Heterogeneous distribution of glucose 6-phosphatase in rat liver microsomal fractions as shown by adaptation of a cytochemical technique

Involvement of microsomal vesicles in part of the sensitivity of carnitine palmitoyltransferase I to malonyl-CoA inhibition in mitochondrial fractions of rat liver

Liver mitochondrial fractions as normally isolated contain only 10-20% of total mitochondria and may not be representative of the whole mitochondrial population. This study was designed to evaluate the dependence of the sensitivity of carnitine palmitoyl-transferase I (CPT I) to malonyl-CoA inhibition in mitochondrial fractions that are not normally studied. Four fractions prepared from rat liver were found to be contaminated to different extents by microsome vesicles, on the basis of marker-enzyme activities and micrographic data. Purification of mitochondrial fractions on a Percoll gradient decreased to some extent the microsomal contamination, which was due in part to the existence of close bonds between microsomes and the outer membranes of mitochondria. A greater degree of contamination of mitochondrial fractions by microsomes was correlated with a greater sensitivity of CPT I to malonyl-CoA inhibition. Attempts were made to enhance the sensitivity of CPT I to malonyl-CoA with the use of microsomes. Measurements performed by adding mitochondria and microsomes in the same CPT I assay failed to demonstrate any significant enhancement of malonyl-CoA inhibition. However, addition of ATP to a mixture of mitochondria and microsomes was shown to trigger the binding of both particles, as assessed by enzymic and micrographic data, and to increase the sensitivity of CPT I to malonyl-CoA inhibition. These results demonstrated that the binding of microsomes to mitochondria, unlike the simple mixing of both particles, was capable of altering the sensitivity of CPT I to malonyl-CoA. The data also suggest that this process could be of physiological importance, owing to the frequency of contiguous zones between mitochondria and endoplasmic reticulum observed in sections of intact liver cells.

Heterogeneity of smooth endoplasmic reticulum from rat liver studied by two-phase partitioning

Smooth microsomal membranes, prepared from rat liver by sucrose-density-gradient centrifugation, were subfractionated by counter-current distribution in an aqueous two-phase system consisting of poly(ethylene glycol) and Dextran T500. A comparison of the distribution curves of marker enzymes, together with theoretically calculated curves, indicated the presence of at least five membrane subfractions, differing in the ratios of the marker enzymes. Glucose-6-phosphatase and arylesterase distributed in one manner, and NADPH-cytochrome c reductase and NADH-ferricyanide reductase in another. Evidence for further heterogeneities in the distribution of marker enzymes in smooth microsomes was obtained by analysing the membrane domain structure using a recently described method [Albertsson (1988) Q. Rev. Biophys. 21, 61-98]. Phenobarbital treatment did not influence the behaviour of the marker enzymes.

On the nature of the interaction between 4,4'-diisothiocyanostilbene 2,2'-disulfonic acid and microsomal glucose-6-phosphatase. Evidence for the involvement of sulfhydryl groups of the phosphohydrolase

The effect of 4,4'-diisothiocyanostilbene 2,2'-disulfonic acid (DIDS) on microsomal glucose 6-phosphate hydrolysis has been reinvestigated and characterized in order to elucidate the topological and functional properties of the interacting sites of the glucose-6-phosphatase. The studies were performed on microsomal membranes, partially purified and reconstituted glucose-6-phosphatase preparations and show the following. (a) DIDS inhibits activity of the glucose-6-phosphatase of native microsomes as well as the partially purified glucose-6-phosphatase. (b) Inhibition is reversed when the microsomes and the partially purified phosphohydrolase, incorporated into asolectin liposomes, are modified with Triton X-114. (c) Treatment of native microsomes with DIDS and the following purification of glucose-6-phosphatase from these labeled membranes leads to an enzyme preparation which is labeled and inhibited by DIDS. (d) Preincubation of native microsomes or partially purified glucose-6-phosphatase with a 3000-fold excess of glucose 6-phosphate cannot prevent the DIDS-induced inhibition. (e) Inhibition of glucose-6-phosphatase by DIDS is completely prevented when reactive sulfhydryl groups of the phosphohydrolase are blocked by p-mecuribenzoate. (f) Reactivation of enzyme activity is obtained when DIDS-labeled microsomes are incubated with 2-mercaptoethanol or dithiothreitol. Therefore, we conclude that inhibition of microsomal glucose 6-phosphate hydrolysis by DIDS cannot result from binding of this agent to a putative glucose-6-phosphate-carrier protein. Our results rather suggest that inhibition is caused by chemical modification of sulfhydryl groups of the integral phosphohydrolase accessible to DIDS attack itself. An easy interpretation of these results can be obtained on the basis of a modified conformational model representing the glucose-6-phosphatase as an integral channel-protein located within the hydrophobic interior of the microsomal membrane [Schulze et al. (1986) J. Biol. Chem. 261, 16,571-16,578].

