Evidence for the translocation of 5'-nucleotidase across hepatic membranes in vivo

Therapeutic potentials of ecto-nucleoside triphosphate diphosphohydrolase, ecto-nucleotide pyrophosphatase/phosphodiesterase, ecto-5'-nucleotidase, and alkaline phosphatase inhibitors

The modulatory role of extracellular nucleotides and adenosine in relevance to purinergic cell signaling mechanisms has long been known and is an object of much research worldwide. These extracellular nucleotides are released by a variety of cell types either innately or as a response to patho-physiological stress or injury. A variety of surface-located ecto-nucleotidases (of four major types; nucleoside triphosphate diphosphohydrolases or NTPDases, nucleotide pyrophosphatase/phosphodiesterases or NPPs, alkaline phosphatases APs or ALPs, and ecto-5'-nucleotidase or e5NT) are responsible for meticulously controlling the availability of these important signaling molecules (at their respective receptors) in extracellular environment and are therefore crucial for maintaining the integrity of normal cell functioning. Overexpression of many of these ubiquitous ecto-enzymes has been implicated in a variety of disorders including cell adhesion, activation, proliferation, apoptosis, and degenerative neurological and immunological responses. Selective inhibition of these ecto-enzymes is an area that is currently being explored with great interest and hopes remain high that development of selective ecto-nucleotidase inhibitors will prove to have many beneficial therapeutic implications. The aim of this review is to emphasize and focus on recent developments made in the field of inhibitors of ecto-nucleotidases and to highlight their structure activity relationships wherever possible. Most recent and significant advances in field of NTPDase, NPP, AP, and e5NT inhibitors is being discussed in detail in anticipation of providing prolific leads and relevant background for research groups interested in synthesis of selective ecto-nucleotidase inhibitors.

Separation of the intracellular secretory compartment of rat liver and isolated rat hepatocytes in a single step using self-generating gradients of iodixanol

A novel method is described for the separation on a single gradient of the major intracellular organelles of the secretory pathway, the Golgi, the smooth endoplasmic reticulum, and the rough endoplasmic reticulum. Total microsomes were prepared from rat liver by differential centrifugation and resuspended in 20% iodixanol. The microsomal suspension was then layered between a 30% iodixanol cushion and a layer of 15% iodixanol and centrifuged in a vertical rotor for 2 h. The microsomes distributed in four visible bands. The gradients were collected by upward displacement and were characterized (i) by determination of UDP galactose-galactosyltransferase (Golgi marker) NADPH-cytochrome c reductase (endoplasmic reticulum marker) and RNA (rough endoplasmic reticulum marker); (ii) by immunoblotting for TGN38 (trans-Golgi marker) and GS28 (cis-Golgi marker) and for protein disulfide isomerase (endoplasmic reticulum lumenal marker); (iii) by determination of the lipid composition; and (iv) by electron microscopy. The results suggest that the top band (density 1.045-1. 090 g/ml), which contains 68% of the galactosyltransferase activity, consists of vesicles derived from the Golgi. The second broad band in the middle of the tube (density 1.130-1.160 g/ml), which contains 54% of the NADPH-cytochrome c reductase activity, consists mainly of vesicles derived from the smooth endoplasmic reticulum, overlapped at the top by a small band of Golgi-derived lamellae. The two bands at the bottom of the tube (density 1.130-1.160 and density 1.180-1. 220 g/ml) appear to contain two subfractions of vesicles derived from the rough endoplasmic reticulum.

