The synthesis and turnover of rat liver peroxisomes. I. Fractionation of peroxisome proteins

Intraparticulate localization of some peroxisomal enzymes related to fatty acid beta-oxidation

Male Wistar rats were given a diet containing 0.05% (w/w) LK-903 (alpha-methyl-p-myristyroxycinnamic acid 1-monoglyceride) for 2 weeks. The activities of four hepatic peroxisomal enzymes involved in the fatty acyl-CoA beta-oxidizing system were determined. The activities of fatty acyl-CoA oxidase, crotonase, beta-hydroxybutyryl-CoA dehydrogenase and thiolase were all increased about three times by administration of LK-903. The intraparticulate localizations of the four enzymes were then investigated by treatment of the purified peroxisomes with Triton X-100, by sonication, and by sucrose-density-gradient centrifugation after Triton X-100 treatment. The results suggest that thiolase is localized in the matrix of peroxisomes, that crotonase and beta-hydroxybutyryl-CoA dehydrogenase are located in the core, and that all or at least part of fatty acyl-CoA oxidase is associated with the core, though its association is weak.

Membranes of rat liver peroxisomes

Membranes of liver peroxisomes from rats fed with clofibrate were purified in a discontinuous gradient using a zonal rotor. The preparation consists of round or oval vesicles mostly devoid of nucleoids with a diameter ranging from 70-700 nm; open sheets are found very infrequently. Mitochondrial profiles as well as vesicles containing cytochemically demonstrable glucose 6-phosphatase are scarce; accordingly, glucose 6-phosphatase is nearly undetectable biochemically. Monoamine oxidase is absent in peroxisomal membranes. Cytochrome b5 is found in a concentration of 0.3 nmoles/mg protein, an order of magnitude comparable to the content of endoplasmic reticulum membranes. Reduction of this cytochrome with palmitoyl-CoA is possible only after recombination of the membranes with the soluble peroxisomal matrix fraction.

A characteristic membrane protein of liver peroxisomes inducible by clofibrate

In sodium dodecyl sulfate polyacrylamide electrophoresis the membranes of rat liver peroxisomes show nine main protein bands (40 000--100 000 dalton); the 40 000-dalton polypeptide cannot be resolved from the membrane by deoxycholate. Treatment of the rats with clofibrate largely increases this protein and another one (about 80 000 dalton) in the peroxisomal but not in the endoplasmic reticulum membrane. Proliferation of peroxisomes seems to be connected with the insertion of specific proteins into the membrane.

Cytochemical localization of catalase and several hydrogen peroxide-producing oxidases in the nucleoids and matrix of rat liver peroxisomes

The distribution of catalase, amino acid oxidase, alpha-hydroxy acid oxidase, urate oxidase and alcohol oxidase was studied cytochemically in rat hepatocytes. The presence of catalase was demonstrated with the conventional diaminobenzidine technique. Oxidase activities were visualized with methods based on the enzymatic or chemical trapping of the hydrogen peroxide produced by these enzymes during aerobic incubations. All enzymes investigated were found to be present in peroxisomes. Catalase activity was found in the peroxisomal matrix, but also associated with the nucleoid. After staining for oxidase activities the stain deposits occurred invariably in the peroxisomal matrix as well as in the nucleoids. In all experiments the activity of both catalase and the oxidases was confined to the peroxisomes. The presence of a hydrogen peroxide-producing alcohol oxidase was demonstrated for the first time in peroxisomes in liver cells. The results imply that the enzyme activity of the nucleoids of rat liver peroxisomes is not exclusively due to urate oxidase. The nucleoids obviously contain a variety of other enzymes that may be more or less loosely associated with the insoluble components of these structures.

