Liver peroxisomes, cytology and function

Biogenesis and Function of Peroxisomes in Human Disease with a Focus on the ABC Transporter

Peroxisomes are indispensable organelles in mammals including humans. They are involved in the β-oxidation of very long chain fatty acids, and the synthesis of ether phospholipids and bile acids. Pre-peroxisomes bud from endoplasmic reticulum and peroxisomal membrane and matrix proteins are imported to the pre-peroxisomes. Then, matured peroxisomes grow by division. Impairment of the biogenesis and function of peroxisomes results in severe diseases. Since I first undertook peroxisome research in Prof. de Duve's laboratory at Rockefeller University in 1985, I have continuously studied peroxisomes for more than 30 years, with a particular focus on the ATP-binding cassette (ABC) transporters. Here, I review the history of peroxisome research, the biogenesis and function of peroxisomes, and peroxisome disease including X-linked adrenoleukodystrophy. The review includes the targeting and function of the ABC transporter subfamily D.

Ethanol-induced oxidative stress: basic knowledge

After a general introduction, the main pathways of ethanol metabolism (alcohol dehydrogenase, catalase, coupling of catalase with NADPH oxidase and microsomal ethanol-oxidizing system) are shortly reviewed. The cytochrome P(450) isoform (CYP2E1) specifically involved in ethanol oxidation is discussed. The acetaldehyde metabolism and the shift of the NAD/NADH ratio in the cellular environment (reductive stress) are stressed. The toxic effects of acetaldehyde are mentioned. The ethanol-induced oxidative stress: the increased MDA formation by incubated liver preparations, the absorption of conjugated dienes in mitochondrial and microsomal lipids and the decrease in the most unsaturated fatty acids in liver cell membranes are discussed. The formation of carbon-centered (1-hydroxyethyl) and oxygen-centered (hydroxyl) radicals during the metabolism of ethanol is considered: the generation of hydroxyethyl radicals, which occurs likely during the process of univalent reduction of dioxygen, is highlighted and is carried out by ferric cytochrome P(450) oxy-complex (P(450)-Fe(3+)O(2) (.-)) formed during the reduction of heme-oxygen. The ethanol-induced lipid peroxidation has been evaluated, and it has been shown that plasma F(2)-isoprostanes are increased in ethanol toxicity.

Pxmp2 is a channel-forming protein in Mammalian peroxisomal membrane

Background

Peroxisomal metabolic machinery requires a continuous flow of organic and inorganic solutes across peroxisomal membrane. Concerning small solutes, the molecular nature of their traffic has remained an enigma.

Methods/Principal Findings

In this study, we show that disruption in mice of the Pxmp2 gene encoding Pxmp2, which belongs to a family of integral membrane proteins with unknown function, leads to partial restriction of peroxisomal membrane permeability to solutes in vitro and in vivo. Multiple-channel recording of liver peroxisomal preparations reveals that the channel-forming components with a conductance of 1.3 nS in 1.0 M KCl were lost in Pxmp2^−/− mice. The channel-forming properties of Pxmp2 were confirmed with recombinant protein expressed in insect cells and with native Pxmp2 purified from mouse liver. The Pxmp2 channel, with an estimated diameter of 1.4 nm, shows weak cation selectivity and no voltage dependence. The long-lasting open states of the channel indicate its functional role as a protein forming a general diffusion pore in the membrane.

Conclusions/Significance

Pxmp2 is the first peroxisomal channel identified, and its existence leads to prediction that the mammalian peroxisomal membrane is permeable to small solutes while transfer of “bulky” metabolites, e.g., cofactors (NAD/H, NADP/H, and CoA) and ATP, requires specific transporters.

