Cytoplasmic ribonucleoprotein components of the Novikoff hepatoma

Isolation of smooth vesicles and free ribosomes from rat liver microsomes

Microsomes, isolated from rat liver homogenate in 0.88 M sucrose, have been fractionated by differential centrifugation. The 2nd microsomal fraction, sedimented between 60 minutes at 105,000 g and 3 hours at 145,000 g, consists mainly of smooth vesicles, free ribosomes, and ferritin. By utilizing the differences in density existing between the membranes and the granular elements it has been possible to separate the smooth membranes from the free ribosomes and ferritin. The procedure is to resuspend the 2nd microsomal fraction in a sucrose solution of 1.21 or 1.25 density and centrifuge it at 145,000 g for 20 or 40 hours. A centripetal migration of membranes and a centrifugal sedimentation of granular elements are obtained. Phospholipids, as well as the enzymatic activities DPNH-cytochrome c reductase, glucose-6-phosphatase and esterase are localized in the membranes. The free ribosomes have been purified by washing. A concentration of 200 microg RNA per mg nitrogen has been reached. RNA is also present in the membranes. These results are discussed in relation to current views on microsomal structure and chemistry.

Physiology of rat-liver polysomes. The stability of messenger ribonucleic acid and ribosomes

Starvation of rats for several days led to marked decrease in cytoplasmic polysomes and accumulation of breakdown products having S values less than 200s. Re-feeding of the starved animals induced a rapid reassembly of polysomes. These newly formed polysomes, in the presence of actinomycin D, decayed in a biphasic fashion: about two-thirds decayed with an apparent half-life of 3-3(1/2)hr. but the other one-third were much more stable. Evidence that polysome decay is an accurate reflexion of messenger RNA stability is presented, and it is concluded that in the presence of large doses of actinomycin D, rat-liver cytoplasm contains messenger RNA classes of widely varying stability, the more stable class having a half-life of at least 80hr. The half-life of liver ribosomes was also determined and was found to be 110-127hr.

The in vivo protein synthetic activities of free versus membrane-bound ribonucleoprotein in a plasma-cell tumor of the mouse

Cytoplasmic extracts of the transplantable RPC-20 plasma-cell tumor were fractionated by sucrose density gradient centrifugation. Four major fractions were distinguished: (a) microsomes and mitochondria; (b) membrane-free polyribosomes; (c) free monomeric ribosomes; and (d) soluble fraction. The fractions were analyzed for RNA and lipid phosphorus, and their particulate components were characterized by electron microscopy. Particular attention was paid to the problem of membrane contamination of the free polyribosome fraction. It was shown that this contamination was small in relation with the total content of ribosomes in the fraction, and that it consisted primarily of smooth-surfaced membranes which were not physically associated with the polyribosomes themselves. In vivo incorporation studies were carried out by injecting tumor-bearing animals intravenously with leucine-C(14), removing the tumors at various times thereafter, and determining the distribution of protein radioactivity among the gradient-separated cytoplasmic fractions. The free polyribosome and the microsome-mitochondria fractions constituted active centers for protein synthesis. It was shown that nascent protein of the free polyribosome fractions was not associated significantly with the contaminating membranes. The kinetics of labeling during incorporation times up to 11 min suggested that protein synthesized on the free polyribosomes was rapidly transferred in vivo to the soluble fraction of the cell, while protein synthesized by the microsomes and mitochondria remained localized within these elements. It was estimated that the free polyribosome fraction and the microsome-mitochondria fraction accounted for approximately equal proportions of the total cytoplasmic protein synthesis in vivo.

Drug induced changes in the cytoplasmic ribonucleoprotein constituents of brain tissue

Studies carried out on the properties of isolated ribosomes from drug-treated brain cortex slices indicate that ribosomes from drug-treated tissues are generally more susceptible to breakdown into smaller components such as proteins, nucleic acids, acid-soluble nucleotides, etc. It seems that some factor or factors responsible for the stability of the complex macromolecular structure of ribonucleoproteins of brain tissue are affected as a result of drug treatment. Ribosomal RNA from drug-treated brain tissue has been isolated under standardized conditions and the secondary structures of RNA have been studied by methods involving thermal hyperchromicity and reaction with formaldehyde. This latter study indicates that, during the action of some of these neuropharmacological drugs, the secondary structure of ribosomal RNA of brain tissue is partially lost. The loss in the stability of cytoplasmic ribonucleoproteins in drug-treated brain tissue may partly be due to the disorganization at the level of the secondary structure of the RNA component.