Degradation of peroxisomal catalase and urate oxidase of rat liver

Urate oxidase and catalase were purified from rat liver peroxisomes, and respective antibodies were prepared from rabbits by the administration of these enzymes. Although urate oxidase generally precipitates in immunoprecipitation-possible pH ranges (pH 4.5--9.5), the enzyme remained soluble in 50 mM glycine buffer (pH 9.5) containing 50% glycerol up to concentration of 0.3 mg/ml. Anti-urate oxidase reacted with purified urate oxidase as well as with the crude preparation. After [3H]leucine was injected to rats, urate oxidase and catalase were purified from rat liver at certain intervals, and further precipitated by respective antibodies. The half-life of the catalase was 39 h and that of urate oxidase, 20 h. When the sonicated light mitochondrial fraction was incubated at 37 degrees C and at pH 7.0 or 5.6, inactivation of catalase did not seem to differ between these pH values, and approximately 80% of the catalase activity remained even after 8 h. Urate oxidase was inactivated very rapidly at pH 5.6; only 30% of its activity survived incubation for 6 h. This inactivation was found to occur by some proteolytic process. From these findings, the turnover rate of urate oxidase was found to be different from that of catalase, and this distinction seemed to be due to different sensitivity to some degradative enzymes.

Synthesis of catalase in two cell-free protein-synthesizing systems and in rat liver

Rat liver polysomal RNA was translated in the rabbit reticulocyte lysate and in the wheat germ cell-free protein-synthesizing systems, using [(35)S]methionine as label. The catalase (hydrogen-peroxide:hydrogen-peroxide oxidoreductase, EC 1.11.1.6) that was synthesized was isolated by immunoprecipitation and characterized by electrophoresis in sodium dodecyl sulfate/polyacrylamide gels followed by fluorography. The catalase made in both systems migrated more slowly during electrophoresis than did purified peroxisomal catalase. By comparison with standards of known molecular mass, the cell-free products were estimated to be about 4000 daltons larger than the purified enzyme. We also investigated the biosynthesis of catalase in vivo by injecting [(35)S]methionine into rats. The precursor of catalase known to be synthesized in liver and found in the high-speed supernatant 8 min later [Lazarow, P. B. & de Duve, C. (1973) J. Cell Biol. 59, 491-506] was isolated immunochemically. For comparison, 1-day-old completed catalase was immunoprecipitated from peroxisomes. The migrations in sodium dodecyl sulfate gels of the 8-min-old precursor and the subunit of the day-old enzyme were indistinguishable and approximately the same as the migration of the cell-free products. These results indicate that catalase's apparent size does not change when it enters peroxisomes but rather decreases during the chemical purification procedure.

Proteins of rough microsomal membranes related to ribosome binding. I. Identification of ribophorins I and II, membrane proteins characteristics of rough microsomes

Rat liver rough microsomes (RM) contain two integral membrane proteins which are not found in smooth microsomes (SM) and appear to be related to the presence of ribosome-binding sites. These proteins, of molecular weight 65,000 and 63,000, were designated ribophorins I and II, respectively. They were not released from the microsomal membranes by alkali or acid treatment, or when the ribosomes were detached by incubation with puromycin in a high salt medium. The anionic detergent sodium deoxycholate caused solubilization of the ribophorins, but neutral detergents led to their recovery with the sedimentable ribosomes. Ribosomal aggregates containing both ribophorins, but few other membrane proteins, were obtained from RM treated with the nonionic detergent Kyro EOB (2.5 X10(-2) M) in a low ionic strength medium. Sedimentation patterns produced by these aggregates resembled those of large polysomes but were not affected by RNase treatment. The aggregates, however, were dispersed by mild trypsinization (10 microgram trypsin for 30 min at 0 degrees C), incubation with deoxycholate, or in a medium of high salt concentration. These treatments led to a concomitant degradation or release of the ribophorins. It was estimated, from the staining intensity of protein bands in acrylamide gels, that in the Kyro EOB aggregates there were one to two molecules of each ribophorin per ribosome. Sedimentable complexes without ribosomes containing both ribophorins could also be obtained by dissolving RM previously stripped of ribosomes by puromycin-KCl using cholate, a milder detergent than DOC. Electron microscope examination of the residue obtained from RM treated with Kyro EOB showed that the rapidly sedimenting polysome-like aggregates containing the ribophorins consisted of groups of tightly packed ribosomes which were associated with remnants of the microsomal membranes.