Isolation and characterization of the ecto-5'-nucleotidase from a rat glioblastoma cell line

5'-Nucleotidase has been purified from rat glioblastoma cells (Rugli cells). The enzyme has been solubilized from plasma membranes by using Triton X-100 and CHAPS. Two affinity chromatographies on concanavalin A and 5'-AMP-Sepharose render the purified enzyme with a high specific activity (76.36 mumol AMP.min-1.mg-1). The purified enzyme gives a single polypeptide band on SDS-PAGE with an apparent molecular mass of 74 kDa. Active forms with an apparent molecular mass of 135 kDa and 268 kDa are observed when the purified enzyme is analyzed by gel filtration in the presence of either 0.6% sodium deoxycholate or 0.1% Triton X-100, respectively. The purified 5'-nucleotidase presents optimum activity at pH 7.8-8.1 either in the presence or in the absence of Mg2+. A linear Arrhenius plot is observed in the 25-46 degrees C temperature range and an activation energy of 33.7 KJ/mol is calculated. The enzyme is inhibited by EDTA; the activity is partially restored by different divalent cations as Zn2+, Mn2+, and Co2+. The hydrolysis of nucleosides 5'-monophosphate shows Michaelis kinetic. The enzyme is inhibited by nucleosides di- and triphosphate. 5'-Nucleotidase is a glycoprotein, being its activity inhibited at different extent by various lectins.

5'-Nucleotidase: molecular structure and functional aspects

Images

The presence and activity in normal and regenerating rat liver postmicrosomal supernatant fraction of an enzyme with properties similar to those of membrane-bound 5'-nucleotidase

An alkaline 5'-nucleotidase with properties similar to those of membrane-bound 5'-nucleotidase was recovered in soluble form in the postmicrosomal supernatant fraction (cytosol) of rat liver. The enzyme seems to constitute a quantitatively distinct fraction, since the activity in postmicrosomal supernatants was increased by a further 10% by additional homogenization of livers. Lysosomal acid phosphatase activity increased similarly, whereas other membrane-bound marker enzymes alkaline phosphatase, phosphodiesterase I and glucose-6-phosphatase showed no increase when homogenization of liver tissue was continued. Gel-permeation chromatography and pH-dependence studies indicated that enzyme activity in the supernatant fraction with 0.3 mM-UMP or -AMP as substrate at pH 8.1 was about 85 or 100% specific respectively. In regenerating liver the enzyme recovered in soluble form showed decreased specific activity, in contrast with alkaline phosphatase measured for comparison. The nucleotidase activity per mg of cytosolic protein was 2.1 nmol/min with AMP as substrate. The total activity measured in the postmicrosomal supernatant was 1.5% of the homogenate activity measured in the presence of detergent.

The membrane topography of ecto-5'-nucleotidase in rat hepatocytes

The transmembrane topography of the rat hepatocyte ectoenzyme 5'-nucleotidase was studied by the use of glycoprotein labelling and limited-proteolysis techniques. Comparison, by one-dimensional peptide mapping, of enzyme iodinated from outside the cell with that iodinated in the solubilized state showed that no additional iodination sites were revealed on solubilization. Incubation of newly synthesized enzyme in a microsomal membrane fraction with proteinase showed that the entire molecule of 5'-nucleotidase was protected from proteolysis. These data suggest that little, if any, of the 5'-nucleotidase molecule is present on the cytoplasmic side of the plasma membrane. No evidence was found for a previously proposed interaction between 5'-nucleotidase and actin, although the ability of preparations of 5'-nucleotidase to prevent inhibition of deoxyribonuclease I by actin was explained by minute traces of ATPase activity. Comparison of peptide maps of enzyme labelled by iodination or by methods specific for carbohydrate showed that in both cases predominantly one section of the molecule was labelled. It is proposed that the enzyme is a short-stalked integral membrane protein without a cytoplasmic domain in which about one-third of the molecule forms the accessible molecular surface.

Protein translocation across and integration into membranes

This review concentrates mainly on the translocation of proteins across the endoplasmic reticulum membrane and cytoplasmic membrane in bacteria. It will start with a short historical review and will pinpoint the crucial questions in the field. Special emphasis will be given to the present knowledge on the molecular details of the first steps, i.e., on the function of the signal recognition particle and its receptor. The knowledge on the signal peptidase and the ribosome receptor(s) will also be summarized. The various models for the translocation of proteins across and the integration of proteins into membranes will be critically discussed. In particular, the function of signal, stop-transfer, and insertion sequences will be dealt with and molecular differences discussed. The cotranslational mode of membrane transfer will be compared with the post-translational transport found for mitochondria and chloroplasts. This review will conclude with open questions and an outlook.