Channel-forming activities of peroxisomal membrane proteins from the yeast Saccharomyces cerevisiae

Highly-purified peroxisomes from the yeast Saccharomyces cerevisiae grown on oleic acid were investigated for the presence of channel (pore)-forming proteins in the membrane of these organelles. Solubilized membrane proteins were reconstituted in planar lipid bilayers and their pore-forming activity was studied by means of multiple-channel monitoring or single-channel analysis. Two abundant pore-forming activities were detected with an average conductance of 0.2 and 0.6 nS in 1.0 m KCl, respectively. The high-conductance pore (0.6 nS in 1.0 m KCl) is slightly selective to cations (P(K+)/P(Cl-) approximately 1.3) and showed an unusual flickering at elevated (> +/-40 mV) holding potentials directed upward relative to the open state of the channel. The data obtained for the properties of the low-conductance pore (0.2 nS in 1.0 m KCl) support the notion that the high-conductance channel represents a cluster of two low-conductance pores. The results lead to conclusion that the yeast peroxisomes contain membrane pore-forming proteins that may aid the transfer of small solutes between the peroxisomal lumen and cytoplasm.

Peroxisomal membrane permeability and solute transfer

The review is dedicated to recent progress in the study of peroxisomal membrane permeability to solutes which has been a matter of debate for more than 40 years. Apparently, the mammalian peroxisomal membrane is freely permeable to small solute molecules owing to the presence of pore-forming channels. However, the membrane forms a permeability barrier for 'bulky' solutes including cofactors (NAD/H, NADP/H, CoA, and acetyl/acyl-CoA esters) and ATP. Therefore, peroxisomes need specific protein transporters to transfer these compounds across the membrane. Recent electrophysiological studies have revealed channel-forming activities in the mammalian peroxisomal membrane. The possible involvement of the channels in the transfer of small metabolites and in the formation of peroxisomal shuttle systems is described.

The rat liver peroxisomal membrane forms a permeability barrier for cofactors but not for small metabolites in vitro

The functional role of the peroxisomal membrane as a permeability barrier to metabolites has been a matter of controversy for more than four decades. The initial conception, claiming free permeability of the membrane to small solute molecules, has recently been challenged by several observations suggesting that the peroxisomal membrane forms a closed compartment. We have characterized in vitro the permeability of rat liver peroxisomal membrane. Our results indicate that the membrane allows free access into peroxisomes for small hydrophilic molecules, such as substrates for peroxisomal enzymes (glycolate, urate), but not to more bulky cofactors (NAD/H, NADP/H, CoA). Although access for cofactors is not prevented completely by the membrane, the membrane barrier severely restricts their rate of entry into peroxisomes. The data lead to conclusion that, in vivo, peroxisomes may possess their own pool of cofactors, while they share a common pool of small metabolites with the cytoplasm. The results also indicate that molecular size plays an important role in in vivo distinction between cofactors and metabolic intermediates.

The behavior of peroxisomes in vitro: mammalian peroxisomes are osmotically sensitive particles

It has been known for a long time that mammalian peroxisomes are extremely fragile in vitro. Changes in the morphological appearance and leakage of proteins from purified particles demonstrate that peroxisomes are damaged during isolation. However, some properties of purified peroxisomes, e.g., the latency of catalase, imply that their membranes are not disrupted. In the current study, we tried to ascertain the mechanism of this unusual behavior of peroxisomes in vitro. Biochemical and morphological examination of isolated peroxisomes subjected to sonication or to freezing and thawing showed that the membrane of the particles seals after disruption, restoring permeability properties. Transient damage of the membrane leads to the formation of peroxisomal "ghosts" containing nucleoid but nearly devoid of matrix proteins. The rate of leakage of matrix proteins from broken particles depended inversely on their molecular size. The effect of polyethylene glycols on peroxisomal integrity indicated that these particles are osmotically sensitive. Peroxisomes suffered an osmotic lysis during isolation that was resistant to commonly used low-molecular-mass osmoprotectors, e.g., sucrose. Damage to peroxisomes was partially prevented by applying more "bulky" osmoprotectors, e.g., polyethylene glycol 1500. A method was developed for the isolation of highly purified and nearly intact peroxisomes from rat liver by using polyethylene glycol 1500 as an osmoprotector.

