Phospholipin metabolism in rabbit-liver cytoplasm

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Abstract
A modification of the method of Hogeboom, Schneider, and Pallade (Jour. Biol. Chem. 172: 619, 1948), for the fractionation of liver cytoplasm by differential centrifugation was used for the prepn. of 3 fractions, large granules, small granules (microsomes) and supernatant, from rabbit-liver cytoplasm. The method of Mirsky and Pollister (Jour. Gen. Physiol. 30: 117. 1946) has been used to prepare rabbit-liver nuclei. The microsome and large granule fractions from 12 rabbit livers were analyzed for total lipid, phospholipin, protein and nucleic acid content. Total lipid, protein and nucleic acid account for 94% of the large granules and 96.6% of the small granules. The total lipid (including phospholipin) content constitutes the main chemical difference between the 2 fractions, the large granules containing 29.6% and the microsomes 43.4%. Approx. 26.4% of the liver cytoplasmic phospholipins was in the large granules, 64.6% in the small granules, and 9% in the supernatant. The metabolism of the phospholipin P of the cytoplasmic fractions and the nucleus was studied with the aid of radioactive P. It was shown that at any time after the injn. of radioactive phosphate into rabbits there exist in the liver cytoplasm at least 3 phospholipin fractions with different specific activities. The original observation of Hevesy (Nature, 156; 534, 1945), that the rate of renewal of phospholipin P in the liver nuclei is slower than in the cytoplasm, was confirmed.
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