Effects of Phenobarbital on the Synthesis and Degradation of the Protein Components of Rat Liver Microsomal Membranes

Further observations on ultrastructural changes in hepatocytes of mice infected with Schistosoma mansoni

The effect of hepatic granulomas initiated by eggs of Schistosoma mansoni on the ultrastructure of hepatocytes of murine hosts was studied. Specimens of infected livers were collected at half week intervals, starting at week 7 postinfection and terminating at week 9 postinfection. Only the hepatocytes adjacent to granulomas showed any alteration in structure. The most obvious change was the proliferation of smooth-surfaced endoplasmic reticulum, especially striking in samples collected 8 1/2 and 9 weeks postinfection. The hypothesis was presented that the material secreted by the egg of the organism might be responsible for what appeared to be a morphologic detoxification response by the hepatocytes. Other alterations evident in the hepatocytes were an increase in the lysosomal population, mitochondrial changes and a slight hypertrophy of the Golgi complexes. Previous related studies by other investigators were explored and the findings compared with the results of this study.

Mutagenic activation of 2-acetylaminofluorene by guinea-pig liver homogenates: essential involvement of cytochrome P-450 mixed-function oxidases

2-Acetylaminofluorene (AAF) was highly mutagenic to Salmonella typhimurium strain TA98, when activated by a liver post-mitochondrial supernatant fraction (S9 fraction) from guinea-pigs, in spite of the resistance of this species to AAF carcinogenesis and the low capacity of the liver of this species for N-hydroxylation of AAF. The mutagenicity was comparable to or higher than that resulting from activation by mouse- or rat-liver S9 fraction, and was not enchanced by treatment with cytochrome P-450 inducers, a combination of phenobarbital and 5,6-benzoflavone. In an attempt to understand this unexpected result we examined whether a cytochrome P-450 mixed-function oxidase system participated in the mutagenic activation of AAF by guinea-pig liver, as it does in the case of mouse liver. The mutagenic activation was: (1) completely dependent on the addition of a co-factor, NADPH, to the mutation assay system, (2) completely suppressed by antiserum against NADPH--cytochrome c reductase, and (3) sensitive to a cytochrome P-450 inhibitor, 7,8-benzoflavone. These results indicate that the cytochrome P-450 enzyme system is essentially involved even in the mutagenic activation of AAF by guinea-pig-liver S9 fraction. Based on both the present and other data, the mechanism of the mutagenic activation is discussed to explain the observed high mutagenic potential of AAF in the presence of guinea-pig-liver S9 fraction.

Studies on the cellular toxicity of polychlorinated biphenyls (PCBs). I. Effect of PCBs on microsomal enzymes and on synthesis and turnover of microsomal and cytoplasmic lipids of rat liver- a morphological and biochemical study

The acute effects of the PCB (polychlorinated biphenyls) mixture (Aroclor 1254) on microsomal enzymes and on synthesis and turnover of microsomal and cytoplasmic lipids of rat liver were investigated. Six daily i.p. injections of 25 and 50 mg PCB/kg body weight resulted in increased liver weight and liver to body weight ratios. When compared to controls PCB treatment resulted in a six-fold increase in amount of cytochrome P-450. Activities of NADPH-cytochrome c reductase, ethylmorphine demethylase and inosine diphosphatase were increased whereas glucose-6-phosphatase values were decreased by PCB exposure. Analysis of liver homogenate and microsomal fraction revealed an increase in lipid in PCB-exposed animals. Phospholipids, cholesterol and triglyceride were significantly increased after PCB exposure; however, the greatest percentage increase was seen in the triglyceride pool. The finding of an increase in microsomal triglyceride to phospholipid ratios with exposure to PCB is suggestive of an increase in membrane-enclosed lipid (liposomes). Studies with labelled glycerol indicated that the PCB-induced fatty liver resulted from increased half life but not increased synthesis of liver lipid moieties. The rate of incorporation of leucine into microsomal membrane and albumin was somewhat enhanced in rats exposed to PCB indicative of increased protein synthesis. Morphological studies showed increased occurrence of lipid material, both in cytoplasmic droplets and within rough and smooth-surfaced endoplasmic reticulum. Proliferation of smooth endoplasmic reticulum and flattened Golgi cisternae with no secretion granules containing lipoprotein particles characterized the liver from animals exposed for 6 days. The increase in lipid within membranes of the endoplasmic reticulum together with the flattened Golgi lacking typical secretory vesicles indicates a defect in transport of lipoproteins from the endoplasmic reticulum to the Golgi apparatus and may be the cause of the PCB-induced fatty liver.

