Electron microscopic cytochemical localization of alpha-hydroxyacid oxidase in rat liver. Association with the crystalline core and matrix of peroxisomes

Peroxisome determination in optical microscopy: a useful tool derived by a simplification of an old ultrastructural technique

Peroxisomes are able to respond to changes in the cellular environment by adapting their number, morphology and metabolic functions. Recently interest in peroxisomes and their possible roles in physiological and pathological processes have significantly increased. In order to identify peroxisomes, several cytochemical techniques have been developed that require fairly complex procedures or are too expensive to be used for screening. In this paper we show that it is possible to label peroxisomes in several cell lines and in tissues by a simple and cheap technique based on 3,3'-diaminobenzidine (DAB) reactivity. The number of peroxisomes detected with this technique in each cell line was similar to that shown by catalase immunoreaction. The technique appears specific because it was able to detect increased number of peroxisomes after treatment with the specific PPARγ antagonist G3335. Gomori's technique for acid phosphatase activity was used to demonstrate that the DAB positive organelles were not lysosomes. The DAB technique has also been applied to transmission electron microscopy, where it labels round structures that are identified as peroxisomes on the basis of morphology, size and localization. The DAB technique has proved to be specific, simple, fast and cheap, which make it ideal to screen possible peroxisome changes in physiological and pathological conditions.

Be different--the diversity of peroxisomes in the animal kingdom

Peroxisomes represent so-called "multipurpose organelles" as they contribute to various anabolic as well as catabolic pathways. Thus, with respect to the physiological specialization of an individual organ or animal species, peroxisomes exhibit a functional diversity, which is documented by significant variations in their proteome. These differences are usually regarded as an adaptational response to the nutritional and environmental life conditions of a specific organism. Thus, human peroxisomes can be regarded as an in part physiologically unique organellar entity fulfilling metabolic functions that differ from our animal model systems. In line with this, a profound understanding on how peroxisomes acquired functional heterogeneity in terms of an evolutionary and mechanistic background is required. This review summarizes our current knowledge on the heterogeneity of peroxisomal physiology, providing insights into the genetic and cell biological mechanisms, which lead to the differential localization or expression of peroxisomal proteins and further gives an overview on peroxisomal biochemical pathways, which are specialized in different animal species and organs. Moreover, it addresses the impact of proteome studies on our understanding of differential peroxisome function describing the utility of mass spectrometry and computer-assisted algorithms to identify peroxisomal target sequences for the detection of new organ- or species-specific peroxisomal proteins.

Purification and characterization of recombinant human liver glycolate oxidase

Glycolate oxidase, an FMN-dependent peroxisomal oxidase, plays an important role in plants, related to photorespiration, and in animals, where it can contribute to the production of oxalate with formation of kidney stones. The best studied plant glycolate oxidase is that of spinach; it has been expressed as a recombinant enzyme, and its crystal structure is known. With respect to animals, the enzyme purified from pig liver has been characterized in detail in terms of activity and inhibition, the enzyme from human liver in less detail. We describe here the purification and initial characterization of the recombinant human glycolate oxidase. Its substrate specificity and the inhibitory effects of a number of anions are in agreement with the properties expected from previous work on glycolate oxidases from diverse sources. The recombinant enzyme presents an inhibition by excess glycolate and by excess DCIP, which has not been documented before. These inhibitions suggest that glycolate binds to the active site of the reduced enzyme, and that DCIP also has affinity for the oxidized enzyme. Glycolate oxidase belongs to a family of l-2-hydroxy-acid-oxidizing flavoenzymes, with strongly conserved active-site residues. A comparison of some of the present results with studies dealing with other family members suggests that residues outside the active site influence the binding of a number of ligands, in particular sulfite.