Heterogeneity of cyclic nucleotide phosphodiesterases in liver endoplasmic reticulum

A microsomal fraction from rat liver was subfractionated into three rough endoplasmic reticulum fractions RIII, RII and RI, together with a smooth endoplasmic reticulum plus Golgi fraction. Cyclic nucleotide phosphodiesterase activity was found in all fractions. Subsequently it was shown that Golgi fractions were essentially devoid of cyclic AMP phosphodiesterase activity and the activity resided in the smooth endoplasmic reticulum fraction. The activity of the endoplasmic reticulum constituted some 20% of the homogenate activity, with the major fraction of this being associated with the RII fraction and the least with the RI fraction. With the exception of the activity of the RI fraction, which was a peripheral enzyme, all of the other enzyme activities were integral, requiring detergent or repeated freeze-thawing to effect solubilization. All of the activities appeared to be exposed at the external surface of the endoplasmic reticulum, as they were inactivated by trypsin under conditions where glucose 6-phosphatase was not. All of these activities displayed distinct sensitivities to both thermal and trypsin inactivation, yielding activity decays consistent with a single enzyme species being present in each case. The freeze-thaw-solubilized enzymes yielded single symmetrical peaks on sucrose-density-gradient centrifugation and polyacrylamide-gel electrophoresis. The sedimentation coefficients for the enzymes in the smooth-endoplasmic-reticulum-plus-Golgi, RIII, RII and RI fractions were 3.2S, 4.2S, 4.5S and 4.5S respectively. Whereas the activity in the smooth-endoplasmic-reticulum-plus-Golgi fraction exhibited normal Michaelis kinetics, those in the other fractions yielded kinetics indicative of apparent negative co-operativity. All of the enzymes exhibited low Km values towards cyclic AMP. The enzymes did not appear to be regulated by Ca2+ or calmodulin. ZnCl2 was found to be a potent non-competitive inhibitor of the enzyme in all fractions. NaF was a weak non-competitive inhibitor. The bilayer fluidizing agent benzyl alcohol exerted dissimilar effects on the enzyme activities. It is concluded that the endoplasmic reticulum displays lateral heterogeneity, with single, rather distinct, cyclic AMP phosphodiesterases being found in the different fractions.

Regulation of glucose 6-phosphatase in hepatic microsomes by thyroid and corticosteroid hormones

The regulation of glucose 6-phosphatase in hepatic microsomes by thyroid and corticosteroid hormones has been studied following the administration of 3,3',5-triiodo-L-thyronine and/or triamcinolone to hypophysectomized rats. The apparent Km for glucose-6-P in isolated ("intact") microsomes increased following administration of either hormone; there was little or no difference in the apparent Km when microsomes were treated with sodium deoxycholate ("disrupted"). In intact microsomes, triiodothyronine caused a 2.3-fold increase in the Vmax of glucose 6-phosphatase; triamcinolone, a 4-fold increase; and both hormones together, a 4.4-fold increase. Corresponding values for disrupted microsomes were: triiodothyronine, 3.7-fold; triamcinolone, 1.8-fold; both hormones, 3.3-fold. After triiodothyronine treatment, disruption of microsomes caused an over 5-fold increase in Vmax; after triamcinolone treatment, the increase was only 1.5-fold. This difference could not be explained by a change in the energy of activation of glucose 6-phosphatase in either intact or disrupted microsomes following hormone treatment. Glucose 6-phosphatase was localized by a cytochemical procedure; the reaction product was associated with 90% of the profiles in all microsomal preparations, except for those from triiodothyronine-treated rats, where less than 50% contained lead precipitate. Vesicles free of lead phosphate were isolated from sucrose gradients and accounted for less than 10% of the protein and glucose 6-phosphatase in all preparations, again except for those from triiodothyronine-treated rats, where they represented 40% of both the protein and glucose 6-phosphatase. The results are consistent with a model for glucose 6-phosphatase in which the substrate is transported across the microsomal membrane by a specific carrier before hydrolysis within the cisternae by a phosphohydrolase. It is suggested that the effect of triiodothyronine is mainly on the activity of the phosphohydrolase, and triamcinolone, on that of the carrier.

