114 III. OXIDATION AND METABOLISM 



bean lipoxidase for the oxidation of methyl hnoleate at 25°C. was 6.5. 

 Bergstrom and Holman^^^ recorded the optimum temperature as 30°C. 

 The activity of the enzyme dechnes precipitously above this temperature; 

 the temperature-accentuated inactivation is caused by contact with either 

 the substrate or the reaction products. Tappel et al.--'' found that the ac- 

 tivity of lipoxidase is reduced by freezing to less than 1% of that fomid in 

 the liquid system at the same temperature. The lipoxidase-catalyzed oxi- 

 dation of linoleate was found to have the relatively low activation energy 

 of 4300 cal. per mole. 227 



Holman225 reported that pure lipoxidase is not inhibited by pyrophos- 

 phate, fluoride, cyanide, azide, mercury ions, p-chloromercuribenzoate or di- 

 ethyldithiocarbamic acid, even when these are present in high concentra- 

 tions. This indicates that neither heavy metals nor sulfhydryl groups are 

 concerned with the functional activity of the enzyme. Although Hummel 

 and Mattill228 have suggested that lipoxidase is an aerobic dehydrogenase. 

 Balls and Kies^^^ are of the opinion that the fact that lipoxidase is not in- 

 hibited by a specific oxidase inhibitor, sodium cyanide (NaCN), is not suf- 

 ficient reason for classifymg it in this category. 



The action of antioxidants on lipoxidase offers another interesting ap- 

 proach to the problem of linoleate oxidation. Thus, although Bergstrom 

 and Holman^^'' note that some mhibition of lipoxidase activity is caused by 

 a-naphthol and by a-tocopherol, they suggest that it is due to antioxidant 

 activity rather than to true enzymic inhibition. Tappel and associates^^^ 

 found that two well-kno^\^l antioxidants were effective inhibitors of lip- 

 oxidase, namely nordihydroguaiaretic acid and propyl gallate. Moreover, 

 the effective concentrations were sufficiently low to render the use of these 

 antioxidants practical for food protection. On the other hand, it was showii 

 that a-tocopherol was rapidly oxidized by the hpoxidase system. Berg- 

 strom and Holman'^* note that some substrate competition is exhibited, in 

 decreasing order, by elaidolinolenic acid, 10,12-linoleic acid, oleic acid, and 

 octanoic acid. 



Although Balls et al.-^^ and Theorell and co-workers^^o believed that lip- 

 oxidase is activated by a polypeptide present in soybeans^^*' which could aug- 

 ment the enzyme activity^^''-^" by as much as 300%, the activator is not 



22' A. I.. Tappel, W. O. Lundberg, and P. D. Boyer, Arch. Biochem. Biophys., ^2, 293- 

 304 (1953). 



228 J. P. Hummel and H. A. Mattill, Proc. Soc. Exptl. Biol. Med., 65, 31-33 (1944). 



229 A. K. Balls and M. W. Kies, /. Biol. Chem., 153, 337-338 (1944). 



230 M. W. Kies, J. Biol. Chem., 170, 121-132 (1947). 

 2" M. W. Kies, Federation Proc, 6, 267 (1947). 



