Organic Substances 101 



D) For unsaturated fatty acicfs.— Unsaturated fats are 

 slowly oxidized even when injected into the tissues; more 

 readily on exposure to atmospheric oxygen; and very rapidly 

 in the presence of various oxidants and catalysts. The prod- 

 ucts of oxidation may include a large variety of com- 

 pounds^^"^^ such as peroxides, epoxides, aldehydes, possibly 

 ketones, hydroxyacids, various fragments and polymers. 

 Some of the latter may be colored and/or insoluble in lipid 

 solvents. The insoluble fraction may exhibit surprising stain- 

 ing reactions, such as acid-fastness and aflBnity to orcein and 

 resorcinol-fuchsin.^^ The chemistry of the oxidative process 

 is poorly understood; the factors which influence the rate of 

 oxidation and the nature of the end-products are obscure. 



Methods for unsaturated fatty acids are relatively specific 

 for phospholipids and cholesterol esters because of the 

 marked unsaturation of these compounds. 



a) Osmium tetroxide (osmic acid),— This substance has 

 been employed for the demonstration of triolein and oleic 

 acid since 1895.^^ While oxidizing the ethylenic double 

 bonds, it is reduced to a black substance (probably a mixture 

 of lower oxides ) . It is of some value only if used according 

 to the specifications of Cain^ (well- washed frozen sections 

 fixed briefly in formalin-CaCL and treated with a solution of 

 osmic acid for about 1 hour; avoidance of dichromate and 

 alcohol). Even so, phospholipids containing oleic acid may 

 react feebly or not at all. 



b ) Dichromate methods.— Fotassiuin dichromate has been 



37. Cummings, M. J., and Mattill, H. A.: J. Nutrition, 3:421, 1930-31. 



38. Bloor, W. R.: Biochemistry of the fatty acids (New York: Reinhold 

 Pub. Co., 1943). 



39. Markley, K. S.: Fatty acids (New York and London: Interscience 

 Publishers, 1947); Hilditch, T. P.: The chemical constitution of natural fats 

 (New York: John Wiley & Sons, 1947). 



40. Ralston, A. W.: Fatty acids and their derivatives (New York: John 

 Wiley & Sons, 1948). 



41. Hass, G. M.: Arch. Path., 27:15, 1939; Hass, G. M.: ibid., 28:177, 

 1939. 



42. Starke, J.: Arch. f. Physiol., p. 70, 1895. 