Localization of somatostatin receptors in secretion vesicles in anterior pituitary cells and pancreatic islets

Secretion vesicles in anterior pituitary cells and pancreatic islets appear to translocate somatostatin receptors from the cell interior to the plasma membrane. In this study we attempted to localize somatostatin receptors on either the cytoplasmic or the intraluminal surface of the secretion vesicles. 125I-somatostatin binding was determined in intact secretion vesicles and vesicles disrupted either by sonication or solubilization. The binding of 125I-somatostatin was identical in intact and disrupted vesicles, indicating cytoplasmic orientation of somatostatin receptors. Pronase treatment of intact secretion vesicles removed approximately 90% of specific somatostatin binding. Sonication of pronase treated secretion vesicles did not reveal latent somatostatin binding sites. Gold-conjugated somatostatin binds to isolated secretion vesicles confirming the presence of somatostatin binding sites on the outer surface of these vesicles. We conclude that somatostatin binding sites are located on the cytoplasmic surface of secretion vesicles isolated from anterior pituitary cells and pancreatic islets.

Mechanisms of glycosylation and sulfation in the Golgi apparatus: evidence for nucleotide sugar/nucleoside monophosphate and nucleotide sulfate/nucleoside monophosphate antiports in the Golgi apparatus membrane

The mechanism of translocation in vitro of sugar nucleotides and adenosine 3'-phosphate 5'-phosphosulfate (PAPS) into the lumen of rat liver Golgi apparatus vesicles has been studied. It has been previously shown that the Golgi apparatus membrane has specific carrier proteins for PAPS and sugar nucleotides. We now report that translocation of the above nucleotide derivatives across Golgi membranes occurs via a coupled equimolar exchange with the corresponding nucleoside monophosphates. An initial incubation of Golgi vesicles with GDP-fucose radiolabeled in the guanidine ring resulted in accumulation within the lumen of radiolabeled GMP. Exit of GMP from these vesicles was specifically dependent on the entry of (additional) GDP-fucose into the vesicles (GDP-mannose and other sugar nucleotides had no effect). GDP-fucose-stimulated exit of GMP was temperature dependent, was blocked by inhibitors of GDP-fucose transport, such as 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, and appeared to be equimolar with GDP-fucose entry. Preliminary evidence for specific, equimolar exchange of CMP-N-acetylneuraminic acid with CMP, PAPS with 3'-AMP, and UDP-galactose and UDP-N-acetylglucosamine with UMP was also obtained. These results strongly suggest the existence of different antiport proteins within the Golgi membrane that mediate the 1:1 exchange of sugar nucleotides or PAPS with the corresponding nucleoside monophosphate. Such proteins may have a regulatory role in glycosylation and sulfation reactions within the Golgi apparatus.

Regional and subcellular distribution in mammalian brain of the enzymes producing adenosine

The activities of 5'-nucleotidase, 2'-nucleotidase, alkaline phosphatase, and acid phosphatase were measured in rat and autopsied human brains. The four phosphatases in the rat brain showed little change in activity after death. The activities of adenosine-producing enzymes were compared in various parts of rat and human brains. When phosphatase activity was measured at pH 7.5, 5'-nucleotidase showed the highest activity in the most parts of the brain. The activity of 2'-nucleotidase and that of nonspecific phosphatase were almost the same at pH 7.5. However, higher phosphatase activity was observed in all parts of the brain when nonspecific phosphatase activity was measured at pH 10.0 or 5.5. High specific activity of 5'-nucleotidase in the brain was detected in the membranous components, especially in the synaptic membranes. The activity of 2'-nucleotidase was distributed in the soluble and synaptosomal fractions. The highest activity of both alkaline and acid phosphatases was recovered in the crude mitochondrial fraction, with the highest specific activity in the microsomal fraction. Phosphatase activity was distributed widely in the rat brain. The activity of 5'-nucleotidase was high in the medulla oblongata, thalamus, and hippocampus, but low in the peripheral nerve, spinal cord, and occipital lobe. The activity of 2'-nucleotidase was high in the vermis and frontal lobe. The highest acid and alkaline phosphatase activities were detected in the frontal lobe and in the olfactory bulb, respectively. The distribution of the four phosphatases in the autopsied human brain was similar to that in the rat brain. The highest 5'-nucleotidase activity was observed in the temporal lobe and thalamus.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation of plasma membrane, golgi apparatus, and endoplasmic reticulum fractions from single homogenates of mouse liver