Isolation and characterization of rough endoplasmic reticulum associated with mitochondria from normal rat liver

A subfraction of rough endoplasmic reticulum (RER) characterized by its close association with mitochondria (MITO) was isolated from low speed pellets of normal rat liver homogenate under defined ionic conditions. This fraction enriched in MITO-RER complexes contained 20% of cellular RNA, 20% of glucose-6-phosphatase and 47% of cytochrome c oxidase activities. Morphologically, the isolated MITO-RER complexes closely resembled physiological associations between the two organelles commonly seen in intact liver. Partial dissociation of RER from mitochondria of the MITO-RER fraction was achieved by either EDTA (0.5 mM) or by hypotonic/hypertonic treatment of MITO-RER complexes. With the latter procedure approx. 70% of RER (RERmito) with 50% of ribosomes still attached could be separated from the inner compartments of mitochondria. This RERmoto exhibited a higher glucose-6-phosphatase activity than RER isolated as rough microsomes from the postmitochondrial supernatant. Isopycnic centrifugation on linear metrizamide gradients revealed that the mitochondria-associated part of RER corresponds to the high density, ribosome-rich subfraction of rough microsomes isolated in cation-free sucrose solution. The combined data demonstrate that a morphologically and biochemically distinct portion of RER is associated with mitochondria and support the concept of considerable intracellular heterogeneities in distribution of enzymes and enzyme systems along the lateral plane of the endoplasmic reticulum membrane system.

Integrated stereological and biochemical studies on hepatocytic membranes. I.V. Heterogeneous distribution of marker enzymes on endoplasmic reticulum membranes in fractions

The purpose of the study was to consider quantitatively the relationships between the surface area of the endoplasmic reticulum (ER) and constituent marker enzyme activities, as they occur in fractions collected from rat liver homogenates. The ER surface area was estimated in five membrane-containing fractions by use of a combined cytochemical-stereological technique (5), while, at the same time, ER marker enzymes were assayed biochemically. Fraction/homogenate recoveries for the ER enzymes averaged 100%, total membrane surface area 98%, and ER surface area 96%. Relative specific activities, which compare the relative amounts of ER marker enzyme activities to the relative ER surface area in the membrane-containing fractions, indicate variable distributions for glucose-6-phosphatase and NADPH cytochrome c reductase, but not for esterase.

Subcellular distribution of delta 5-3 beta-hydroxy steroid dehydrogenase in the granulosa cells of the domestic fowl (Gallus domesticus)

1. The distribution of 3 beta-hydroxy steroid dehydrogenase was examined in the subcellular fractions of granulosa cells collected from the ovary of the domestic fowl. 2. 3 beta-hydroxy steroid dehydrogenase activity was observed in the mitochondrial (4000g for 20min) and microsomal (105 000g for 120min) fractions. 3. Approximately three times more 3 beta-hydroxy steroid dehydrogenase activity was associated with the cytochrome oxidase activity (a mitochondrial marker enzyme) in anteovulatory-follicle granulosa cells than with that of the postovulatory follicle. 4. Comparison of the latent properties of mitochondrial 3 beta-hydroxy steroid dehydrogenase with those of cytochrome oxidase and isocitrate dehydrogenase indicated that 3 beta-hydroxy steroid dehydrogenase is located extramitochondrially. 5. This apparent distribution of 3 beta-hydroxy steroid dehydrogenase is explained on the basis that the mitochondrial activity is either an artefact caused by a redistribution in the subcellular location of the enzyme, occurring during homogenization, or by the existence of a functionally heterogeneous endoplasmic reticulum that yields particles of widely differing sedimentation properties.

Nucleotide metabolism by microsomal UDP-glucuronyltransferase and nucleoside diphosphatase as determine by 31P nuclear-magnetic-resonance spectroscopy

31P n.m.r. spectroscopy was used to study the nucleotide kinetics of UDP-glucuronyltransferase and associated reactions in the liver microsomal fraction. The effects of Mg2+ and EDTA on these reactions were investigated qualitatively. It was found that the rabbit microsomal fraction has no nucleoside pyrophosphatase activity, that UDP was immediately hydrolysed and that it was released from the microsomal surface. Reverse glucuronyltransferase could be demonstrated. The results are discussed with reference to functional coupling of UDP-glucuronyltransferase to other enzymes and the effects of Mg2+ and EDTA on the system.