Localization of cytosolic NADP-dependent isocitrate dehydrogenase in the peroxisomes of rat liver cells: biochemical and immunocytochemical studies

Two types of NADP-dependent isocitrate dehydrogenases (ICDs) have been reported: mitochondrial (ICD1) and cytosolic (ICD2). The C-terminal amino acid sequence of ICD2 has a tripeptide peroxisome targeting signal 1 sequence (PTS1). After differential centrifugation of the postnuclear fraction of rat liver homogenate, approximately 75% of ICD activity was found in the cytosolic fraction. To elucidate the true localization of ICD2 in rat hepatocytes, we analyzed the distribution of ICD activity and immunoreactivity in fractions isolated by Nycodenz gradient centrifugation and immunocytochemical localization of ICD2 antigenic sites in the cells. On Nycodenz gradient centrifugation of the light mitochondrial fraction, ICD2 activity was distributed in the fractions in which activity of catalase, a peroxisomal marker, was also detected, but a low level of activity was also detected in the fractions containing activity for succinate cytochrome C reductase (a mitochondrial marker) and acid phosphatase (a lysosomal marker). We have purified ICD2 from rat liver homogenate and raised a specific antibody to the enzyme. On SDS-PAGE, a single band with a molecular mass of 47 kD was observed, and on immunoblotting analysis of rat liver homogenate a single signal was detected. Double staining of catalase and ICD2 in rat liver revealed co-localization of both enzymes in the same cytoplasmic granules. Immunoelectron microscopy revealed gold particles with antigenic sites of ICD2 present mainly in peroxisomes. The results clearly indicated that ICD2 is a peroxisomal enzyme in rat hepatocytes. ICD2 has been regarded as a cytosolic enzyme, probably because the enzyme easily leaks out of peroxisomes during homogenization. (J Histochem Cytochem 49:1123-1131, 2001)

Permeability properties of peroxisomes in digitonin-permeabilized rat hepatocytes. Evidence for free permeability towards a variety of substrates

In order to investigate the permeability properties of rat-liver peroxisomes in situ, we selectively permeabilized hepatocytes with digitonin in a medium mimicking the cytosol. This system permitted us to study the latency of peroxisomal oxidases by means of measurement of their activities in permeabilized compared to disrupted hepatocytes. The activity of peroxisomal oxidases was studied using three different methods: (1) measurement of the oxidase-mediated production of H2O2 in a system containing homovanillic acid, horseradish peroxidase and azide; (2) measurement of the rate of substrate utilization or product formation; (3) measurement of the production of H2O2 via the peroxidative action of catalase in the presence of an excess of methanol. The results obtained depended on which system was used to measure the activity of the different oxidases. Our observations lead us to conclude that method 1 cannot be used for latency studies, whereas methods 2 and 3 are suitable under defined circumstances. Based on the results of methods 2 and 3, we conclude that urate oxidase, L-alpha-hydroxyacid oxidase A and D-amino acid oxidase show no structure-linked latency in digitonin-permeabilized hepatocytes, suggesting that the substrates for these enzymes permeate freely through the peroxisomal membrane.

Metabolic pathways in mammalian peroxisomes

This article summarizes our current knowledge of the metabolic pathways present in mammalian peroxisomes. Emphasis is placed on those aspects that are not covered by other articles in this issue: peroxisomal enzyme content and topology; the peroxisomal beta-oxidation system; substrates of peroxisomal beta-oxidation such as very-long-chain fatty acids, branched fatty acids, dicarboxylic fatty acids, prostaglandins and xenobiotics; the role of peroxisomes in the metabolism of purines, polyamines, amino acids, glyoxylate and reactive oxygen products such as hydrogen peroxide, superoxide anions and epoxides.

