A sub-population of rat liver membrane-bound ribosomes that are detached in vitro by carcinogens and centrifugation

4-Dimethylaminoazobenzene interferes with the binding of ribosomes to reticular membranes

Treatment of chemically degranulated rough endoplasmic reticulum (RER) with 4-dimethylamino-azobenzene (4-DAB) partly damages binding of fresh polyribosomes to both the lighter and heavier fractions of reconstituted RER. Also, treatment of ribosomes with 4-DAB seemed to inhibit their binding to the chemically stripped RER (DRER) appearing both as lighter and heavier rough membrane layers after reconstitution. The interaction of 4-DAB with stripped microsomal fraction and ribosomes might be introducing certain conformational constraints into their proteins, thus crippling the binding of the ribosomes to reticular membranes.

Lipid peroxidation and mechanisms of toxicity

Aerobic organisms by definition require oxygen, and the importance of iron in aerobic respiration has long been recognized, but despite their beneficial roles, these elements can pose a real threat to the organism. During oxygen reduction, reactive species such as O2-. and H2O2 are formed readily. Iron can combine with these species, or with molecular oxygen itself, to generate free radicals which will attack the polyunsaturated fatty acids of membrane lipids. This oxidative deterioration of membrane lipids is known as lipid peroxidation. To protect itself against this form of attack, the organism possesses several types of defense mechanisms. Under normal conditions, these defenses appear to offer adequate protection for cell membranes, but the possibility exists that certain foreign compounds may interfere with or even overwhelm these defenses, and herein could lie a general mechanism of toxicity. This possible cause of toxicity is discussed in relation to other suggested causes.

Microsomal membrane subfractionation by a lectin affinity method

Concanavalin A-agarose treatment of rat liver post-mitochondrial supernatant removes a fraction rich in cholesterol and 5'-nucleotidase activity but low in glucose-6-phosphatase. At the same time, radiolabel associated with the cell surface is removed. We interpret these findings as evidence that concanavalin A binds to, and under these circumstances will remove, fragments of plasma membrane present in the microsomal fraction and believe that this may be of use in the gentle, and rapid subfractionation of microsomal membranes.

Efficient prediction of chemical carcinogenicity by microsomal degranulation

Microsomes prepared at low 'g' force (10 000) have been used to evaluate the carcinogenicity of 25 coded and 14 noncoded compounds to validate the microsomal degranulation technique for its use as one of the tests in a battery of 5 to 6 tests. The method correctly predicted 35 compounds out of 39 randomly selected carcinogens and non-carcinogens showing a higher predictive efficiency in comparison to that achieved by using rough endoplasmic reticulum prepared at high 'g' forces (105 000 X g or above). The use of this technique is advocated on a large scale for the screening of environmental carcinogens.

Preparation and properties of a complex from rat liver of polyribosomes with components of the cytoskeleton

Gel filtration with 1% agarose (Bio-Gel A-150m) separates polyribosomes bound to microsomal membranes from 'free' polyribosomes when these fractions are prepared by standard centrifugal techniques. However, when polyribosomes contained in an unfractionated postmitochondrial supernatant are run on an identical column, over 90% of the total polyribosomes are present as aggregates, designated 'membrane-cytomatrix', which are eluted in the column void volume. Polyribosomes are not released from these aggregates on removal of microsomal phospholipids by treatment of postmitochondrial supernatant with 1% Triton X-100, a neutral detergent. The aggregates are disrupted by the usual ultracentrifugation techniques used in subcellular fractionation. After treatment of membrane-cytomatrix with Triton X-100 to remove phospholipids and membrane proteins, 58% of the polyribosomes still remain associated with protein-containing complexes in the form of a cytomatrix and are not 'free'. Preparations of both membrane-cytomatrix and cytomatrix are capable of sustained protein synthesis. Sodium dodecyl sulphate/polyacrylamide-gel electrophoresis revealed that the cytoskeletal proteins actin and myosin are present in the cytomatrix. Incubation of cytomatrix preparations with the actin-depolymerizing agent deoxyribonuclease I caused release of the polyribosomes. Polyribosome release by deoxyribonuclease I was prevented by prior incubation with phalloidin, which is known to stabilize F-actin. Thus polyribosomes are associated with cytoskeletal elements in rat liver, and this association is dependent on polymeric forms of actin.

Studies on the degranulation test for carcinogens

The radiometric assay of degranulation of the hepatic endoplasmic reticulum by chemical carcinogens has been re-examined. Both 1,2,3,4,- and 1,25,6-dibenzanthracenes caused degranulation of rough membranes in vitro; with acetamidofluorenes and naphthylamines the carcinogenic analogues caused moderately greater degranulation. Degranulation by 1,2,3,4-dibenzantracene was rapid and was maximal after 5 min incubation. Pretreatment of animals with phenobarbital or methylcholanthrene increased the fraction of rough membranes, but these were not fully granulated. The assay is limited in specificity and sensitivity because the 1.35 M sucrose gradient does not effectively separate rough and smooth membranes, and sedimented membranes are contaminated with aggregates of free ribosomes.

A gel filtration approach to the study of ribosome-membrane interactions

1. Gel filtration on agarose can be used to investigate ribosome-membrane interactions without exposing the materials to the high, and possibly perturbing, hydrostatic pressures experienced during centrifugation procedures to separate free ribosomes from membrane vesicles. 2. After treatment of microsomes with degranulating agents, degranulated membranes are isolated from Sepharose 2B columns at the void volume, while displaced ribosomes elute at the total column volume. This provides a convenient method for monitoring degranulation in vitro. 3. Centrifugation of rough microsomes or ribosomes into dense pellets or layers, followed by resuspension, leads to preparations which will not pass rapidly or quantitatively through Sepharose 2B columns. 4. Methods are described for the isolation of degranulated microsomes and ribosomes which are eluted rapidly from Sepharose 2B at the void volume and total column volume, respectively. These materials are suitable for the investigation of ribosome-membrane binding in vitro, using a gel filtration separation to monitor binding. 5. Incubation of 3H-labelled ribosomes with degranulated microsomes in vitro, leads to specific binding, demonstrated by the elution of the bound ribosomes at the void volume.

Synthesis and secretion of albumin in rats during treatment with a carcinogenic dose of N-2-acetylaminofluorene

The chronic administration of N-2-acetylaminofluorene (N-2-AAF) to rats causes a loss of hepatic cytoplasmic RNA, particularly from the endoplasmic-membrane fractions. At the end of the complete carcinogenic dose, the level of amino-acid incorporation into proalbumin is normal, despite the loss of 35% of membrane-bound RNA. The secretion of albumin, however, is inhibited. This inhibition of secretion is apparently the result of a change in membrane flow and differentiation; transfer of nascent protein from smooth-surfaced vesicles to the Golgi apparatus is blocked. The significance of these findings is discussed.

The separation of microsomal membranes and free polyribosomes by gel filtration

The integrity of polyribosome-membrane complexes in microsomal preparations is dependent on the centrifugal conditions used for their preparation. This paper describes techniques based on gel filtration that enable the rapid and gentle separation of microsomal membranes from both free polyribosomes and soluble protein.