Fractionation of microsomal membranes on the basis of their surface properties

Partition in dextran-poly(ethylene glycol) aqueous-phase systems can be used for both membrane subfractionation and gaining information on membrane surface properties [H. Walter (1977) in Methods of Cell Separation (Catsimpoolas, N., ed.), vol. 1, pp. 307-354, Plenum, New York]. Smooth, light rough and heavy rough rat liver microsome (obtained by sucrose-density-gradient centrifugation) were subjected to countercurrent distribution in such a system. Smooth microsomal membranes had the highest, heavy rough microsomal membranes the lowest and light rough microsomal membranes an intermediate partition coefficient. The separation is based primarily on hydrophobic differences in the membrane surfaces of the three preparations and is thus due to microsomal properties not previously utilized in their fractionation. The method permits additional subfractionations of microsomes.

Two fractions of rough endoplasmic reticulum from rat liver. II. Cytoplasmic messenger RNA's which code for albumin and mitochondrial proteins are distributed differently between the two fractions

Subcellular fractions were obtained from rat liver homogenates under conditions which prevented degradation of polysomes (pH 8.5 and high ionic strength). Rough endoplasmic reticulum (RER) was recovered in high yields from a low-speed nuclear pellet (rapidly sedimenting endoplasmic reticulum, RSER) and from a postmitochondrial supernate (rough microsomes). The polysomal RNA content of these two fractions was very similar. When polyA+-RNA's were translated inthe mRNA-dependent wheat embryo cell-free system, both fractions yielded polypeptide products which had similar electrophoretic patterns on sodium dodecyl sulfate (SDS)-polyacrylamide gels. Activities of messenger RNA's which code for albumin and for polypeptides destined for transport to the inner membrane and matrix of mitochondria (i.e. 'mitoplasts') were assayed by translating in the more active rabbit reticulocyte cell-free system followed by immunoprecipitation of radioactive products and coelectrophoresis with immunoprecipitated marker proteins on SDS-polyacrylamide gels. These tests indicated that albumin mRNA is about equally distributed between the two fractions of RER, or slightly enriched in the RSER fraction when activity is expressed as a percentage of total polypeptide synthesis. Activities of cytoplasmic mRNA's which code for at least some mitoplast proteins could be detected in both fractions, but all were enriched in the rough microsome fraction, not the RSER (two- to threefold when corrected for differences in total polypeptide synthesis in the lysate). Comparisons of mRNA's from free vs. membrane-bound polysomes indicated that most of the albumin mRNA activity (86-91%) and mitoplast protein mRNA activities (75%) were present in the bound fraction. Assuming that RSER and rough microsomes do not derive exclusively from different cells types, the evidence suggests that, compared to albumin and most other membrane-bound mRNA's, cytoplasmic mRNA's coding for mitoplast proteins may be preferentially segregated or compartmentalized within the cell on the microsomal class of RER.

Enzyme and phospholipid asymmetry in liver microsomal membranes

The transverse distribution of enzyme proteins and phospholipids within microsomal membranes was studied by analyzing membrane composition after treatment with proteases and phospholipases. Upon trypsin treatment of closed microsomal vesicles, NADH- and NADPH-cytochrome c reductases as well as cytochrome b5 were solubilized or inactivated, while cytochrome P-450 was partially inactivated. When microsomes were exposed to a concentration of deoxycholate which makes them permeable to macromolecules but does not disrupt the membrane, the detergent alone was sufficient to release four enzymes: nucleoside diphosphatase, esterase, beta-glucuronidase, and a portion of the DT-diaphorase. Introduction of trypsin into the vesicle lumen inactivated glucose-6-phosphatase completely and cytochrome P-450 partially. The rest of this cytochrome, ATPase, AMPase, UDP-glucuronyltransferase, and the remaining 50% of DT-diaphorase activity were not affected by proteolysis from either side of the membrane. Phospholipase A treatment of intact microsomes in the presence of albumin hydrolyzed all of the phosphatidylethanolamine, phosphatidylserine, and 55% of the phosphatidylcholine. From this observation, it was concluded that these lipids are localized in the outer half of the bilayer of the microsomal membrane; Phosphatidylinositol, 45% of the phosphatidylcholine, and sphingomyelin are tentatively assigned to the inner half of this bilayer. It appears that the various enzyme proteins and phospholipids of the microsomal membrane display an asymmetric distribution in the transverse plane.

