GLYOXYSOME BIOGENESIS VIA CYTOSOLIC POOLS IN CUCUMBER

Peroxisomes: Organelles at the crossroads

Recent years have seen remarkable progress in our understanding of the function of peroxisomes in higher and lower eukaryotes. Combined genetic and biochemical approaches have led to the identification of many genes required for the biogenesis of this organelle. This review summarizes recent, rather surprising, results and discusses how they can be incorporated into the current view of peroxisome biogenesis.

Purification and properties of glycolate oxidase from plants with different photosynthetic pathways: Distinctness of C4 enzyme from that of a C 3 species and a C 3-C 4 intermediate

Glycolate oxidase (GO; EC 1.1.3.1) was purified from the leaves of three plant species:Amaranthus hypochondriacus L.(NAD-ME type C4 dicot),Pisum sativum L. (C3 species) andParthenium hysterophorus L. (C3-C4. intermediate). A flavin moiety was present in the enzyme from all the three species. The enzyme from the C4 plant had a low specific activity, exhibited lower KM for glycolate, and required a lower pH for maximal activity, compared to the C3 enzyme. The enzyme from the C4 species oxidized glyoxylate at <10% of the rate with glycolate, while the GO from the C3 plant oxidized glyoxylate at a rate of about 35 to 40% of that with glycolate. The sensitivity of GO from C4 plant to α-hydroxypyridinemethane sulfonate, 2-hydroxy-3-butynoate and other inhibitors was less than that of the enzyme from C3 source. The properties of GO fromParthenium hysterophorus, were similar to those of the enzyme fromPisum sativum. The characteristics of glycolate oxidase from leaves of a C4 plant,Amaranthus hypochondriacus are different from those of the C3 species or the C3-C4 intermediate.

Malate dehydrogenase isoenzymes: cellular locations and role in the flow of metabolites between the cytoplasm and cell organelles

Malate dehydrogenases belong to the most active enzymes in glyoxysomes, mitochondria, peroxisomes, chloroplasts and the cytosol. In this review, the properties and the role of the isoenzymes in different compartments of the cell are compared, with emphasis on molecular biological aspects. Structure and function of malate dehydrogenase isoenzymes from plants, mammalian cells and ascomycetes (yeast, Neurospora) are considered. Significant information on evolutionary aspects and characterisation of functional domains of the enzymes emanates from bacterial malate and lactate dehydrogenases modified by protein engineering. The review endeavours to give up-to-date information on the biogenesis and intracellular targeting of malate dehydrogenase isoenzymes as well as enzymes cooperating with them in the flow of metabolites of a given pathway and organelle.

Transition form of microbodies. Overlapping of two sets of marker proteins during the rearrangement of glyoxysomes into leaf peroxisomes

Several forms of microbodies have been characterized on the basis of their biochemical functions. We have investigated cucumber cotyledons which house two different microbody forms during their development. In these cells, a shift from organelles with the enzymes of beta-oxidation and glyoxylate cycle to peroxisomes with the enzymes of the photosynthetic C2-cycle can be induced by light. The transition state and the time course of changes was studied at different levels of gene expression during the first 2 days of illumination, by quantifying the rate of de novo protein synthesis in cotyledons and by measuring the mRNA activities in vitro. Synthesis and turnover of particular proteins were determined during the transition stage by immunoprecipitation of malate synthase, isocitrate lyase, catalase, multifunctional protein, and thiolase, and quantified by fluorography. From the mRNA activities and the rate of protein synthesis, gene expression for enzymes of the glyoxylate cycle and beta-oxidation started to decrease 24-36 h after onset of continuous light. At that time the rate of synthesis of glycolate oxidase, a leaf peroxisomal marker, is already maximal. By pulse-chase experiments 0-48 h after the onset of light the speed and intensity of protein turnover were measured. Rates of proteolytic degradation of individual enzymes indicated that the different enzymes were not lost simultaneously or all at once. This excludes a destruction of the whole organelle by the lytic compartment.