Peroxisomes and oxidative stress

The discovery of the colocalization of catalase with H2O2-generating oxidases in peroxisomes was the first indication of their involvement in the metabolism of oxygen metabolites. In past decades it has been revealed that peroxisomes participate not only in the generation of reactive oxygen species (ROS) with grave consequences for cell fate such as malignant degeneration but also in cell rescue from the damaging effects of such radicals. In this review the role of peroxisomes in a variety of physiological and pathological processes involving ROS mainly in animal cells is presented. At the outset the enzymes generating and scavenging H2O2 and other oxygen metabolites are reviewed. The exposure of cultured cells to UV light and different oxidizing agents induces peroxisome proliferation with formation of tubular peroxisomes and apparent upregulation of PEX genes. Significant reduction of peroxisomal volume density and several of their enzymes is observed in inflammatory processes such as infections, ischemia-reperfusion injury and hepatic allograft rejection. The latter response is related to the suppressive effects of TNFalpha on peroxisomal function and on PPARalpha. Their massive proliferation induced by a variety of xenobiotics and the subsequent tumor formation in rodents is evidently due to an imbalance in the formation and scavenging of ROS, and is mediated by PPARalpha. In PEX5-/- mice with the absence of functional peroxisomes severe abnormalities of mitochondria in different organs are observed which resemble closely those in respiratory chain disorders associated with oxidative stress. Interestingly, no evidence of oxidative damage to proteins or lipids, nor of increased peroxide production has been found in that mouse model. In this respect the role of PPARalpha, which is highly activated in those mice, in prevention of oxidative stress deserves further investigation.

Reconstruction of human hepatocyte glyoxylate metabolic pathways in stably transformed Chinese-hamster ovary cells

Failure to detoxify the intermediary metabolite glyoxylate in human hepatocytes underlies the metabolic pathology of two potentially lethal hereditary calcium oxalate kidney stone diseases, PH (primary hyperoxaluria) types 1 and 2. In order to define more clearly the roles of enzymes involved in the metabolism of glyoxylate, we have established singly, doubly and triply transformed CHO (Chinese-hamster ovary) cell lines, expressing all combinations of normal human AGT (alanine:glyoxylate aminotransferase; the enzyme deficient in PH1), GR/HPR (glyoxylate/hydroxypyruvate reductase; the enzyme deficient in PH2), and GO (glycolate oxidase). We have embarked on the preliminary metabolic analysis of these transformants by studying the indirect toxicity of glycolate as a simple measure of the net intracellular production of glyoxylate. Our results show that glycolate is toxic only to those cells expressing GO and that this toxicity is diminished when AGT and/or GR/HPR are expressed in addition to GO. This finding indicates that we have been able to reconstruct the glycolate-->glyoxylate, glyoxylate-->glycine, and glyoxylate-->glycolate metabolic pathways, catalysed by GO, AGT, and GR/HPR respectively, in cells that do not normally express them. These results are compatible with the findings in PH1 and PH2, in which AGT and GR/HPR deficiencies lead to increased oxalate synthesis, due to the failure to detoxify its immediate precursor glyoxylate. These CHO cell transformants have a potential use as a cell-based bioassay for screening small molecules that stabilize AGT or GR/HPR and might have use in the treatment of PH1 or PH2.

Mammalian peroxisomes and reactive oxygen species

The central role of peroxisomes in the generation and scavenging of hydrogen peroxide has been well known ever since their discovery almost four decades ago. Recent studies have revealed their involvement in metabolism of oxygen free radicals and nitric oxide that have important functions in intra- and intercellular signaling. The analysis of the role of mammalian peroxisomes in a variety of physiological and pathological processes involving reactive oxygen species (ROS) is the subject of this review. The general characteristics of peroxisomes and their enzymes involved in the metabolism of ROS are briefly reviewed. An expansion of the peroxisomal compartment with proliferation of tubular peroxisomes is observed in cells exposed to UV irradiation and various oxidants and is apparently accompanied by upregulation of PEX genes. Significant reduction of peroxisomes and their enzymes is observed in inflammatory processes including infections, ischemia-reperfusion injury, and allograft rejection and seems to be related to the suppressive effect of tumor necrosis factor-alpha on peroxisome function and peroxisome proliferator activated receptor-alpha. Xenobiotic-induced proliferation of peroxisomes in rodents is accompanied by the formation of hepatic tumors, and evidently the imbalance in generation and decomposition of ROS plays an important role in this process. In PEX5-/- knockout mice lacking functional peroxisomes severe alterations of mitochondria in various organs are observed which seem to be due to a generalized increase in oxidative stress confirming the important role of peroxisomes in homeostasis of ROS and the implications of its disturbances for cell pathology.