Effects of trypsin and phospholipases A2 and C on enzyme organization in testis microsomes

Pregnenolone and progesterone concentrated in the microsomal fraction of cryptorchid mouse testis compared with mitochondria and cytosol. While the concentrating mechanisms had high capacity and low association constants the effect did not seem to be due to nonspecific solubility in the lipid components since 17-hydroxyprogesterone, dehydroepiandrosterone, androstenedione and testosterone did not show differential concentration. Also digestion with phospholipases A2 and C to the point where most of the phospholipids were specifically split, only lowered the differential binding of pregnenolone and progesterone by less than half. Trypsin had a greater effect, short digestion at 0 degrees C lowering the specific binding to 35-40% and decreasing the steroid dehydrogenases to a similar extent. The members of the mixed function oxidase system in the testis microsomes were particularly sensitive to trypsin, cytochrome P-450 and, as a consequence, 17alpha-hydroxylase and 17, 20-lyase activity being eliminated under tha same conditions while liver microsomal cytochrome P-450 was hardly affected. Bonds split by trypsin seem to play a more important role in the hydroxylase activity of testis microsomes than in the hepatic system.

Morphological and biochemical changes in the hepatic endoplasmic reticulum and golgi apparatus of male Xenopus laevis after induction of egg-yolk protein synthesis by oestradiol-17 beta

This report describes morphological and biochemical changes accompanying oestrogen induced synthesis of the egg-yolk protein precursor, vitellogenin, in male Xenopus liver. Extensive proliferation of the rough and smooth endoplasmic reticulum and the Golgi apparatus occurs between 3 and 9 days after administration of oestradiol-17 beta. Subcellular fractionation showed that microsomal fractions have an increased number of ribosomes available for protein synthesis, hormone treatment enhances the in vitro protein synthetic capacity per unit of RNA; both in microsome and ribosome preparations. Polypeptides synthesized in vitro by ribosome preparations show an enrichment in serine content after hormone treatment and an increased proportion of ribosomes can be immunoprecipitated by antibodies directed against vitellogenin. Our data are consistent with the proposal that vitellogenin is synthesized on the ribosomes of the rough endoplasmic reticulum and processed and packaged for secretion in the smooth endoplasmic reticulum and Golgi apparatus. Response to hormonal induction of vitellogenin involves an early phase in which membrane proliferation occurs in order to increase the cellular capacity to synthesize, process and secrete large quantities of egg-yolk protein precursor.

Effect of glutaraldehyde and lead on the activity of hepatic glucose-6-phosphatase. A biochemical and cytochemical study

The sensitivity of mouse liver glucose-6-phosphatase activity towards glutaraldehyde fixation has been analysed by biochemical and cytochemical means. The degree of enzymatic inhibition and various enzymatic properties have been studied. Several differences have been observed in the Km determination, the sensitivity to pH 5 and the activity related to pH between fixed and unfixed enzymes. The role of Pb++ ions in the cytochemical media has also been estimated. It is concluded that several enzymatic differences appear between fixed and unfixed enzymes and that the inhibition by Pb ions is dependent on the buffer and on the amount of substrate used.

On the involvement of a glucose 6-phosphate transport system in the function of microsomal glucose 6-phosphatase

A model for microsomal glucose 6-phosphatase (EC 3.1.3.9) is presented. Glucose 6-phosphatase is postulated to be resultant of the coupling of two components of the microsomal membrane: 1) a glucose 6-phosphate - specific transport system which functions to shuttle the sugar phosphate from the cytoplasm to the lumen of the endoplasmic reticulum; and 2) a catalytic component, glucose-6-P phosphohydrolase, bound to the luminal surface of the membrane. A large body of existing data was shown to be consistent with this hypothesis. In particular, the model reconciles well-documented differences in the kinetic properties of the enzyme of untreated and modified microsomal preparations. Characteristic responses of the enzyme to changes in nutritional and hormonal states may be attributed to adaptations which alter the relative capacities of the transport and catalytic components.