β-Oxidation of fatty acids in algae: Localization of thiolase and acyl-CoA oxidizing enzymes in three different organisms

In the algae Mougeotia, Bumilleriopsis and Eremosphaera, recently shown to possess the enzymes hydroxyacyl-CoA dehydrogenase (EC 1.1.1.35) and enoyl-CoA hydratase (EC 4.2.1.17), the presence of thiolase (EC 2.3.1.9) and acyl-CoA-oxidizing enzymes can also be demonstrated, indicating that β-oxidation of fatty acids is possible in these organisms. The compartmentation of enzymes is different in the various algae. In Mougeotia, both thiolase and the acyl-CoA-oxidizing enzyme are located exclusively in the peroxisomes. The latter enzyme was found to be an oxidase using molecular oxygen as an electron acceptor. On the other hand, in Bumilleriopsis all enzymes of the fatty-acid β-oxidation pathway tested are constituents only of the mitochondria, and acyl-CoA is oxidized by a dehydrogenase incapable of reducing oxygen. Finally, in Eremosphaera thiolase and acyl-CoA-oxidizing enzymes were found in the peroxisomes as well as in the mitochondria. In the peroxisomes, oxidation of acyl-CoA is catalyzed by an oxidase, whereas the corresponding enzyme in the mitochondria is a dehydrogenase. The acyl-CoA oxidases/dehydrogenases of the three algae differ not only by their capability for oxidation of acyl-CoA of different chain lengths but also with regard to their Km values and substrate specificities. Indications were obtained that the oxygen is reduced to water rather than to H2O2 by the algal acyl-CoA oxidases. When cells of Eremosphaera were cultured with hypolipodemic substances in the growth medium the activities of the peroxisomal enzymes, but not those of the mitochondrial enzymes of the fatty-acid β-oxidation pathway, were increased by a factor of two to three.

Chemically induced proliferation of peroxisomes: implications for risk assessment

An increasing number of beneficial and economically important drugs, industrial chemicals, and agrichemicals are being found to cause a dose-related hepatomegaly in rodent species which is associated with the proliferation of the subcellular organelle, the peroxisome. The prolonged proliferation of hepatocellular peroxisomes and the enhanced production of the normal peroxisomal metabolic byproduct, hydrogen peroxide, in these animals during chronic bioassays has been hypothesized to account for the tumorigenicity of several of these compounds, most of which lack any measurable genotoxicity in in vitro and in vivo assays. This paper briefly reviews the basic morphology and enzymology of the peroxisome and its relationship to specific pathologic changes in animals. The potential impact of the mechanism of action of peroxisome proliferators upon the design of toxicity studies and, in conjunction with interspecies sensitivity data, upon risk assessment is discussed.

Characterization of two forms of the multifunctional protein acting in fatty acid beta-oxidation

The enzymatic apparatus of fatty acid beta-oxidation in peroxisomes and glyoxysomes includes a multifunctional protein. Two forms of this protein were detected in extracts from cotyledons of germinating cucumber seeds and separated on hydroxylapatite. The two proteins purified to apparent homogeneity possessed enoyl-CoA hydratase, 3-hydroxyacyl-CoA epimerase, and 3-hydroxyacyl-CoA dehydrogenase activity; the proteins are therefore trifunctional. Analysis of molecular structures and kinetic parameters of the two enzyme forms revealed significant differences in size and amino acid composition. The two proteins were characterized as monomers exhibiting molecular weights of 74,000 and 76,500. Likewise, the data obtained with limited proteolysis proved the occurrence of two independent proteins. Immunological comparisons were performed with antibodies raised against the 76.5-kDa protein. They indicated a weak relationship between the two proteins. From that we conclude that within one type of organelle, i.e., glyoxysome, two isoenzymes with multiple functions are located.

Effect of abscisic and gibberellic acids on malate synthase transcripts in germinating castor bean seeds

Several clones complementary to malate synthase mRNA have been identified in a complementary-DNA library to mRNA from castor bean endosperm. One of these clones has been used as a probe to measure levels of transcripts during seed germination and the effects of gibberellic acid and abscisic acid on these levels have been examined.Malate synthase transcripts increased during germination and GA3 advanced their appearance in the endosperm. Exogenously applied ABA inhibited the accumulation of transcripts over a time course of germination but the addition of GA3 counteracted its inhibitory effects. The data confirmed previous reports which indicated that the action of both growth regulators was on transcript accumulation and that there is a coordinated induction of the enzymes involved in the lipid metabolism in oil seeds.

