THE RELATION BETWEEN THE INTRACELLULAR RIBONUCLEIC ACID DISTRIBUTION AND AMINO ACID INCORPORATION IN THE LIVER OF THE DEVELOPING CHICK EMBRYO

The RNA-P and DNA-P content of the nucleus and the RNA-P content of the whole cell of the livers of 8- to 20-day chick embryos and of adult fowls have been determined. The DNA-P content of the liver nuclei was slightly higher in the 8- and 10-day embryo than in all the other stages examined. A significant decrease in the RNA content of the cell occurred during embryonic development. The RNA content of the adult cell was the same as that of the 14- to 16-day embryo. The proportion of the cellular RNA contributed by the nucleus also decreased during development. In respect to both nuclear RNA content and distribution of RNA between nucleus and cytoplasm, the adult resembled the 8- to 12-day embryo. Examination of the fine structure of the cell showed that, as development progressed, free ribosomes decreased in number and the rough membranes increased. Slices of 8-, 14-, and 20-day embryonic livers and of adult livers were incubated with ¹⁴C-leucine, and the amount of labeled amino acid incorporated into whole tissue protein and into the proteins of the subcellular fractions was measured. Embryonic liver incorporated ¹⁴C-leucine 15 to 30 times more rapidly than adult liver. The microsomal protein was always more highly labelled than the protein in any other subcellular fraction; however, in the 8-day embryonic and the adult liver the proportion of total counts found in the nuclear fraction was considerably higher than in the 14- or 20-day embryonic liver. The significance of an apparent correlation between the proportion of the cell's RNA contributed by the nucleus and the proportion of total counts in the nuclear fraction is discussed.

A NUCLEAR MEMBRANE CHANGE AFTER PARTIAL HEPATECTOMY

Partial hepatectomy (67 per cent extirpation) of the rat leads to a change in the membrane of liver nuclei (purified with citric acid) detectable as an increase in electrophoretic mobility. No change is detectable 2 hours after the operation, but between 2 and 6 hours about a 1.4-fold increase in mobility occurs after which the mobility becomes constant at the elevated level. Removal of only 10 per cent of the liver causes no detectable change in 6 hours. Bilateral adrenalectomy immediately before partial hepatectomy does not affect the development of the nuclear change. Actinomycin D and p-fluorophenylalanine, but not noradrenalin, ionizing radiation, or EDTA, suppress the increase in electrophoretic mobility. The level of actinomycin D required to block the nuclear membrane change is 6 times greater than that necessary to prevent the rate increase in hepatic RNA metabolism that follows removal of part of the liver. This discrepancy and the difference in the response to noradrenalin indicate that, at least initially, the nuclear membrane change and the change in the rate of RNA synthesis are independent processes. The inability of EDTA to block the nuclear membrane change shows that the Zn⁺⁺ requirement for DNA replication is not related to the events that lead to the alteration in the electrokinetic properties of liver nuclei.

CALCIUM ION ACCUMULATION AND VOLUME CHANGES OF ISOLATED LIVER MITOCHONDRIA. REVERSAL OF CALCIUM ION-INDUCED SWELLING

1. The excessive accumulation of Ca(2+) by mitochondria suspended in an iso-osmotic buffered potassium chloride medium containing oxidizable substrate and phosphate led to extensive swelling and release of accumulated Ca(2+) from the mitochondria. When the Ca(2+) was removed from the medium by chelation with ethylene glycol bis(aminoethyl)tetra-acetate, the swelling was reversed in a respiration-dependent contraction. The contracted mitochondria were shown to have regained some degree of respiratory control. 2. The respiration-dependent contraction could be supported by electron transport through a restricted portion of the respiratory chain, and by substrates donating electrons at different levels in the respiratory chain. 3. Respiratory inhibitors appropriate to the substrate present completely inhibited the contraction. Uncoupling agents, and the inhibitors oligomycin and atractyloside, were without effect. 4. When the reversal of swelling had been prevented by respiratory inhibitors, the addition of ATP induced a contraction of the mitochondria. In the absence of added chelating agent the contraction was very slow. The ATP-induced contraction was completely inhibited by oligomycin and atractyloside, was incomplete in the presence of uncoupling agents and was unaffected by respiratory inhibitors. 5. The relationship between the energy requirements of respiration-dependent contraction and the requirements of ion transport and other contractile systems are discussed.

