism and most 

 noncarbohydrate metabolism, is meager and gappy, and the problem is com- 

 plicated by the interrelationships between glycolysis and almost all other 

 metabolic pathways, so that even though the block is specific for 3-PGDH, 

 quite possibly lipid metabolism will be altered secondarily by changes in 

 the level of ATP or acetyl-CoA. In addition, one must always consider the 

 direct action of iodoacetate on the fatty acid helix, transfers of two-carbon 

 fragments, and the various biosynthetic pathways, although very few studies 

 on these systems have been done. Table 1-23 presents a number of enzymes 

 involved in lipid metabolism which are sensitive enough to warrant consid- 

 eration even when iodoacetate is used at the low concentrations sufficient 

 to block the EM pathway. The discussion will be divided into two parts: 

 lipid catabolism and lipid synthesis. 



Lipid Catabolism and Fatty Acid Oxidation 



The oxidation of fatty acids is inhibited fairly potently by iodoacetate, 

 as shown by Singer and Barron (1945) in suspensions of £". coli, 1 niM iodo- 

 acetate reducing the oxidation of stearate and oleate around 45%, and con- 

 firmed by Waltman and Rittenberg (1954) in Serratia marcescens, where 

 some inhibition was noted at 0.1 mM. The Conversion of hexanoate and 

 Zl^-hexenoate to acetoacetate in rat liver homogenates is also inhibited by 

 iodoacetate, 1.3 mM depressing 31% and 60%, respectively (Witter et al., 

 1950). The site of this inhibition is unknown. Most of the helix enzymes 

 have not been examined for sensitivity to iodoacetate; crotonase seems to 

 be fairly resistant. One possible site is the initial activation of fatty acids, 

 since Jencks and Lipmann (1957) found this system in pig liver to be inhib- 

 ited 55-60% by 3 mM iodoacetate. In most of these test systems, ATP is 

 added but, in the cell, iodoacetate may further depress this initial step by 

 reducing the ATP available. In plants these oxidations may be less sensitive; 

 at least the formation of C^^O.^ from paImitate-1-C^* in extracts of peanut 

 cotyledons is unaffected by 1 mM iodoacetamide (Castelfranco et al, 1955). 



The interesting studies of Geyer and his co-workers (1950 a, b) on fatty 

 acid breakdown in rat liver slices provide evidence that perhaps a major 

 portion of the iodoacetate inhibition is exerted on the hehx. First, pyruvate 

 does not counteract the effect of iodoacetate, which it should do if the site 

 of action is the EM pathway and ATP is reduced (the rather high concen- 

 tration of iodoacetate used, 5 mM, may have inhibited pyruvate oxidation, 

 however). Second, the formation of labeled acetoacetate from labeled fatty 

 acids is inhibited (see accompanying tabulation). The greater inhibition of 

 C^^Og formation may reflect some effect on the cycle. Since 5 mM iodoacetate 

 depresses the helix only 45%, in liver at least it is likely that concentrations 