Factors influencing the latency of the peroxisomal enzyme dihydroxyacetone-phosphate acyltransferase (DHAP-AT) in permeabilized human skin fibroblasts

In selectively permeabilized fibroblasts suspended in a medium mimicking the composition of the cytosol the peroxisomal enzyme dihydroxyacetone-phosphate acyltransferase (DHAP-AT) was found to exhibit about 80% latency (Wolvetang, E.J., Tager, J.M. and Wanders, R.J.A. (1990) Biochem. Biophys. Res. Commun. 1035, 6-11). We investigated which components of the cytosol mimicking medium are important for latency of DHAP-AT and unmasking of latent DHAP-AT activity by ATP. We show that the latency of DHAP-AT is critically dependent upon the presence of reduced glutathione in the medium and that the in vivo prevailing GSH/GSSG ratio is sufficient to maintain DHAP-AT latency. Although thiol-groups in the peroxisomal membrane seem to be essential for the integrity of peroxisomes in selectively permeabilized fibroblasts no latency of DHAP-AT is observed in buffered sucrose media or in cell homogenates, irrespective of the presence of GSH in the medium used. We suggest that during homogenization irreversible damage is inflicted upon the peroxisomal membrane and/or that more factors than at present investigated are involved in maintaining peroxisomal integrity. Furthermore, we demonstrate that cations play a role in the stimulatory effect of ATP on latent DHAP-AT activity while a proton gradient is not directly involved in the stimulatory effect of ATP on latent DHAP-AT activity.

Topography of very-long-chain-fatty-acid-activating activity in peroxisomes from rat liver

We have investigated the localization of palmitoyl-CoA (hexadecanoyl-CoA) synthetase (EC 6.2.1.3) and cerotoyl-CoA (hexacosanoyl-CoA) synthetase in peroxisomes isolated from rat liver. Palmitoyl-CoA and cerotoyl-CoA synthetases, like acyl-CoA: dihydroxyacetone phosphate acyltransferase (EC 2.3.1.42), are present in the peroxisomal membrane. Trypsin treatment of intact peroxisomes led to the disappearance of both palmitoyl-CoA and cerotoyl-CoA synthetase activities but had little, if any, effect on L-alpha-hydroxy-acid oxidase (EC 1.1.3.15), D-amino acid oxidase (EC 1.4.3.3) or acyl-CoA:dihydroxyacetone phosphate acyltransferase. The latter three enzymes were inactivated if the trypsin treatment was preceeded by disruption of the peroxisomes by sonication. These results show that the active site, or at least domains essential for the activity of cerotoyl-CoA synthetase, like that of palmitoyl-CoA synthetase, is located on the cytosolic face of the peroxisomal membrane.

Latency of the peroxisomal enzyme acyl-CoA:dihydroxyacetonephosphate acyltransferase in digitonin-permeabilized fibroblasts: the effect of ATP and ATPase inhibitors

We have studied the activity of acyl-CoA:dihydroxyacetonephosphate acyltransferase (DHAP-AT) in fibroblasts treated with low concentrations of digitonin so that the cytoplasmic compartment was freely accessible to the substrates of DHAP-AT while intracellular membranes remained intact. DHAP-AT activity exhibited 70% latency under these conditions. This latency could be overcome by addition of ATP, resulting in a four-fold stimulation of DHAP-AT activity. Virtually no stimulatory effect of ATP on DHAP-AT activity was observed in sonicated fibroblasts or when a non-hydrolyzable ATP analogue was used. Furthermore the stimulatory effect of ATP was prevented in part by DCCD. N-ethylmaleimide and high concentrations of oligomycin; bafilomycin had no effect. This pattern of inhibitor sensitivity is similar to that of the ATPase activity in peroxisomal fractions from rat liver. We conclude that peroxisomes in situ exhibit structure linked latency and that ATP is required for the transport of at least one of the substrates of DHAP-AT.