Peroxisomal targeting of mammalian hydroxyacid oxidase 1 requires the C-terminal tripeptide SKI

Peroxisomal proteins are post-translationally imported into peroxisomes after recognition by specific receptors. The best-defined peroxisomal targeting signal is a C-terminal tripeptide SKL. Different functional variants of this tripeptide have been defined, but mutants with a SKI sequence were recognized as being inefficiently targeted to peroxisomes. Recently, we have cloned a cDNA for the mouse hydroxyacid oxidase 1 (Hao1), a protein that seems to be localized in peroxisomes. Interestingly, the mouse Hao1 sequence comprises a C-terminal SKI tripeptide. We have analyzed the subcellular localization of Hao1 and tested whether its SKI sequence acts as a targeting signal. Ltk(-) and Cos-7 cells were transfected with vectors expressing a fusion protein of green fluorescence protein and Hao1, as well as mutants thereof. Targeting to peroxisomes of the fusion protein with the wild-type SKI sequence was highly selective and as complete as with the peroxisome-specific SKL sequence. By contrast, targeting was lost in a mutant with the sequence CKM. The data show that mammalian Hao1 is a peroxisomal protein and that the C-terminal sequence SKI acts as the targeting signal.

Current cytochemical techniques for the investigation of peroxisomes. A review

The past decade has witnessed unprecedented progress in elucidation of the complex problems of the biogenesis of peroxisomes and related human disorders, with further deepening of our understanding of the metabolic role of this ubiquitous cell organelle. There have been many recent reviews on biochemical and molecular biological aspects of peroxisomes, with the morphology and cytochemistry receiving little attention. This review focuses on the state-of-the-art cytochemical techniques available for investigation of peroxisomes. After a brief introduction into the use of the 3,3'-diaminobenzidine method for localization of catalase, which is still most commonly used for identification of peroxisomes, the cerium technique for detection of peroxisomal oxidases is discussed. The influence of the buffer used in the incubation medium on the ultrastructural pattern obtained in rat liver peroxisomes in conjunction with the localization of urate oxidase in their crystalline cores is discussed, particularly since Tris-maleate buffer inhibits the enzyme activity. In immunocytochemistry, quantitation of immunogold labeling by automatic image analysis enables quantitative assessment of alterations of proteins in the matrix of peroxisomes. This provides a highly sensitive approach for analysis of peroxisomal responses to metabolic alterations or to xenobiotics. The recent evidence suggesting the involvement of ER in the biogenesis of "preperoxisomes" is mentioned and the potential role of preembedding immunocytochemistry for identification of ER-derived early peroxisomes is emphasized. The use of GFP expressed with a peroxisomal targeting signal for the investigation of peroxisomes in living cells is briefly discussed. Finally, the application of in situ hybridization for detection of peroxisomal mRNAs is reviewed, with emphasis on a recent protocol using perfusion-fixation, paraffin embedding, and digoxigenin-labeled cRNA probes, which provides a highly sensitive method for detection of both high- and low-abundance mRNAs encoding peroxisomal proteins. (J Histochem Cytochem 47:1219-1232, 1999)

Molecular cloning of mouse glycolate oxidase. High evolutionary conservation and presence of an iron-responsive element-like sequence in the mRNA