A STRUCTURAL STUDY OF RAT LIVER RIBOSOMES

Polyribosomes, ribosomes, and ribosomal subunits were prepared from rat liver using sodium deoxycholate and a variety of ionic media. They were examined in the electron microscope, mainly as negatively or positively stained preparations, and in the analytical ultracentrifuge. The polyribosomes consist of up to twelve or more ribosomes linked by a fine strand, 10 to 15 A in diameter, probably of RNA. The ribosomes are approximately spherical with diameters of 250 to 300 A, and are estimated to be about 50 per cent porous. Possibly because of their high protein content, whole ribosomes show no cleavage furrows. Ribosomes were dissociated in phosphate buffer and the subunits separated on sucrose density gradients containing 10 per cent formalin. Three classes of subunit were obtained with sedimentation coefficients of 71S, 50S, and 31S respectively. The smallest, 31S subunit is about 250 A long by 100 A wide. The largest subunits appear to be clusters of smaller particles. It is estimated from their linear dimensions in electron micrographs that the whole 83S ribosome could contain up to six 31S subunits, or their equivalent.

EFFECTS OF TETRAZOLIUM SALTS ON OXIDATIVE PHOSPHORYLATION IN RAT-LIVER MITOCHONDRIA

1. The effects of five different tetrazolium salts on oxidative phosphorylation in rat-liver mitochondria have been investigated. 2. In all cases the mitochondria were uncoupled by very low concentrations of the tetrazolium salts. Further, the transition from a system just exhibiting respiratory control to one in which the mitochondria were totally uncoupled has been shown to occur over very small concentration ranges of the tetrazolium salts. 3. The effectiveness of the five tetrazolium salts as uncoupling agents is discussed in the light of their standard electrode potentials and effectiveness as electron acceptors in dehydrogenase-linked reactions.

METABOLISM OF TISSUE CELLS IN SUSPENSION. SYNTHESIS OF RIBONUCLEIC ACID BY LEIVER CELLS IN SUSPENSION

1. Liver cells in suspension are shown to incorporate several RNA precursors into their RNA. 2. The incorporation of [(32)P]phosphate and [(14)C]adenine into the RNA of the cell suspension is usually of the same order as that in the perfused (or unperfused) liver slices. However, the initial lag in the incorporation of adenine into the RNA of the cell suspensions is much longer than that obtained for the tissue slices, and the optimum incorporation of adenine in the former, unlike that in the latter, needs exogenous glucose and probably a high concentration of phosphate. 3. The cell suspensions also differ from the tissue slices in being unable to incorporate [(14)C]orotic acid into their RNA, and resemble tumour tissues in incorporating uracil into their RNA at a rate significantly higher than that obtained with the tissue slices. 4. The above differences in the metabolic behaviour of liver-cell suspensions and tissue slices are considered to be due to the different levels of organization of the liver cells in the two tissue preparations.

FURTHER OBSERVATIONS ON THE PRODUCTION OF AMINO ACIDS BY RAT-LIVER MITOCHONDRIA AND OTHER SUBCELLULAR FRACTIONS

1. Rat-liver mitochondria showed a decrease in amino acid production after preparation in 0.25m-sucrose containing EDTA (1mm), but an increase in water content. When EDTA was replaced by Mn(2+) (1mm) or succinate (1mm), both amino acid production and water content were lowered, whereas preparation in 0.9% potassium chloride caused an increase in both. 2. Amino acid production by rat-liver homogenates prepared in 0.9% potassium chloride or 0.25m-sucrose was similar (q(amino acid) 0.047 and 0.042 respectively aerobically). After freezing-and-thawing q(amino acid) values were approximately doubled, and approached that of a homogenate prepared in water. 3. All cations tested inhibited amino acid production by mitochondria, Hg(2+) and Zn(2+) being the most effective in tris-hydrochloric acid buffer. In phosphate buffer Mg(2+) and Mn(2+) had no effect. Of the anions tested only pyrophosphate and arsenate had any inhibitory effect at final concn. 1mm. 4. Iodosobenzoate (1mm) and p-chloromercuribenzenesulphonate (1mm) inhibited mitochondrial amino acid production by 70-80%, whereas soya-bean trypsin inhibitor, EDTA and di-isopropyl phosphorofluoridate inhibited by a maximum of 30%. Respiratory inhibitors had no effect. 5. Rat-liver homogenate and subcellular fractions each showed an individual pattern of inhibition when a series of inhibitors was tested. 6. Amino acid production by mitochondria was decreased by up to 50% in the presence of oxidizable substrate, apart from alpha-glycerophosphate and palmitate, which had no effect. CoA stimulated amino acid production in tris-hydrochloric acid but not in phosphate buffer, alpha-oxoglutarate abolishing the stimulation. 7. Cysteine and glutathione stimulated amino acid production by whole mitochondria by 30%, but only reduced glutathione stimulated production in broken mitochondria. 8. Adrenocorticotrophic hormone and growth hormone stimulated mitochondrial amino acid production by 21-24%, whereas insulin inhibited production by 25%. 9. Coupled oxidative phosphorylation increased amino acid production by up to 154% at 25 degrees and 40 degrees . The increase was abolished by 2,4-dinitrophenol. 10. Amino acid incorporation in mitochondria was accompanied by an increase in amino acid production, both being decreased by chloramphenicol. 11. Mitochondrial production of ninhydrin-positive material was increased in the presence of albumin. The biggest increase was noted for the soluble fraction of broken mitochondria. No increase was found in the presence of (14)C-labelled algal protein or denatured mitochondrial protein.