Procedures to isolate plasma membrane, Golgi apparatus, and endoplasmic reticulum from a single homogenate of mouse liver are described. Fractions contain low levels of contaminating membranes as determined from morphometry and analyses of marker enzymes. The method requires only 2-3 gm of liver as starting material and yields approximately 0.7, 0.7, and 0.5 mg protein/gm liver, respectively, for endoplasmic reticulum, Golgi apparatus, and plasma membrane. Golgi apparatus fractions show high levels of galactosyltransferase activity and consist of cisternal stacks and associated secretory vesicles and tubules. Endoplasmic reticulum fractions are enriched in both glucose-6-phosphatase and nicotinamide adenine dinucleotide phosphate (reduced) (NADPH)-cytochrome c reductase and contain membrane vesicles with attached ribosomes. K+-stimulated p-nitrophenyl phosphatase and (Na+K+) adenosine triphosphatase activity are enriched in the plasma membrane fraction. This fraction consists of membrane sheets, many with junctional complexes, and bile canaliculi that are representative of the total hepatocyte plasma membrane. The fractionation procedure is designed to utilize small amounts of tissue (e.g., with liver slices), to reduce the total time required for fractionation, and to permit comparisons of constituents of plasma membrane, Golgi apparatus, and endoplasmic reticulum prepared from the same starting homogenates.

The transverse distribution of phospholipids in the membranes of Golgi subfractions of rat hepatocytes

The transverse distribution of phospholipids in the membranes of subfractions of the Golgi complex was investigated by using phospholipase C and 2,4,6-trinitrobenzenesulphonic acid as probes. In trans-enriched Golgi membranes, 26% of the phosphatidylethanolamine is available for reaction with trinitrobenzenesulphonate or for hydrolysis by phospholipase C, and 72% of the phosphatidylcholine is hydrolysed by phospholipase C. In cis-enriched Golgi membranes, 45% of the phosphatidylethanolamine is available for reaction with trinitrobenzenesulphonate and for hydrolysis by phospholipase C, and 95% of the phosphatidylcholine is hydrolysed by phospholipase C. Under the conditions used with either probe the contents of the Golgi vesicles labelled with either [3H]palmitic acid or [14C]leucine were retained. Galactosyltransferase activity of the membrane vesicles was partially inhibited by the experimental procedures used to investigate the transverse distribution of phospholipids. However, the residual activity was latent, suggesting that the vesicles remained closed. Trinitrobenzenesulphonic acid caused no detectable morphological change in either Golgi fraction. Phospholipase C treatment caused morphological changes, including fusion of vesicles and the appearance of 'signet-ring' profiles in some vesicles; however, the vesicles remained closed and the bilayer was retained. It appears, therefore, that neither probe causes major disruption of the Golgi vesicles nor gains access to the inner surface of the membrane bilayer. These observations suggest that phospholipids have a transverse asymmetry in Golgi membranes, that this distribution differs in trans and cis membranes, and that the phospholipid structure of Golgi membranes is inconsistent with a simple flow of membrane bilayer from endoplasmic reticulum to Golgi membranes to plasma membrane.