Iron regulatory proteins (IRPs) control the synthesis of several proteins in iron metabolism by binding to iron-responsive elements (IREs), a hairpin structure in the untranslated region (UTR) of corresponding mRNAs. Binding of IRPs to IREs in the 5' UTR inhibits translation of ferritin heavy and light chain, erythroid aminolevulinic acid synthase, mitochondrial aconitase, and Drosophila succinate dehydrogenase b, whereas IRP binding to IREs in the 3' UTR of transferrin receptor mRNA prolongs mRNA half-life. To identify new targets of IRPs, we devised a method to enrich IRE-containing mRNAs by using recombinant IRP-1 as an affinity matrix. A cDNA library established from enriched mRNA was screened by an RNA-protein band shift assay. This revealed a novel IRE-like sequence in the 3' UTR of a liver-specific mouse mRNA. The newly identified cDNA codes for a protein with high homology to plant glycolate oxidase (GOX). Recombinant protein expressed in bacteria displayed enzymatic GOX activity. Therefore, this cDNA represents the first vertebrate GOX homologue. The IRE-like sequence in mouse GOX exhibited strong binding to IRPs at room temperature. However, it differs from functional IREs by a mismatch in the middle of its upper stem and did not confer iron-dependent regulation in cells.

Peroxisomes in adrenal steroidogenesis

Peroxisomes, cytoplasmic organelles limited by a single membrane and with a matrix of moderate electron density, are present in a great number of cells, namely in adrenal cortex and other steroid-secreting organs. Presently peroxisomes are considered to be involved in important metabolic processes. They intervene in: (1) the production and degradation of H2O2; (2) biosynthesis of ether-phospholipids, cholesterol, dolichol, and bile acids; (3) oxidation of very long chain fatty acids, purines, polyamines, and prostaglandins; (4) catabolism of pipecolic, phythanic and glyoxylic acids; and (5) gluconeogenesis. Recent studies demonstrated that the experimental alterations in the normal steroidogenesis, produce significant morphological and biochemical changes in peroxisomes. Besides this, the presence of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (the key enzyme in the de novo cholesterol synthesis from acetate) and of sterol carrier protein-2 (SCP2), which is involved in the cholesterol metabolism and steroid metabolic pathways, are located in peroxisomes of steroid-secreting cells. In addition, patients with peroxisome diseases present deficiency in steroidogenesis, as well as reduced levels of SCP2. These data pointed out the important role of peroxisomes in steroid biosynthesis.

In situ heterogeneity of peroxisomal oxidase activities: an update

Oxidases are a widespread group of enzymes. They are present in numerous organisms and organs and in various tissues, cells, and subcellular compartments, such as mitochondria. An important source of oxidases, which is investigated and discussed in this study, are the (micro)peroxisomes. Oxidases share the ability to reduce molecular oxygen during oxidation of their substrate, yielding an oxidized product and hydrogen peroxide. Besides the hydrogen peroxide-catabolizing enzyme catalase, peroxisomes contain one or more hydrogen peroxide-generating oxidases, which participate in different metabolic pathways. During the last four decades, various methods have been developed and elaborated for the histochemical localization of the activities of these oxidases. These methods are based either on the reduction of soluble electron acceptors by oxidase activity or on the capture of hydrogen peroxide. Both methods yield a coloured and/or electron dense precipitate. The most reliable technique in peroxisomal oxidase histochemistry is the cerium salt capture method. This method is based on the direct capture of hydrogen peroxide by cerium ions to form a fine crystalline, insoluble, electron dense reaction product, cerium perhydroxide, which can be visualized for light microscopy with diaminobenzidine. With the use of this technique, it became clear that oxidase activities not only vary between different organisms, organs, and tissues, but that heterogeneity also exists between different cells and within cells, i.e. between individual peroxisomes. A literature review, and recent studies performed in our laboratory, show that peroxisomes are highly differentiated organelles with respect to the presence of active enzymes. This study gives an overview of the in situ distribution and heterogeneity of peroxisomal enzyme activities as detected by histochemical assays of the activities of catalase, and the peroxisomal oxidases D-amino acid oxidase, L-alpha-hydroxy acid oxidase, polyamine oxidase and uric acid oxidase.