LIVER PARENCHYMAL CELL INJURY. I. INITIAL ALTERATIONS OF THE CELL FOLLOWING POISONING WITH CARBON TETRACHLORIDE

The structure of the endoplasmic reticulum, plasma membrane, mitochondria, and Golgi apparatus of the liver parenchymal cell is strikingly altered within 1 hour following the administration of a single oral dose of carbon tetrachloride to rats. Progressive loss of glucose-6-phosphatase activity accompanies dispersal of the ergastoplasm. Electron microscopy reveals that these changes are associated with vacuolization of the cisternae of the granular endoplasmic reticulum, degranulation of its membranes, and the appearance of increased number of free ribosomes in the adjacent cytoplasmic matrix. Concomitantly, calcium enters the liver parenchymal cell and is sequestered by mitochondria. First increased at 30 minutes, calcium content is maximal at 1 hour and returns to normal at 2 hours. Although succinic and glutamic dehydrogenase activity patterns within the liver lobule are unaffected, liver cell mitochondria enlarge and some appear to fuse or assume cup-like configurations. Microvilli lining the space of Disse become irregularly indistinct and increasingly pleomorphic by 30 minutes when the plasma membrane becomes increasingly permeable to calcium. Golgi vesicles swell and discharge their granules during the period of poisoning studied. Although all the changes observed may be the result of direct interaction of carbon tetrachloride with the membranes of the cytoplasmic constituents of the liver parenchymal cell, the possibility that the irreversible changes observed in the granular endoplasmic reticulum may be due to the chemical interaction between the poison and this system is discussed.

STUDIES ON ESSENTIAL FATTY ACID DEFICIENCY. EFFECT OF THE DEFICIENCY ON THE LIPIDS IN LIVER MITOCHONDRIA AND OXIDATIVE PHOSPHORYLATION

1. Dietary deficiency of essential fatty acids results in a twofold increase in the neutral lipid content of liver mitochondria as compared with the corresponding value for stock-fed rats. 2. Deficiency produces changes in the pattern of the constituent fatty acids of the main phospholipid fractions of liver mitochondria which are similar to those previously reported for the lipids of whole liver. There is a fall in the content of C(18:2) acid and to a smaller extent of C(20:4) acid associated with a rise of C(16:1), C(18:1) and C(20:3) acids. 3. Deficiency results in small decreases in the phosphorylation quotients of liver mitochondria during oxidation of succinate and pyruvate, but the values lie within the range reported for normal mitochondria. Mitochondrial respiration with succinate is decreased as a result of deficiency but no change was observed with pyruvate as substrate.

ENZYMIC CONVERSION OF 3-HYDROXYANTHRANILIC ACID INTO CINNABARINIC ACID BY THE NUCLEAR FRACTION OF RAT LIVER

1. An enzyme solely localized in the nuclear fraction of rat liver was found to convert 3-hydroxyanthranilic acid into a red product that was isolated and crystallized from the reaction mixture. The product was identified as cinnabarinic acid (2-amino-3-oxo-3H-phenoxazine-1,9-dicarboxylic acid) by comparing its properties with synthetic cinnabarinic acid. 2. The enzyme had optimum pH at 7.2. Heavy-metal ions like Ag(+), Hg(2+), MoO(4) (2-), Fe(2+) and Cu(2+) were inhibitory; Mn(2+) activated the reaction to a considerable extent. 3. The reaction was inhibited by mercaptoethanol, GSH and cysteine, and activated by p-hydroxymercuribenzoate and sodium arsenite, which may suggest the involvement of disulphide groups in the reaction.