Characterization of microsomal subfractions from porcine endometrium cells

Crude microsomes from porcine endometrium and three subfractions obtained by a modification of Rothschild's technique were characterized by RNA/protein ratio, marker enzyme activities and morphological appearance. The microsomes were devoid of glucose-6-phosphatase activity. They contained approximately 10% of arylesterase-, approximately 30% of both NADPH-cytochrome reductase- and UDPgalactose-N-acetyl-glucosamine beta-D-galactosyltransferase- and approximately 60% of 5'-nucleotidase activities present in the homogenates. Subfraction I (smooth membranes) had twice the galactosyltransferase activity of Subfraction II (smooth and rough membranes + free ribosomes); both subfractions were rich in 5'-nucleotidase and cytochrome reductase activities. Subfraction III (rough membranes) had very low marker activities but exhibited the highest RNA/protein ratio, which was lowest in I.

A histochemical study of variations in the localization of 5'-nucleotidase activity in the acinar cell of the rat exocrine pancreas over the twenty-four hour period

The circadian variation of 5'-nucleotidase (AMPase) activity was studied in rat pancreatic exocrine cells. The localization of this enzyme, often associated with the plasmalemma, was studied by ultracytochemical methods at six time points over the 24-h period. The localization of AMPase activity exhibited a clear-cut circadian variation. During the light span strong activity was observed on the luminal plasmalemma, negative or weak activity on the baso-lateral plasmalemma and clearly visible activity on intracellular structures such as cytoplasmic vacuoles (fragmentation-like vesicles), dilated rims of the Golgi cisternae (or cisternal ends of the Golgi stacks), condensing vacuoles and lysosomal bodies. During the dark span the activity was detectable only on the baso-lateral plasmalemma. The fact that AMPase activity could not be found on the luminal plasmalemma during the dark span suggests that the luminal membranes may be replaced by the membranes of secretory granules, which do not display AMPase activity. The intracellular localization of AMPase activity during the light span, especially at 08.00 h, includes all cytoplasmic compartments which have hitherto been associated with the intracellular pathway for membrane retrieval from the plasmalemma. Moreover, the appearance of the activity in the dilated rims of the Golgi stack and condensing vacuoles indicates that these compartments may constitute a functional unit.

Hepatic Golgi fractions resolved into membrane and content subfractions

Golgi fractions isolated from rat liver homogenates have been resolved into membrane and content subfractions by treatment with 100 mM Na2CO3 pH 11.3. This procedure permitted extensive extraction of content proteins and lipoproteins, presumably because it caused an alteration of Golgi membranes that minimized the reformation of closed vesicles. The type and degree of contamination of the fractions was assessed by electron microscopy and biochemical assays. The membrane subfraction retained 15% of content proteins and lipids, and these could not be removed by various washing procedures. The content subfraction was contaminated by both membrane fragments and vesicles and accounted for 5 to 10% of the membrane enzyme activities of the original Golgi fraction. The lipid compositions of the subfractions was determined, and the phospholipids of both membrane and content were found to be uniformly labeled with [33P]phosphate administered in vivo.

Analytical study of microsomes and isolated subcellular membranes from rat liver VIII. Subfractionation of preparations enriched with plasma membranes, outer mitochondrial membranes, or Golgi complex membranes