Ultrastructure of periportal and centrilobular hepatocytes in human fatty liver of various aetiology

The ultrastructure of periportal and centrilobular hepatocytes of fatty liver from 2 alcoholic patients, 2 diabetic patients and 2 obese patients is described and compared with that of hepatocytes in 2 patients with normal liver. A striking difference between periportal and centrilobular hepatocytes in all cases of fatty liver is deomonstrated. The periportal hepatocytes contain abnormal mitochondria, while centrilobular hepatocytes have normal mitochondria, but fatty vacuoles, peroxisomes and lysosomes are more abundant. The nature and degree of ultrastructural changes showed no correlation with aetiology of the fatty liver. The normal livers showed no such changes or distinct differences between periportal and centrilobular hepatocytes.

Hepatic effect of two hypolipidemic drugs in rats

Some hepatic effects of the hypolipidemic agents 3,9-di-3-pyridyl-2,4,8,10-tetraoxaspiro-5,5-undecane (compound A) and 2-(4-dibenzofuranyloxy)-2-methylpropionic acid (compound B) were investigated in male rats. The animals were treated orally with these drugs and a reference compound-clofibrate for 10 weeks, the daily doses being 250, 300 and 300 mg/kg body weight respectively. All three drugs caused hepatomegaly with a normal microscopic appearance in liver cells. In rats given compound A, part of some liver cells could be occupied by numerous membranes of smooth endoplasmic reticulum. The hepatocytes of the rats treated with compound B or clofibrate showed a marked increase in microbody profiles and an elevated hepatic catalase activity in comparison to the control animals. Neither the microbodies nor the catalase activity were affected by compound A. Hypolipidemic effects were demonstrated with all three compounds, the most potent activity being shown by compound B. Treatment with this agent resulted in significantly higher catalse activity than with clofibrate.

An electron microscopic and enzymic study of rat liver peroxisomal nucleoid core and its association with urate oxidase

The appearance of the characteristic crystalloid core of rat liver peroxisomes is emulated by the electron microscopic (EM) appearance of highly purified urate oxidase prepared from the same tissue. The purity of the enzyme preparation was established by gel electrophoresis under various conditions and the specific enzyme activity was at least as high as any previously reported. The amino acid composition of urate oxidase was determined. As additional evidence for close association of the peroxisomal core with urate oxidase, it was demonstrated that the biphasic changes in rat liver urate oxidase activity in response to prolonged starvation were paralleled by changes in the EM appearance of peroxisomes. Under comparable conditions catalase, another peroxisomal enzyme, did not show the same changes in activity as did urate oxidase. Evidence for the possible identity of urate oxidase with the peroxisomal crystalloid of rat liver has been presented, all materials having been obtained from, and experiments performed with, the rat.

Fine structure of a virilizing adrenocortical adenoma

An adrenocortical adenoma associated with adrenogenital syndrome in a two-year-old boy was investigated light and electron microscopically. Urinary 17-ketosteroid excretion was considerably elevated and unresponsive to dexamethasone administration. The level returned to normal after surgical removal of the tumour. Adenomatous cells display striking cellular and nuclear pleomorphism. Megalocytes with huge nuclei and nucleoli frequently occur. Deep cytoplasmic indentations cause nuclear pseudoinclusions and bizarre shape of the nuclei. True nuclear inclusions are also seen, as well as nuclear fragmentation. Cytoplasmic organelles show striking morphological alterations. Mitochondria with lamellar and tubular cristae are transformed into round or ovoid organelles of vesicular type. Their internal compartment is reduced, matrix material increases relatively, and mitochondrial inclusion bodies develop. Mitochondrial inclusions are identified as corresponding to fuchsinophil (siderophil or argyrophil) granules seen in the light microscope. Their staining properties indicate their glycoprotein nature. Vesicular profiles of smooth endoplasmic reticulum predominate and stacks of rough endoplasmic reticulum are transformed into tubules and vesicles. In Golgi regions, only vesicular elements are enriched. Lipid droplets are scarce. It was not possible to demonstrate histochemically catalase activity in microbodies. Dense bodies only occur in small, undifferentiated tumour cells. Multivesicular bodies, autophagosomes and residual bodies are rare. Lipofuscin is absent. Tumour cells are thought to derive from a population of undifferentiated cells ("germinative tumour cells"). Their morphological features and organelle equipment during a hypothetical course of differentiation and following dedifferentiation is described and discussed with respect to exceeding androgen synthesis.