Alterations in the integrity of peroxisomal membranes in livers of mice treated with peroxisome proliferators

Catalase leakage from its particulate compartment within the light mitochondrial fraction of liver was used as an index of the integrity of peroxisomes in untreated mice and in mice treated with the peroxisome proliferators clofibrate(ethyl-p-chlorophenoxyisobutyrate), Wy-14,643(4-chloro-6[2,3-xylidino)-2-pyrimidinylthio]acetic acid) and DEHP(di-(2-ethylhexyl)phthalate). Catalase leakage represented about 2% of the total catalase activity when fractions from untreated mice were incubated at 4 degrees C, increasing to about 5% during 60 min incubation at 37 degrees C. In fractions from livers of mice treated with peroxisome proliferators, catalase leakage was significantly higher, being 7-11% at 4 degrees C and increasing to approximately 20% after 60 min incubation at 37 degrees C. The pattern of release was similar for all proliferators. Parallel data were obtained for catalase latency in these fractions, i.e. following 60 min incubation at 37 degrees C, free (non-latent) catalase activity was 18% in control mice and 65, 67, and 83% in fractions from clofibrate-, Wy-14,643- and DEHP-treated mice, respectively. Differences in catalase leakage from peroxisomes in fractions from untreated mice and clofibrate-treated mice were also apparent following treatments designed to effect membrane permeabilization, as in freeze-thawing, osmotic rupture, and extraction with Triton X-100 and lysophosphatidylcholine. These data are consistent with a significant alteration in the integrity of the membranes of peroxisomes in livers of mice which have been treated with peroxisome proliferators, and furthermore indicate a commonality of effect of these agents.

Properties of the ATPase activity associated with peroxisome-enriched fractions from rat liver: comparison with mitochondrial F1F0-ATPase

Highly purified peroxisomal fractions from rat liver contain ATPase activity (18.8 +/- 0.1 nmol/min per mg, n = 6). This activity is about 2% of that found in purified mitochondrial fractions. Measurement of marker enzyme activities and immunoblotting of the peroxisomal fraction with an antiserum raised against the beta-subunit of mitochondrial ATPase indicates that the ATPase activity in the peroxisomal fractions can not be ascribed to contamination with mitochondria or other subcellular organelles. From the sensitivity of the ATPase present in the peroxisomal fraction towards a variety of ATPase inhibitors, we conclude that it displays both V-type and F-type features and is distinguishable from both the mitochondrial F1F0-ATPase and the lysosomal V-type ATPase.

Fluorometric assay of peroxisomal oxidases

The present paper deals with the adaptation of the fluorometric measurement of H2O2 originally described by Guilbault et al. (1967, Anal. Chem. 39, 271) for the assay of the peroxisomal oxidation of D-amino acids, L-alpha-hydroxyacids, uric acid, and acyl-CoA esters. The present work essentially covers three facets: (i) the general kinetics of the assay of peroxisomal oxidases and the influence of each component of the assay medium on these kinetics; (ii) the measurement of peroxisomal oxidase activities in subcellular fractions and tissues from human and untreated and clofibrate-treated rodents; and (iii) the comparison between the oxidase activities measured by the fluorometric and spectrophotometric methods.

Single-channel analysis of a large conductance channel in peroxisomes from rat liver

Native membranes and Triton X-100 solubilized integral membrane proteins of peroxisomes from rat liver were reconstituted in liposomes. With the patch clamp technique, a channel was detected with a conductance of 420 +/- 30 pS and a PK/PC1 of about 3. The channel in native membrane fractions was weakly voltage dependent, residing most of the time in an open state with the possibility to shift to different substates. Solubilization changed the kinetic properties. The channel became strongly voltage dependent and closed at voltages negative to -20 mV. The estimated diameter of the channel is about 1.7 nm and might explain, at least partially, the permeability properties of the peroxisomal membrane.