Purification and characterization of heme-containing low-activity form of catalase from greening pumpkin cotyledons

In germinating pumpkin seeds, catalase is synthesized as a precursor (59-kDa) form, with molecular mass larger than the mature molecule (55 kDa). Both the precursor and mature forms of catalase are localized in the microbodies, i.e., glyoxysomes and leaf peroxisomes [Proc. Natl. Acad. Sci. USA 81, 4809-4813 (1984)]. We have now purified the 59-kDa catalase precursor and compared its properties with those of the 55-kDa mature molecule. The molar catalytic activity of the 59-kDa catalase was tenfold lower than that of the 55-kDa molecule, whereas the heme content was found to be same, with both forms containing four hematin groups per molecule. It is inferred from these results that the low activity of the 59-kDa molecule is not related to the binding of heme to the protein, but presumably involves conformational differences between the 59-kDa and 55-kDa molecules. We have further found that the reduction of total catalase activity in pumpkin cotyledons during greening was due to a decrease in the amount of the enzymically active 55-kDa catalase accompanying an increase in the 59-kDa molecule.

The action of exogenous abscisic acid on malate-synthase synthesis in germinating castor-bean seeds

The presence of 30 μM abscisic acid inhibited development of malate-synthase activity in the endosperm of germinating castor-bean seeds. Malate synthase was purified from castor-bean endosperms and an antibody to it was prepared from rabbit serum. This antibody was used to measure the amounts of malate-synthase mRNA using an in-vitro translation system. The effect of abscisic acid appeared to be greater on malate-synthase mRNA than on the bulk of mRNA, indicating some specificity of abscisic-acid action. The extent of the inhibition of malate-synthase activity and of malate-synthase mRNA accumulation were similar. This indicates that abscisic acid inhibits malate-synthase activity by lowering levels of translatable malate-synthase mRNA rather than by affecting the translation rate of this mRNA.

Ontogeny of glyoxysomes in maturing and germinated cotton seeds-a morphometric analysis

Morphometric procedures were used with light and electron microscopy to examine glyoxysome number, volume, shape and distribution as well as mesophyll cell volume, in cotyledons of mature (50 d postanthesis), imbibed (5h) and germinated (24 and 37 h) cotton (Gossypium hirsutum L.) seeds. Additionally, activities of five glyoxysomal marker enzymes in cotyledon extracts were assayed at each of the above ages. Cell volume was determined from photomicrographs of Epon-embedded sections by the point-counting procedure. Analysis of variance showed that cell volume was not different among the tissue segments studied. Glyoxysomes were cytochemically stained for catalase (EC 1.11.1.6) activity with the 3,3'-diaminobenzidine-tetrahydrochloride procedure. Analyses involving both phase and electron microscopy, and two separate sterologic calculations for determining the number of glyoxysomes per cell, indicate that glyoxysomes are numerous in mature seeds, persist through desiccation and imbibition, then increase dramatically in volume (seven fold) but not number (a maximum of 1.5-fold), when enzyme activities increase two to six times (depending on the enzyme). During the entire period of increase in glyoxysomal enzyme activities, no ultrastructural evidence was found for glyoxysome formation or destruction. Our data, in contrast to some proposals in the literature, indicate that cottonseed glyoxysomes form during seed maturation, then develop following seed imbibition into pleomorphic organelles by posttranslational accumulation of proteins from the cytosol and transfer of membrane components probably from the endoplasmic reticulum.

Synthesis in vitro of precursor-type carnitine acetyltransferase with messenger RNA from Candida tropicalis

Carnitine acetyltransferase was synthesized in vitro in the mRNA-dependent reticulocyte system with mRNA from alkane-grown or propionate-grown cells of Candida tropicalis. The protein synthesized in vitro was isolated by immunoprecipitation with antibody against peroxisomal or mitochondrial carnitine acetyltransferase and was compared with peroxisomal carnitine acetyltransferase (Mr of subunits, 64 000 and 57 000) and the mitochondrial enzyme (Mr of subunits, 64 000 and 52 000) of C. tropicalis by electrophoresis in the presence of sodium dodecyl sulfate. Nascent carnitine acetyltransferase prepared in vitro showed a hetero-oligomeric property, like the peroxisomal and mitochondrial enzymes isolated from C. tropicalis. The molecular weights of the subunits of nascent carnitine acetyltransferase were estimated to be 71 000 and 57 000, indicating the existence of the precursor form of the enzyme. By sucrose density gradient centrifugation of total mRNA, these two subunit proteins were shown to be synthesized with respective mRNAs of different sizes. The same precursor-type of carnitine acetyltransferase was obtained with the mRNAs from the alkane-grown cells and the propionate-grown cells. The results obtained suggest that a common precursor will be post-translationally modified to form the peroxisomal and mitochondrial enzymes.

