The metabolism of isolated rat-liver nuclei

AN ELECTRON MICROSCOPE STUDY OF CALF THYMUS NUCLEAR PREPARATIONS ISOLATED IN SUCROSE SOLUTIONS

This work was carried out with the intent of developing a method capable of routinely evaluating calf thymus nuclear preparations with the electron microscope. Examination of small random samples, pre-embedded in agar after fixation with permanganate, were found to give results comparable to those obtained with much larger samples withdrawn randomly from pellets and embedded and sectioned conventionally. Results obtained by this pre-embedding technique with acrolein, osmium tetroxide, or permanganate fixations were equivalent. Calf thymus nuclear preparations isolated in sucrose by the method prevalently used (see 1) are contaminated only slightly with intact cells, to a degree which varies with each preparation. However, intact cells, damaged cells, or nuclei with some cytoplasm constitute together about 30 per cent of the preparation. Particles other than intact cells are not readily distinguishable from one another by light or phase microscope techniques. These preparations can be purified further by centrifuging through a dense sucrose layer. In our hands, however, contamination with some cytoplasm still remains in approximately 10 per cent of the particles. Incubation of the particles prepared without purification procedures, under conditions frequently used, results in the extensive breakdown of particles. Under at least one set of conditions, nuclei are selectively disrupted, leaving primarily damaged cells in the preparation.

Properties of mammalian nuclear-envelope nucleoside triphosphatase

The nucleoside triphosphatase activities of the nuclear envelopes from rat liver, pig liver and simian-virus-40-transformed mouse-embryo 3T3 cells were shown to exhibit similar parperties. All three preparations hydrolyse ATP, 2'-dATP, 3'-dATP, GTP, CTP and UTP in the presence of Mg2+, Ca2+, Mn2+ and Co2+ with a pH optimum of 8.0, are sensitive to inhibition by mercurials, arsenicals, quercetin, proflavin and adenosine 5'-[gamma-thio]triphosphate and are partially inactivated by exposure to high ionic strength. The kinetic behaviour is similar for all substrates irrespective of the source of material. The typical Eadie-Hofstee plot, which is concave upwards at pH 8.0 when the ionic strength is 20mM, becomes linear when the pH is increased to 8.5 or the ionic strength to 160mM. The overall evidence, particularly the labelling of only one polypeptide by [gamma-32P]ATP, suggests that under the conditions of preparation and assay used only one class of nucleoside triphosphatase active sites is detectable in nuclear envelopes. The importance of these results for an understanding of the role of the enzyme in vivo is discussed.

Ultrastructure of mitochondria-containining nuclei in human myocardial cells

The ultrastructure of myocardial cell nuclei was examined in a group of 12 patients undergoing cardiopulmonary bypass surgery. In one patient (aged 60 years) with rheumatic heart disease, myocardial cells were observed which contained mitochondria with well preserved membranes within their nuclei. Such intranuclear mitochondria were observed with equal frequency in all of the three stages examined, i.e. at the start and end of aortic crossclamping and after 20 min of reperfusion. Approximately 2-3% of the total number of nuclei studied in this patient contained mitochondria. A partial disintegration of the membranes of the affected nuclei was sometimes seen. These findings are discussed in relation to the hypothesis of a de novo synthesis of mitochondria inside the nucleus, and also, in relation to the "trapping theory" of mitochondria through the nuclear envelope.

Molecular characteristics of some carcinogenic hydrocarbons

The chemical reactivity and presumably the carcinogenic activity induced in aromatic hydrocarbons by hydroxylating enzymes may be due to the generation of electrophilic centers in some (benzo[a]pyrene) or nucleophilic centers in others (7,12-dimethylbenz[a]anthracene or 3-methylcholanthrene). These centers, which are at positions complementary to the points of activation by acid-derived protons (models of the positive oxygen of the hydroxylating enzymes), may react simultaneously with corresponding cellular components.

The effect of tannic acid on the synthesis of protein and nucleic acid by rat liver

1. As early as 1hr. after the intraperitoneal administration of tannic acid to rats, it could be demonstrated in the liver. At 3hr. the nuclear fraction contained the largest amount of tannic acid. 2. Nuclear RNA synthesis was inhibited in vivo 2hr. after the administration of tannic acid. Induction by cortisol of tryptophan pyrrolase was 90% inhibited at 24hr. 3. Incorporation of [1-(14)C]leucine into protein by liver slices from treated rats was decreased by 50% after 24hr. Its incorporation into postmitochondrial supernatant from treated animals was not inhibited. Incorporation into slices and postmitochondrial supernatants were inhibited in vitro by tannic acid. 4. The sequence of events: concentration of tannic acid in nuclei, inhibition of nuclear RNA synthesis, inhibition of protein synthesis and production of necrosis, is discussed.

The hepatotoxic action of allyl formate

The hepatotoxic action of allyl formate on rat liver has been investigated. Biochemical changes can be detected in the liver cell many hours before the histological changes and it would appear that the toxin has a direct action on the liver parenchymal cell. The results suggest that allyl formate is not the toxic agent but that it is converted via allyl alcohol into acrolein. This reaction requires the presence of alcohol dehydrogenase. Histochemical studies have shown that this enzyme is localized in the periportal region of the liver lobule, and may explain why allyl formate solely produces a periportal necrosis. As glutathione and 1,4-dithiothreitol protect against the early biochemical changes produced by the poison, it is probable that acrolein alkylates proteins and nucleic acids.

The effect of the aflatoxins B1, G1, and G2 on protein and nucleic acid synthesis in rat liver

A comparison has been made of the difference spectra obtained by causing various aflatoxins (B(1), G(1) and G(2)) to interact with calf-thymus DNA. The effect of these toxins on RNA and protein synthesis by rat-liver slices has been measured. The extent of their inhibitory action on the synthetic reactions was proportional to the degree of spectral shift obtained with their interaction with DNA. It is proposed that their toxicity depends on this interaction. It was demonstrated that the RNA polymerase of nucleoli isolated from the livers of aflatoxin B(1)-poisoned rats was inhibited. This finding is in agreement with the proposed mechanism for the hepatotoxic action of aflatoxin.

The action of aflatoxin B1 on the rat liver

The administration of a single dose of aflatoxin B(1) to the rat (7mg./kg. body wt.) results in the slow development of a periportal necrosis. Hepatic enzymes are released into the serum in the second 24hr. of the poisoning, closely preceding the onset of the necrosis, which is followed by a rise in serum alkaline-phosphatase activity and bilirubin concentration. Aflatoxin B(1) has been detected in the nucleus of the poisoned liver cell and in vitro it has been shown to interact with DNA. The toxin inhibits the production of nuclear RNA, probably by preventing the transcription of DNA by the RNA polymerase. It is proposed that the interaction of the toxin with DNA gives rise to its inhibitory action on mitosis and its necrogenic action.