Histochemistry of peroxisomal enzyme activities: a tool in the diagnosis of Zellweger syndrome

The localization of the activity of the peroxisomal enzymes D-amino acid oxidase and hydroxyacid oxidase was studied at the light-microscopical level in livers and kidneys of control subjects and patients with Zellweger syndrome, an inherited disease characterized by a lack of intact peroxisomes. D-Amino acid oxidase and hydroxyacid oxidase activities were demonstrated in unfixed cryostat sections with the cerium-diaminobenzidine-cobalt-hydrogen peroxide procedure, in which cerium ions capture hydrogen peroxide, the product of both enzymes. In a second step reaction decomposition of cerium perhydroxide gives rise to a diaminobenzidine polymer complexed with cobalt ions. D-Amino acid oxidase and hydroxyacid oxidase activities were found in peroxisomes of liver parenchymal cells, and only D-amino acid oxidase in peroxisomes of proximal tubular cells of kidneys of control humans. The activities of these enzymes were not detectable in livers and kidneys of Zellweger patients. It is concluded that the cerium-diaminobenzidine-cobalt-hydrogen peroxide procedure enables the demonstration of peroxisomal enzyme activities in human tissues at the light-microscopical level and is an important tool in detecting patients with Zellweger syndrome.

A new staining method for the detection of activities of H2O2-producing oxidases on gels and blots using cerium and 3,3'-diaminobenzidine

Cerium chloride (CeCl3) was used to trap the hydrogen peroxide generated by several oxidases on native gels and blots. The pale yellow color of cerium perhydroxide formed is converted to a brown-black precipitate by the subsequent reaction with 3,3'-diaminobenzidine. The suitability of this method for the detection of the activity of several oxidases on gels and on blots under nondenaturing conditions, employing different electrophoretic systems and resolving techniques, is demonstrated. Moreover, this method has proven to be highly suitable for the assessment of the substrate and stereospecificity of oxidases, the determination of their molecular weights, and the isoelectric points of isoforms.

Light microscopical detection of H2O2-generating oxidases using cerium ions and aqueous incubation media

Light microscopical procedures were optimized and tested for specificity for the histochemical demonstration of D-amino acid oxidase, alpha-hydroxy acid oxidase, monoamine oxidase, and xanthine oxidase using cerium ions and a visualization step originally described by Angermüller and Fahimi (1988a, b), and modified for D-amino acid oxidase by Gossrau et al. (1989). The visualization medium contained diaminobenzidine, cobalt ions, and small amounts of hydrogen peroxide. Procedures of pretreatment of cryostat sections, types of substrate, concentrations of substrates, and cerium ions were varied. Optimum procedures are reported for the detection of these oxidases in different rat tissues. The results are compared with those obtained with other methods described for the demonstration of hydrogen peroxide-generating oxidases such as the tetrazolium, Hatchett Brown, and coupled peroxidatic methods.

Peroxisomal oxidases: cytochemical localization and biological relevance

(1) alpha-HAOX has a broad substrate specificity. In rat kidney, the enzyme reacts with aliphatic and aromatic alpha-hydroxy acids, in rat liver, however, only with aliphatic ones. (2) The best substrate for the demonstration of alpha-HAOX activity in rat and human liver is glycolate. (3) alpha-hydroxy butyric acid is the best substrate in the luminometric assay for the demonstration of alpha-HAOX activity in the rat kidney, whereas glycolate is not catalysed by the enzyme. (4) In the proximal tubulus epithelial cells of the rat kidney alpha-HAOX is concentrated in the peripheral matrix of the peroxisomes.