Effects of clofibrate (alpha-p-chlorophenoxyisobutyryl-ethyl-ester) on male rat liver

In male rats, fed 0.5% clofibrate in their diet for 8 days and 21 days, the ultrastructural morphometric alterations of the hepatocytes were evaluated and compared with the biochemical data. The morphologic alterations of the microbodies were particularly related to the changes of the catalase activity of the liver homogenates. The results showd a marked hypertrophy of the liver and an increase in the volume of the individual hepatocyte. The numerical density and, even more pronounced, the volume density of the microbodies increased excessively during the treatment. The numerical density of the mitochondria decreased markedly after 21 days of administration. The surface of the rough endoplasmic reticulum showed a significant decrease, whereas the surface of the smooth endoplasmic reticulum showed a hypertrophy. The catalase activity of the liver homogenates increased 2-fold after 8 days and remained at this new steady-state after 21 days of treatment. The results suggest that the enzyme content of the microbodies changed after treatment with clofibrate, and support the suggestion that clofibrate may induce the synthesis of a yet unidentified peroxisomal protein.

A fatty acyl-CoA oxidizing system in rat liver peroxisomes; enhancement by clofibrate, a hypolipidemic drug

Purified rat liver peroxisomes contain a cyanide-insensitive fatty acyl-CoA oxidizing system that uses O2 and NAD as electron acceptors. The system was detected by the ability of added palmitoyl-CoA to elicit O2 consumption, H2O2 production, and O2-dependent NAD reduction. The activity of this system is increased approximately one order of magnitude in rats treated with clofibrate, a hypolipidemic drug known to cause peroxisomal proliferation.

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.

Hepatic peroxisome proliferation: induction by two novel compounds structurally unrelated to clofibrate

Two hypolipidemic compounds [ 4-chloro-6(2,3-xylidino)-2-pyrimidinyl-thio] acetic acid, and 2-chloro-5(3,5-dimethylpiperidinosufony)benzoic acid (tibric acid) greatly increased the number of peroxisomes (microbodies) in liver cells of rats and mice. This augmented peroxisome population was accompanied by significant elevation of liver catalase activity. These two hypolipidemic peroxisome proliferators are structurally different from ethyl a-p-chlorophenozyisobutyrate (clofibrate) and other hypolipidemic, arylocyisobutyrate derivatives which cause hepatic peroxisome proliferation. Induction of peroxisome proliferation by these structurally unrelated hypolipidemic compounds suggests a possible relation between hepatic peroxisome proliferation and hypolipidemia.

Structure, composition, physical properties, and turnover of proliferated peroxisomes. A study of the trophic effects of Su-13437 on rat liver

Peroxisome proliferation has been induced with 2-methyl-2-(p-[1,2,3,4-tetrahydro-1-naphthyl]-phenoxy)-propionic acid (Su-13437). DNA, protein, cytochrome oxidase, glucose-6-phosphatase, and acid phosphatase concentrations remain almost constant. Peroxisomal enzyme activities change to approximately 165%, 50%, 30%, and 0% of the controls for catalase, urate oxidase, L-alpha-hydroxy acid oxidase, and D-amino acid oxidase, respectively. For catalase the change results from a decrease in particle-bound activity and a fivefold increase in soluble activity. The average diameter of peroxisome sections is 0.58 +/- 0.15 mum in controls and 0.73 +/- 0.25 mum after treatment. Therefore, the measured peroxisomal enzymes are highly diluted in proliferated particles. After tissue fractionation, approximately one-half of the normal peroxisomes and all proliferated peroxisomes show matric extraction with ghost formation, but no change in size. In homogenates submitted to mechanical stress, proliferated peroxisomes do not reveal increased fragility; unexpectedly, Su-13437 stabilizes lysosomes. Our results suggest that matrix extraction and increased soluble enzyme activities result from transmembrane passage of peroxisomal proteins. The changes in concentration of peroxisomal oxidases and soluble catalase after Su-13437 allow the calculation of their half-lives. These are the same as those found for total catalase, in normal and treated rats, after allyl isopropyl acetamide: about 1.3 days, a result compatible with peroxisome degradation by autophagy. A sequential increase in liver RNA concentration, [14C]leucine incorporation into DOC-soluble proteins and into immunoprecipitable catalase, and an increase in liver size and peroxisomal volume per gram liver, characterize the trophic effect of the drug used. In males, Su-13437 is more active than CPIB, another peroxisome proliferation-inducing drug; in females, only Su-13437 is active.