D-aspartate oxidase in rat, bovine and sheep kidney cortex is localized in peroxisomes

D-Aspartate oxidase (EC 1.4.3.1) was assayed in subcellular fractions and in highly purified peroxisomes from rat, bovine and sheep kidney cortex as well as from rat liver. During all steps of subcellular-fractionation procedures, D-aspartate oxidase co-fractionated with peroxisomal marker enzymes. In highly purified preparations of peroxisomes, the enrichment of D-aspartate oxidase activity over the homogenate is about 32-fold, being comparable with that of the peroxisomal marker enzymes catalase and D-amino acid oxidase. Disruption of the peroxisomes by freezing and thawing released more than 90% of the enzyme activity, which is typical for soluble peroxisomal-matrix proteins. Our findings provide strong evidence that in these tissues D-aspartate oxidase is a peroxisomal-matrix protein and should be added as an additional flavoprotein oxidase to the known set of peroxisomal oxidases.

Coenzyme A in purified peroxisomes is not freely soluble in the matrix but firmly bound to a matrix protein

On subfractionation of purified rat liver peroxisomes in matrical, peripheral membrane, integral membrane and core protein fractions, the endogenous peroxisomal CoA was released together with the matrix proteins. The released CoA could not be measured by an enzymatic cycling assay unless the matrix proteins were denatured by acid treatment or by heating at alkaline pH. The cofactor could not be removed by dialysis of the matrix proteins unless salt was added. It was not displaced by exogenous CoA. It migrated into sucrose density gradients together with a protein of approximately 80 kDa. The results indicate that peroxisomal CoA is firmly bound to a matrix protein and that the presence of CoA inside purified peroxisomes does not necessarily imply that the peroxisomal membrane is impermeable to this cofactor.

Distribution of alkylglycerone-phosphate synthase in subcellular fractions of rat liver

Subcellular fractions of rat liver were isolated by density-gradient centrifugation on a linear Metrizamide gradient and were assayed for marker enzymes of peroxisomes, lysosomes, microsomes and mitochondria. Alkylglycerone-phosphate synthase catalysing the formation of the ether bond in glycerolipids was also determined along the gradient. The enzyme was found to be enriched in the peroxisomal and the microsomal fractions thus, displaying a bimodal distribution pattern. Two reaction-products each, alkylglycerone phosphate and alkylglycerone were obtained in the enzymic assays performed, the ratio of which was clearly dependent upon the fraction employed. Alkylglycerone phosphate was mainly synthesized by the 'peroxisomal synthase', whereas an inverse proportion was observed assaying the microsomal counterpart. Furthermore, comparing the mean specific activities of both the enzymes the microsomal one was shown to be roughly twice as active in metabolizing 1-O-palmitoylglycerone 3-phosphate, simultaneously displaying a somewhat different sensitivity to NaF. These findings provide a first line of evidence, that two separate synthases, one in microsomes and another one in peroxisomes might be engaged in the biosynthesis of 1-O-alkyl-glycerolipids in rat liver.

Isolation and characterization of peroxisomes from the liver of normal untreated rats

The classic method of Leighton et al. [(1968) J. Cell Biol. 37, 482-513] for the isolation of peroxisomes from rat liver involves the use of Triton WR-1339 which alters the biochemical properties of this organelle and requires the specialized type Beaufay-rotor which is not easily available. We have employed Metrizamide as the gradient medium and a commercial type vertical rotor to obtain highly purified and structurally well-preserved peroxisomes from normal untreated animals. The livers were homogenized in buffered 0.25 M sucrose and a slightly modified 'light mitochondrial fraction' was prepared by differential centrifugation. This was loaded on top of a linear Metrizamide gradient (1.12-1.26 g/cm3) and subjected to an integrated force of 1.252 X 10(6) X (g X min) using a Beckman VTi 50 vertical rotor. Peroxisomes banded at the density of 1.245 g/cm3. In the isolated fraction 95% of the protein was contributed by peroxisomes, which exhibited a strong activity for cyanide-insensitive lipid beta-oxidation. The purity of fractions was also confirmed by morphometry, which revealed that 98% of isolated particles consisted of peroxisomes. The latency for catalase was about 90% indicating a high degree of peroxisomal integrity. This corresponded to the low level of extraction of catalase in 3,3'-diaminobenzidine-stained filter preparations. The entire procedure took about five hours. Highly purified and structurally well preserved peroxisomes should be useful in further elucidation of the function of this organelle and especially in studies of peroxisomal enzymes with multiple intracellular localizations.

