I 



PHOSPHOLIPIDS 55 



creased rate. In some cases, the rate of synthesis was doubled in the 

 denervated muscle, as contrasted with the corresponding contralateral in- 

 tact muscle. Although a decrease in phospholipid content has been ob- 

 served in inanition-^^ and in muscular atrophy and muscular dystrophy, ^^^ 

 the changes reported for denervated muscles occurred before any atrophy 

 had occurred. Hevesy and Smedley-MacLean^" noted an increased turn- 

 over rate for phospholipids in the muscles of rats maintained on a fat- 

 deficient diet. 



e. Brain. The rate of synthesis of phospholipids would appear to be 

 slowest in the intact brain. Before radioisotope technics were available, 

 McConnell and Sinclair-^* reported that brain incorporates elaidic acid into 

 the lecithins and cephalins at a slower rate than do muscle and liver. 

 With the advent of P'^ as an indicator of phospholipid synthesis, many 

 investigations, including those of Hevesy and Hahn,-*'''^^^ Artom et al.,'^*^ 

 and the Chaikoff group-*^'-*"-'^"-^^^ have confirmed the earlier opinion as to 

 the sluggishness of the synthesis of phospholipids in the brain. Some 

 workers^"'-^"'^^^"^^' attributed this retarded phospholipid synthesis in brain 

 to a slow penetration of the labeled phosphate into the brain and into its 

 extracellular spaces. In the experiments of Changus, Chaikoff, and 

 Ruben, 2^" incorporation of P*- into brain phospholipids occurred at a 

 progressive rate for as long as 200 hours following the administration of the 

 isotopic phosphate. After the maximum content of P^- phospholipid was 

 attained, it was retained for a prolonged period ; as long as 800 hours after 

 the administration of P^- the brain still contained 70% of the amount 

 which had been noted at the 200-hour interval.'^'' Cephalin and sphingo- 

 myelin were found to be more readily replaced than was lecithin. ^^''■^^^ 



The abihty of the brain to synthesize phospholipids is dependent upon 

 age. Thus, the maximum incorporation of P^^ in the whole brain phos- 

 pholipids of the rat was shown by Fries et aZ."^-^^* to occur between birth 

 and the age at which the animal attained a weight of 50 g. This is also 

 the period of maximum myelinization of the brain. -'^^ In their later work, 



»5 C. Ciaccio, Arch, farmacol. sper., U, 231-254 (1917). 

 ^ W. R. Bloor, /. Biol. Chem., 119, 451-465 (1937). 



^^ G. C. Hevesy and I. Smedley-MacLean, Biochem. J., 34, 903-905 (1940). 

 *« K. P. McConnell and R. G. Sinclair, J. Biol. Chem., 118, 131-136 (1937). 

 ^^ L. Hahn and G. Hevesy, Skand. Arch. Physiol., 77, 148-157 (1937). 

 "« G. W. Changus, I. L. Chaikoff, and S. Ruben, /. Biol. Chem., 126, 493-500 (1938). 

 "IB. A. Fries and I. L. Chaikoff, J. Biol. Chem., Ul, 479-485 (1941). 

 "2 W. E. Cohn and D. M. Greenberg, /. Biol. Chem., 123, 185-198 (1938). 

 2" H. B. Jones, I. L. Chaikoff, and J. H. Lawrence, Am. J. Cancer, 40, 243-250 (1940). 

 "« B. A. Fries, C. Entenman, G. W. Changus, and I. L. Chaikoff, /. Biol. Chem., 137, 

 303-310 (1941). 



*'* J. B. Watson, Animal Education, Univ. Chicago Press, Chicago, 1903. 



