Phosphorus compounds in the cell. 4. The incorporation of radioactive phosphorus into liver cell fractions

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Abstract
Rat and rabbit liver cell cytoplasms were separated by the technic of differential centrifugation in 0.25 M sucrose into 2 particulate fractions and a non-sedimentable supernatant fluid corresponding to the cell sap. Isolated cell nuclei were also prepared. When characterized by Janus Green staining and by electron microscopy the particulate fractions were shown to consist of 2 independent populations of particles corresponding to the mitochondria and microsomes. The incorporation of p32 into the phospholipids and ribonucleic acid (RNA) of the isolated nuclei and 3 cytoplasmic fractions and into the desoxyribonucleic acid (DNA) of the isolated nuclei was studied in normal, regenerating and weanling rat liver and in normal, maternal and fetal rabbit liver. In all cases, the most rapid incorporation was found in nuclear RNA. In the cytoplasm the cell sap invariably showed a higher activity than the granules. In fetal rabbit liver the microsomes showed a higher activity than the mitochondria . In all other forms of liver tissue studied the activity of the microsomes was always of the same order or slightly lower than that of the mitochondria. The incorporation of P32 into RNA in the various morphological fractions of normal rabbit liver was detd. at various time intervals. Nuclear RNA showed max. incorporation in about 15 hrs. after admn., the 3 cytoplasmic fractions in about 30 hrs. Incorporation of P32 into DNA in resting liver tissue was very low. In rat liver regenerating after partial hepatectomy the activity of the DNA was greatly increased, and in fetal rabbit liver it increased above that of the cytoplasmic RNA.