Peroxisomes in pigment epithelium and Müller cells of amphibian retina possess D-amino acid oxidase as well as catalase

In this paper we identify peroxisomes in Müller cells and retinal pigment epithelial cells of Rana pipiens and Xenopus laevis by virtue of their content of cytochemically stainable catalase. These organelles have the form of an extensive, branched system of tubules in the retinal pigment epithelium and appear as discrete ovoid structures in the Müller cells. In both the pigment epithelium and the Müller cells a second peroxisomal enzyme, D-amino acid oxidase, can be detected in the same structures as catalase by means of a cerium-based cytochemical staining procedure. This oxidase is active only against nonpolar and polar, uncharged D-amino acids.

Rat kidney L-2-hydroxyacid oxidase. Structural and mechanistic comparison with flavocytochrome b2 from baker's yeast

Hydroxyacid oxidase from rat kidney is an FMN-dependent enzyme that catalyzes the oxidation of L-alpha-hydroxy acids as well as, more slowly, that of L-alpha-amino acids. We report here a modified purification method for the enzyme, which is found to possess one cofactor per subunit of Mr 39,000. Determination of its N-terminal sequence suggests the protein is homologous to spinach glycolate oxidase and baker's yeast lactate dehydrogenase. In the presence of a hydroxy acid and of bromopyruvate, under anaerobic conditions, the enzyme is found to catalyze both transhydrogenation and reductive bromide ion elimination. It had previously been observed that hydroxyacid oxidase could not catalyze chloride elimination from chlorolactate in the presence of oxygen [Cromartie, T.H., & Walsh, C.T. (1975) Biochemistry 14, 3482-3490]. The behavior of this enzyme toward halogeno substrates is therefore similar to that of baker's yeast L-lactate dehydrogenase and in part different from that of Mycobacterium smegmatis lactate oxidase and porcine kidney D-amino-acid oxidase. These findings can be rationalized on the basis of a common mechanism for all these enzymes, implying formation of a carbanion as a first step, with different rate-limiting steps in the overall reaction.

Electron microscopic cytochemical localization of alpha-hydroxyacid oxidase in rat kidney cortex. Heterogeneous staining of peroxisomes

The substrate specificity of alpha-hydroxyacid oxidase in the rat kidney has been investigated cytochemically by the cerium technique and biochemically with a luminometric assay applied to isolated renal peroxisomes. Rat kidneys were fixed by perfusion via the abdominal aorta with a low concentration (0.25%) of glutaraldehyde. Vibratome sections were incubated for 60 min at 37 degrees C in a medium containing 3 mM CeCl3, 100 mM NaN3 and 5 mM of an alpha-hydroxyacid in 0.1 M Pipes or 0.1 M Tris-maleate buffer both adjusted to pH 7.8. Ten aliphatic alpha-hydroxyacids with chain lengths between 2 and 8 carbon atoms and two aromatic substrates were tested. The alpha-hydroxyacid oxidase in the kidney exhibited a markedly different substrate specificity than the corresponding enzyme in the liver. Thus glycolate gave a negative reaction while two aromatic substrates, mandelic acid and phenyllactic acid, stained prominently. With aliphatic substrates a stronger reaction was obtained in Pipes than in the Tris-maleate buffered incubation media. The best reaction in the kidney was obtained with hydroxybutyric acid. These cytochemical findings were confirmed by the luminometric determination of the oxidase activity in isolated purified peroxisome fractions. By electron microscopy the electron dense reaction product of cerium perhydroxide was found in the matrix of peroxisomes in the proximal tubules. The intensity of reaction varied markedly in neighbouring epithelial cells but also in different peroxisomes within the same cell. Thus heavily stained particles were seen next to lightly reacted ones. These observations establish the substrate specificity of alpha-hydroxyacid oxidase in the rat kidney and demonstrate the marked heterogeneity in the staining of renal peroxisomes for this enzyme.