Intramembraneous localization of rat liver microsomal hexose-6-phosphate dehydrogenase and membrane permeability to its substrates

A method for purifying hexose-6-phosphate dehydrogenase (beta-D-glucose: NAD(P) -oxidoreductase, EC 1.1.1.47) from rat liver microsomes is described. The purified enzyme was shown to be homogeneous by sodium dodecyl sulfate (SDS)-polyacrylamide electrophoresis. It is shown that the enzyme is bound to the inner surface of microsomal membranes, and that glucose 6-phosphate, but not NADP, penetrates almost freely into the membranes at 37 degrees C.

Induction of liver lysosomal enzymes during the autophagic phase following phenobarbital treatment of rat

Phenobarbital was given to male rats as a single injection and as repetitive injections for 7 days. The effects of treatment on the lysosomal hydrolases acid phosphatase, cathepsin D, and aryl sulfatase were analyzed at different intervals ranging from 1 to 15 days after seven injections, and from 1 to 48 h after a single injection. In both cases, microsomal protein and NADPH-cytochrome c reductase were measured to ensure proper induction. After a single injection, a slight decrease in hydrolytic activities was observed. Repetitive administration of phenobarbital gave rise to a marked decrease of lysosomal enzyme activities 1 day after cessation of treatment. This decrease was followed by a continuous increase in activity up to day 3 and 4. One or 2 weeks after treatment, enzyme activities declined to control values. The increase in activity of lysosomal hydrolytic enzymes was correlated with the onset of induced autophagy of endoplasmic reticulum membranes described as occurring in liver upon cessation of phenobarbital exposure. It is concluded that phenobarbital treatment per se decreases lysosomal enzyme activities, whereas the induced autophagy following cessation of exposure is associated with enhanced levels of lysosomal hydrolases in rat liver.

Localization of enzymes in specialized regions of the microsomal membrane

Microsomal vesicles were centrifuged through sucrose density gradients containing deoxycholate. With 0.15% detergent electron transport enzymes and phosphatases could be separated. Increasing the deoxycholate concentration to 0.19% resulted in separation of the microsomal material into five bands containing (in order from the top of the gradient) adenosine monophosphatase, inosine diphosphatase and some glucose-6-phosphatase (band 1); NADH-linked (band 2) and NADH-linked (band 3) electron transport enzymes; and glucose-6-phosphatase (bands 4 and 5). It appears that enzymes are arranged in specialized patches in the microsomal membrane.