Water- and solute-accessible spaces of purified peroxisomes. Evidence that peroxisomes are permeable to NAD+

Peroxisomes were purified from liver homogenates from rats, treated with the peroxisome proliferator clofibrate, by a combination of differential centrifugation and isopycnic centrifugation in iso-osmotic self-generating Percoll gradients. Structural integrity of the peroxisomes appeared to be preserved as evidenced by a high degree of catalase latency, the absence of catalase release during purification and the exclusion of inulin (mol.wt. +/- 5000). Spaces for water and solutes were measured after incubation of the peroxisomes in iso-osmotic sucrose with radioactive water or solutes and separation of the organelles from their media by centrifugation through an organic layer. Extraperoxisomal water was corrected for by the use of radioactive dextran or inulin. The sucrose, glucose, urea, methanol and acetate-accessible spaces were identical, suggesting that these spaces represent the volume in which molecules that can cross the membrane distribute. This volume equalled 50-65% of the water space. Urate and NAD+, a cofactor of peroxisomal beta-oxidation of fatty acids, also distributed in this volume, but were also partly bound. Urate and NAD+ binding was not abolished by sonication, which released the bulk of matrix catalase activity, but NAD+ binding was seriously diminished. The peroxisomal water and sucrose spaces were estimated to be 107 microliters and 55 microliters per g of liver tissue from a clofibrate-treated rat. From quantitative morphometric data [Anthony, Schmucker, Mooney & Jones (1978) J. Lipid Res. 19, 154-165] and our marker enzyme analyses, as well as from our experimentally determined water spaces of mitochondrial and microsomal fractions, it could be calculated that the volume contamination by lysosomes, mitochondria and microsomes did not exceed 1, 8 and 6% respectively. Our data indicate that apparently intact peroxisomes are permeable to a number of small molecules, including NAD+. Whether the NAD+-binding sites in sonicated peroxisomes mirror the likely existence of a membrane carrier requires further investigation.

Subcellular localization of chicken liver xanthine dehydrogenase. A possible source of cytoplasmic reducing equivalents

Classical fractionation studies showed that xanthine dehydrogenase (EC 1.2.1.37) was exclusively cytosolic in chicken liver. Fumarase (EC 4.2.1.2) and malate dehydrogenase (EC 1.1.1.37) were also found to have major cytosolic locations. These data indicate that urate synthesis in chicken liver produces substantial quantities of cytoplasmic NADH which may supply reducing equivalents of gluconeogenesis and other processes.

Analytical fractionation of cultured hepatoma cells (HTC cells)

Homogenates of HTC cells have been fractionated by differential centrifugation (in four particulate fractions: N, M, L, P, and a supernatant S) or isopycnic banding in linear sucrose gradients. On this basis, the following subcellular organelles may be characterized: (i) Mitochondria, detected by cytochrome oxidase and succinodehydrogenase, are collected in the M and L fractions, and equilibrate, as a narrow band, at a median buoyant density of 1.18 g/cm3. (ii) Lysosomes, detected by the latent hydrolases beta-glycerophosphatase and N-acetyl-beta-glucosaminidase, are largely sedimented in the M and L fractions, and display a broad density distribution pattern with a median value of 1.17 g/cm3. This density is decreased or increased after cultivation of the cells in presence of Triton WR-1339 or Dextran 500, respectively. The behavior of cathepsin D is somewhat at variance with that of the two other hydrolases. (iii) Plasma membrane is tentatively detected by alkaline phosphodiesterase I. Largely recovered in the P fraction, this enzyme equilibrates at a median density close to that of the lysosomal hydrolases; the bulk of cholesterol and about half of the leucyl-2-naphthylamidase are closely associated with alkaline phosphodiesterase I; HTC cells do not contain typical 5'-nucleotidase. (iv) Catalase-bearing particles, of high buoyant density (1.22 g/cm3) are present, but 30-40% of the catalase is also found readily soluble. NADPH- and NADH: cytochrome c reductase, and RNA show more complex distributions. It is suggested that the former enzyme is associated with the endoplasmic reticulum; as in liver, NADH reductase activity is shared between the endoplasmic reticulum and the mitochondria; half of the RNA is associated with free ribosomes of polysomes. True glucose-6-phosphatase could not be detected.