Solubilization of enzymes from glyoxysomes of maize scutellum

Glyoxysomes isolated from maize scutella (Zea mays L.) were subjected to several disruptive treatments (osmotic shock, resuspension in an alkaline medium, addition of detergent). The damaged glyoxysomes were centrifuged at 89,500g for 40 minutes and several enzymic activities (isocitratase, malate synthetase, catalase, citrate synthetase, malate dehydrogenase) were measured in the supernatant fraction and in the pellet. Isocitratase is the most easily released of all glyoxysomal enzymes closely followed by malate synthetase. Citrate synthetase is in all instances the most insoluble enzyme. All of the enzymes had higher specific activities in the supernatant than in the pellet. These findings suggest that in corn scutellum glyoxysomes none of these enzymes is truly membrane-bound.

Nafenopin-induced hepatic microbody (peroxisome) proliferation and catalase synthesis in rats and mice. Absence of sex difference in response

Nafenopin (2-methyl-2[p-(1,2,3,4-tetrahydro-1-naphthyl)phenoxy]-propionic acid; Su-13437), a potent hypolipidemic compound, was administered in varying concentrations in ground Purina Chow to male and female rats, wild type (Cs(a) strain) mice and acatalasemic (Cs(b) strain) mice to determine the hepatic microbody proliferative and catalase-inducing effects. In all groups of animals, administration of nafenopin at dietary levels of 0.125% and 0.25% produced a significant and sustained increase in the number of peroxisomes. The hepatic microbody proliferation in both male and female rats and wild type Cs(a) strain mice treated with nafenopin was of the same magnitude and was associated with a two-fold increase in catalase activity and in the concentration of catalase protein. The increase in microbody population in acatalasemic mice, although not accompanied by increase in catalase activity, was associated with a twofold increase in the amount of catalase protein. The absence of sex difference in microbody proliferative response in nafenopin-treated rats and wild type mice is of particular significance, since ethyl-alpha-p-chlorophenoxyisobutyrate (CPIB)-induced microbody proliferation and increase in catalase activity occurred only in males. Nafenopin can, therefore, be used as an inducer of microbody proliferation and of catalase synthesis in both sexes of rats and mice. The serum glycerol-glycerides were markedly lowered in all the animals given nafenopin, which paralleled the increase in liver catalase. All the above effects of nafenopin were fully reversed when the drug was withdrawn from the diet of male rats. During reversal, several microbody nucleoids were seen free in the hyaloplasm or in the dilated endoplasmic reticulum channels resulting from a rapid reduction in microbody matrix proteins after the withdrawal of nafenopin from the diet. Because of microbody proliferation and catalase induction with increasing number of hypolipidemic compounds, additional studies are necessary to determine the interrelationships of microbody proliferation, catalase induction, and hypolipidemia.

A zonal rotor method for the preparation of microperoxisomes from epithelial cells of guinea pig small intestine

A method is described for the preparation of catalase particles from homogenates made from suspensions of epithelial cells of the small intestine of the guinea pig. Electron microscope examination of the preparations revealed the presence of small diaminobenzidine-positive particles measuring 0.1-0.3 nm in diameter and resembling the microperoxisomes observed by Novikoff and Novikoff (1972. J. Cell Biol.53:532.). Analytical data upon which the method is based are presented. The technique consisted of a rate zonal separation of microperoxisomes from large particles followed by an isopycnic separation from less dense organelles. Application of the method yielded microperoxisomes purified between 20- and 30-fold.

