LIPIDS PRESENT IN LIVER 719 



A number of factors may alter the rate of turnover of liver lipids. Thus, 

 in the case of rats with fatty livers due to choline deficiency, Boxer and 

 Stetten" reported that, in spite of the fact that the amount of choline may 

 be normal, 100,101 the rate of turnover of choline is depressed. The normal 

 half-life of choline, instead of being six days, was shown to be eighteen days 

 in rats with severe fatty livers. The daily rate of replacement decreased 

 from a normal value of 3.9 mg. per rat per day to a level of 1.3 mg. per rat 

 per day. Thus, phospholipids are not effectively utilized in rats having 

 fatty livers ; apparently, the animal requires a continuous supply of choline 

 derived either from the diet or by synthesis, to insure a normal rate of turn- 

 over of phospholipids and so to maintain a normal level of liver fat. 



Bollman, Flock, and Berkson 102 determined that the proportional turn- 

 over rate for hepatic phospholipid approximates 5% per hour. Since the 

 hepatic phospholipid phosphorus is 132 mg./lOO g. liver, the mass turnover 

 rate is 6 mg. P/hr./lOO g. liver, or 0.2 mg. P/100 g. body weight. In a 

 later communication from the above workers, 103 it was reported that the 

 rate of phospholipid turnover in the liver of rats was increased by thyroxine 

 and decreased by thiouracil. The rate of turnover of phospholipid was 

 likewise found to be essentially the same in young rats as in adult animals. 

 However, since the relative size of the liver is greater in young rats than 

 in older ones, more phospholipid is formed in proportion to body weight in 

 the young rat than in the adult. 



There has been some question as to the effect of palmitic acid on phos- 

 pholipid turnover in the liver. When a large amount was given to rats 

 along with a low-protein, high-fat diet, liver fat was shown by Campbell 

 et al. Wi to be increased, together with a concomitant release of depot fats. 

 The metabolism of oleic acid did not impose a similar demand upon the fat 

 depots. However, the rate of phospholipid turnover as determined with 

 P 32 did not show any correlation with the amount of liver lipids or with gross 

 changes in fat metabolism caused by palmitic acid feeding. Either the 

 phospholipid turnover rates are not involved, or the rate of exchange of P 32 

 is not a measure of that of fatty acid in the phospholipid molecule. 



d. Diurnal Variation in Liver Lipids. It has already been shown that 

 liver lipids have an especially short half-life as contrasted with carcass 



99 G. E. Boxer and De W. Stetten, Jr., /. Biol. Chem., 153, 617-625 (1944). 



100 H. P. Jacobi, C. A. Baumann, and W. J. Meek, /. Biol. Chem., 138, 571-582 (1941). 



101 H. P. Jacobi and C. A. Baumann, J. Biol. Chem., 142, 65-76 (1942). 



102 J. L. Bollman, E. V. Flock, and J. Berkson, Proc. Soc. Exptl. Biol. Med., 67, 308-313 

 (1948). 



103 E. V. Flock, J. L. Bollman, and J. Berkson, Am. J. Physiol., 155, 402-408 (1948). 



104 I. G. Campbell, J. Olley, and M. Blewett, Biochem. J., 45, 105-112 (1949). 