Oligomerization of malate synthase during glyoxysome biosynthesis

The octameric malate synthase, found in glyoxysomes of plants, is synthesized as monomeric precursor in the cytoplasm. The precursor form does not possess a different subunit molecular weight than the mature organellar enzyme, but differs from the organellar protein by not oligomerizing and aggregating. This was shown by synthesis in a cell-free reticulocyte lysate system programmed with cucumber poly A+-mRNA followed by immunoprecipitation of the radiolabeled translation products. The precursor form of malate synthase was also detected in vivo in the cytosol of pulse-labeled cucumber cotyledons after immunoprecipitation of the radiolabeled polypeptide. At low salt concentrations, mature malate synthase can be converted into aggregated forms. However, the precursor form obtained either by in vitro translation or by extraction from the cytosol after short pulses of radioactive methionine, could neither be oligomerized into the octameric form nor aggregated into the 100-S form. Processing of malate synthase, assumed to be a requisite for oligomerization, took place rapidly in the glyoxysomes, but proceeded only slowly in the cytosol. This was demonstrated both by the uptake of in vitro-translated malate synthase into glyoxysomes, and by analysis of newly synthesized malate synthase detectable in glyoxysomes in vivo. In both cases the octamer was by far the predominant form.

Malate synthase: aggregation, deaggregation, and binding of phospholipids

Octameric malate synthase is located in the glyoxysomes of cucumber cotyledons. The enzyme is predominantly confined to the organelle's membrane and can be solubilized with Mg2+. Separation of cell structures in a zonal rotor afforded, besides glyoxysomes, two other zones with malate synthase activity, viz., in the gradient supernatant and in the range of the endoplasmic reticulum (ER). Malate synthases of these three fractions were purified to apparent homogeneity and classified according to their molecular weight. Differences in subunit molecular weight, however, could not be detected when malate synthases from the three fractions were compared. Mature malate synthase, as well as malate synthase prepared from fractions sedimenting similarly to the ER, exhibited the following behavior with respect to aggregation and deaggregation: at low salt concentrations and in the absence of Mg2+, the enzyme shifted to aggregated forms (approx 100 S); with 2 mM Mg2+, malate synthase deaggregated and occurred predominantly as an octamer (19 S). By changing buffer conditions, mature forms of malate synthase could be interconverted repeatedly between octameric and aggregated forms, whereas a monomeric form (5 S), prepared from soluble fractions assigned to the cytosol, did not oligomerize. The amphipathic properties of malate synthase were demonstrated by the enzyme's capacity for binding phospholipids.

Striated adrenocortical cells in cerebro-hepato-renal (Zellweger) syndrome

Adrenal glands from eight patients with the cerebro-hepato-renal syndrome, a disease in which there are no morphologically demonstrable peroxisomes, were studied histologically; one of the eight was also examined ultrastructurally. Seven of the eight demonstrated striated adrenocortical cells in the inner portion of the adrenal cortex. Ultrastructural examination confirmed that the striated cells contained the lammellae and lamellar-lipid profiles of very long chain fatty acids-cholesterol esters that are characteristic of adreno-leukodystrophy. This morphologic observation further emphasizes the common pathogenetic features of the cerebro-hepato-renal (Zellweger) syndrome and adreno-leukodystrophy.

Biosynthesis of a microbody matrix enzyme in greening cotyledons : Glycollate oxidase synthesized in vivo and in vitro

Earlier work on microbody biosynthesis has shown that glyoxysomal and liver peroxisomal proteins synthesized in the cytosol are made as precursors which are then transferred into the organelles and processed. Here, it is demonstrated that the unprecessed precursor detected in the cytosol after protein synthesis in vivo for an enzyme at the transition stage between glyoxysomes and leaf peroxisomes is indistinguishable from the product of translation in vitro. It is assumed that the transfer of extraorganellarly made precursor across the glyoxysomal membranes is followed by processing of the precursor and oligomerization to the tetrameric or 16-meric form of the enzyme. Oligomerization was, however, also observed in a portion of the cytosolic form.