Hepatic microbody proliferation and catalase synthesis induced by methyl clofenapate, a hypolipidemic analog of CPIB

The effects of the administration of methyl clofenapate (methyl-2-[4-(p-chlorophenyl)phenoxy]2-methylpropionate) on the inducibility of hepatic microbody (peroxisome) proliferation and catalase synthesis were studied in male rats and in both sexes of wild type (Cs(a) strain) and acatalasemic (Cs(b) strain) mice. These investigations included electron microscopic examination of livers, assay of liver catalase activity, quantitation of catalase protein by immunotitration procedure, and measurements of serum cholesterol and glyceride-glycerol levels. In all groups of animals administration of methyl clofenapate at dietary concentrations of 0.015, 0.05 and 0.125% produced a significant and sustained increase in number of hepatic microbody (peroxisome) profiles. There was no appreciable increase in mitochondrial population, but several mitochondria were markedly enlarged and possessed numerous cristae. The hepatic microbody proliferation in male rats and in both sexes of wild type mice following methyl clofenapate administration was associated with a twofold increase in catalase activity and in the concentration of catalase protein. The increase in microbody population in acatalasemic mice, however, was not accompanied by a significant elevation of the catalase activity, which is due to the unusual heat lability of the mutant catalase enzyme. A marked decrease in serum cholesterol and glyceride-glycerol levels was observed in male rats following methyl clofenapate administration which paralleled the increase in liver catalase activity. In both strains of mice there was a significant reduction in serum glyceride-glycerol concentrations. All the above effects of methyl clofenapate were fully reversed when the drug was withdrawn from the diet of male wild type mice. The demonstration of microbody proliferation and catalase induction with hypolipidemic compounds, CPIB, nafenopin and, in these studies, with methyl clofenapate suggests a possible but as yet unclarified relationship between microbodies and hypolipidemia.

The synthesis and turnover of rat liver of rat liver peroxisomes. IV. Biochemical pathway of catalase synthesis

Early events in the biosynthesis of liver catalase were studied on female rats receiving [(3)H]leucine or [(3)H]delta-aminolevulinic acid or a mixture of [(3)H]leucine with [(14)C]delta-aminolevulinic acid by intraportal injection. Catalase antigen was selectively separated from homogenates by immunoprecipitation, both without and after partial purification of the enzyme. Label from both precursors appeared first in immunoprecipitable material which was lost upon purification of catalase; the label subsequently became associated with material indistinguishable from catalase. Kinetic analysis of the results indicates that the nonpurifiable material identified by early labeling consists of two distinct biosynthetic intermediates, the first lacking heme and representing about 1.6% of the total catalase content or 13 microg/g liver, the second containing heme and representing about 0.5% of the total catalase content or 4 microg/g liver. The first intermediate migrates at the same rate as catalase upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and therefore has a monomeric molecular weight of about 60,000.

Localization of enzymes within microbodies

Microbodies from rat liver and a variety of plant tissues were osmotically shocked and subsequently centrifuged at 40,000 g for 30 min to yield supernatant and pellet fractions. From rat liver microbodies, all of the uricase activity but little glycolate oxidase or catalase activity were recovered in the pellet, which probably contained the crystalline cores as many other reports had shown. All the measured enzymes in spinach leaf microbodies were solubilized. With microbodies from potato tuber, further sucrose gradient centrifugation of the pellet yielded a fraction at density 1.28 g/cm(3) which, presumably representing the crystalline cores, contained 7% of the total catalase activity but no uricase or glycolate oxidase activity. Using microbodies from castor bean endosperm (glyoxysomes), 50-60% of the malate dehydrogenase, fatty acyl CoA dehydrogenase, and crotonase and 90% of the malate synthetase and citrate synthetase were recovered in the pellet, which also contained 96% of the radioactivity when lecithin in the glyoxysomal membrane had been labeled by previous treatment of the tissue with [(14)C]choline. When the labeled pellet was centrifuged to equilibrium on a sucrose gradient, all the radioactivity, protein, and enzyme activities were recovered together at peak density 1.21-1.22 g/cm(3), whereas the original glyoxysomes appeared at density 1.24 g/cm(3). Electron microscopy showed that the fraction at 1.21-1.22 g/cm(3) was comprised of intact glyoxysomal membranes. All of the membrane-bound enzymes were stripped off with 0.15 M KCl, leaving the "ghosts" still intact as revealed by electron microscopy and sucrose gradient centrifugation. It is concluded that the crystalline cores of plant microbodies contain no uricase and are not particularly enriched with catalase. Some of the enzymes in glyoxysomes are associated with the membranes and this probably has functional significance.