Enzyme histochemistry as a link between biochemistry and morphology

The presented paper describes the role of enzyme histochemistry in cell biological investigations. In the first chapter a general discussion has been given about enzyme histochemistry as a connecting link between biochemistry and morphology. The methods available for determination of enzymes in a particular cell or cell compartment have been reviewed. In this respect the characteristics of enzyme histochemistry have been discussed. Furthermore, attention has been paid to the possibilities and limitations of enzyme histochemistry. In chapter two a comparison has been made between histochemically judged and biochemically determined enzyme activities. Some fundamental differences between the biochemical and the histochemical approach in cell biological investigations are dealt with. To correlate histochemically and biochemically determined enzyme activities, a description has been given of the application of histochemical methods on isolated fractions and sucrose-ficoll gradients of these fractions. Several experimental results are described concerning the question whether a relation exists between histochemically and biochemically determined activities of respectively alkaline phosphatase, glucose-6-phosphatase, 5'-nucleotidase and 3ss-hydroxysteroid dehydrogenase. From these results the conclusion could be drawn that in general a good correlation exists between histochemically judged activity per volume (area X thickness) and biochemically determined activity per gram tissue. In chapter three the role of enzymes as markers of cellular particles and as parameters of metabolic pathways is described. Histochemical methods are available for most marker enzymes. Only activities of key enzymes can be regarded as parameters of metabolic pathways. The distribution in sucrose-ficoll gradients of enzymes, regarded as markers of mitochondria, lysosomes, endoplasmic reticulum and plasma membranes has been given. The changes occur ing under different experimental conditions for a number of marker enzymes in rat liver are described. Attention has been given to the contibution of enzyme histochemistry in the study of the heterogeneity of mitochondria, the dual localization of some (lysosomal) enzymes, the complexity of the microsomal fraction, the function of the Golgi apparatus and the heterogeneity and function of plasma membranes. Based on these results and on literature findings the possible role of some marker enzymes in cell metabolism has been discussed. In chapter four problems coherent with species and sex differences in enzyme activities are described. The interpretation of histochemical and biochemical results in view of these differences is discussed. Enzymes characteristic for a given cell type -3ss-hydroxysteroid dehydrogenase in steroid producing cells, ATP-ase in liver plasma membrane surrounding the bile canaliculi - do show less variations between species and sexes than enzymes not directly involved in specialized functions...

Effects of phenobarbital on protein synthesis and polysome levels in rat liver

Administration of phenobarbital to rats increases the rate of synthesis of certain microsomal drug-metabolizing enzymes in a selective manner and promotes proliferation of smooth endoplasmic reticulum in the liver. Phenobarbital increased a number of factors by which protein synthesis could be enhanced in the liver. It produced a 30% increase in the amount of ribosomes and mRNA per cell. The proportion of ribosomes associated with polysomes was increased by 5-10% over normal liver. There was a 10-30% increase in the rate of ploypeptide elongation and a small increase or no change in polysome size, indicating that the rate of polypeptide initiation was increased proportionately. The product of these effects accounts for the 1.5-fold increase in the rate of total protein synthesis previously reported. The average polysome size, and the size of free polysomes in particular, was maintained when actinomycin D was administered to phenobarbital-pretreated rats, suggesting that the rate of mRNA degradation was decreased selectively. Phenobarbital did not, however, affect the distribution of ribosomes between the free and membrane-bound states or the activity of ribonucleases associated with isolated free and bound polysomes. Thus, we conclude that phenobarbital stimulates protein synthesis by expanding the mRNA pool, at least partially through effects on mRNA degradation, and by augmenting the rate of mRNA translation.

Reduced response to phenobarbital by the liver of rats fed a choline-deficient diet

1. The response of the liver to phenobarbital administration was compared in rats fed either laboratory chow, a semipurified choline-supplemented diet or a semipurified choline-deficient diet. 2. The liver contents of proteins, lipids and cytochrome P-450, as well as the activity of aminopyrine and ethylmorphine demethylases, were measured after 5 days of feeding and five daily injections of phenobarbital. Liver sections were examined electron microscopically. 3. In rats fed the choline-supplemented diet, phenobarbital administration caused increases in cellular constituents, enzyme activities and smooth endoplasmic reticulum membranes as great as those seen in rats fed laboratory chow. However, in rats fed the choline-deficient diet, the response of the liver to phenobarbital administration was severely reduced in comparison to that in rats fed the other diets. 4. It is concluded that dietary choline and an adequate synthesis of lecithins are necessary for the induction of microsomal mixed-function oxidases and the concomitant accumulation of smooth endoplasmic reticulum in hepatocytes.