The peroxisome (microbody) membrane: effects of detergents and lipid solvents on its ultrastructure and permeability to catalase

The effects of detergents, organic lipid solvents, and several adjuvants used in cell fractionation on the ultrastructure of the peroxisomal (microbody) membrane and its permeability to catalase have been investigated. Chopper sections of glutaraldehyde-fixed liver were incubated in the presence of various agents, followed by cytochemical staining for catalase and processed for electron microscopy. Catalase activity was also determined biochemically in the incubation medium. Marked catalase diffusion was found after treatment with 1% or 0.5% Triton X-100 or deoxycholate, as well as with 50% ethanol or acetone or 20% propanol or t-butanol. In contrast, 1% digitonin and lower concentrations of the above agents, as well as sucrose or glycerine caused selective diffusion of catalase from a limited population of peroxisomes. Treatment with 10% polyvinylpyrrolidone (PVP), which has been used as a protective agent in the isolation of microbodies, did not produce any alteration in the fine structure and cytochemical appearance of peroxisomes. These findings concur with earlier biochemical studies on freshly isolated peroxisomes and demonstrate the susceptibility of microbodies, even in glutaraldehyde-fixed rat liver to the effects of various agents which affect the microbody membrane. A close correlation between the ultrastructural integrity of the microbody membrane and its permeability to catalase has been found. The significance of these observations for the assessment of the permeability characteristics of the microbody membrane is discussed.

Cytochemical discrimination between catalases and peroxidases using diaminobenzidine

The influence on diaminobenzidine staining of four variables: prefixation in aldehyde, temperature and pH of incubation, and H2O2 concentration, was investigated in catalase-, as well as in peroxydase-containing material. Catalase from five different sources and five types of peroxidase were examined. It is concluded: (a) when cells are incubated without prior fixation, in a DAB medium at room temperature and pH 7.3 with 0.003% H2O2, peroxidases produce a visible cytochemical stain, while catalases do not; (b) the cytochemical reaction elicited by catalases is stimulated by prior aldehyde fixation in specified conditions, and incubation at 45 degrees C and pH 9.7 with 0.06% H2O2; (c) under the latter circumstances several peroxidases also stain. Ultrastructural preservation is satisfactory in tissues incubated prior to fixation.

Ultrastructural localization of L-alpha-hydroxy acid oxidase in rat liver perioxisomes

The localization of L-alpha-hydroxy acid oxidase in rat liver peroxisomes was studied using slight modifications of the Shnitka and Talibi (1971) method. Best results were obtained with formaldehyde fixation and incubation with glycolate as substrate. Following incubation the copper ferrocyanide reaction product was amplified with 3,3'-diamino-benzidine according to Hanker et al. (1972a,b). Dense reaction product was visible in hepatocyte peroxisomes by light and electron microscopy. Some diffusion of enzyme and/or reaction product into the adjacent cytoplasm occurred around the peroxisomes. Apparent non-specific deposits occurred on the plasmalemma, in the nucleus, and occasionally over mitochondria. Glutaraldehyde fixation severely inhibited enzymatic activity, and the enzyme showed less activity toward L-lactate and DL-alpha-hydroxybutyrate.