Preparations enriched with plasmalemmal, outer mitochondrial, or Golgi complex membranes from rat liver were subfractionated by isopycnic centrifugation, without or after treatment with digitonin, to establish the subcellular distribution of a variety of enzymes. The typical plasmalemmal enzymes 5'-nucleotidase, alkaline phosphodiesterase I, and alkaline phosphatase were markedly shifted by digitonin toward higher densities in all three preparations. Three glycosyltransferases, highly purified in the Golgi fraction, were moderately shifted by digitonin in both this Golgi complex preparation and the microsomal fraction. The outer mitochondrial membrane marker, monoamine oxidase, was not affected by digitonin in the outer mitochondrial membrane marker, monoamine oxidase, was not affected by digitonin in the out mitochondrial membrane preparation, in agreement wit its behavior in microsomes. With the exception of NADH cytochrome c reductase (which was concentrated in the outer mitochondrial membrane preparation), typical microsomal enzymes (glucose-6-phosphatase, esterase, and NADPH cytochrome c reductase) displayed low specific activities in the three preparations; except for part of the glucose-6-phosphatase activity in the plasma membrane preparation, their density distributions were insensitive to digitonin, as they were in microsomes. The influence of digitonin on equilibrium densities was correlated with its morphological effects. Digitonin induced pseudofenestrations in plasma membranes. In Golgi and outer mitochondrial membrane preparations, a few similarly altered membranes were detected in subfractions enriched with 5'-nucleotidase and alkaline phosphodiesterase I. The alterations of Golgi membranes were less obvious and seemingly restricted to some elements in the Golgi preparation. No morphological modification was detected in digitonin-treated outer mitochondrial membranes. These results indicate that each enzyme is associated with the same membrane entity in all membrane preparations and support the view that there is little overlap in the enzymatic equipment of the various types of cytomembranes.

Orientation of glycoprotein galactosyltransferase and sialyltransferase enzymes in vesicles derived from rat liver Golgi apparatus

UDP-galactose: N-acetylglucosamine galactosyltransferase (GT) and CMP-sialic:desialylated transferrin sialyltransferse (ST) activities of rat liver Golgi apparatus are membrane-bound enzymes that can be released by treatment with Triton X-100. When protein substrates are used to assay these enzymes in freshly prepared Golgi vesicles, both activities are enhanced about eightfold by the addition of Triton X-100. When small molecular weight substrates are used, however, both activities are only enhanced about twofold by the addition of detergent. The enzymes remain inaccessible to large protein substrates even after freezing and storage of the Golgi preparation for 2 mo in liquid nitrogen. Accessibility to small molecular and weight substrates increases significantly after such storage. GT and ST activities in Golgi vesicles are not destroyed by treatment with trypsin, but are destroyed by this treatment if the vesicles are first disrupted with Triton X-100. Treatment of Golgi vesicles with low levels of filipin, a polyene antibiotic known to complex with cholesterol in biological membranes, also results in enhanced trypsin susceptibility of both glycosyltransferases. Maximum destruction of the glycosyltransferase activities by trypsin is obtained at filipin to total cholesterol weight ratios of approximately 1.6 or molar ratios of approximately 1. This level of filipin does not solubilize the enzymes but causes both puckering of Golgi membranes visible by electron microscopy and disruption of the Golgi vesicles as measured by release of serum albumin. When isolated Golgi apparatus is fixed with glutaraldehyde to maintain the three-dimensional orientation of cisternae and secretory vesicles, and then treated with filipin, cisternal membranes on both cis and trans faces of the apparatus as well as secretory granule membranes appear to be affected about equally. These results indicate that liver Golgi vesicles as isolated are largely oriented with GT and ST on the luminal side of the membranes, which corresponds to the cisternal compartment of the Golgi apparatus in the hepatocyte. Cholesterol is an integral part of the membrane of the Golgi apparatus and its distribution throughout the apparatus is similar to that of both transferases.

Assembly of proteins into membranes

Two pathways for protein assembly into biological membranes have been proposed. The "signal hypothesis" emphasizes the role of specific membrane proteins in binding the growing polypeptide and conducting it into the bilayer during its synthesis. The "membrane-triggered folding" hypothesis emphasizes self-assembly and the role of changing protein conformation during transfer from an aqueous compartment into a membrane. These ideas provide a framework for reviewing recent data on the biogenesis of membrane proteins.

Differences between rat liver epithelial and fibroblast cells in metabolism of purines

Epithelial and fibroblast cells from adult rat liver were found to differ markedly in their metabolism of the purine hypoxanthine. Both cell types took up hypoxanthine and possessed hypoxanthine-guanine phosphoribosyl transferase for phosphoribosylating the purine. However, in the transferase assay, lysates from epithelial cells converted hypoxanthine predominantly to inosine monophosphate, with small amounts of the nucleoside inosine as product, whereas fibroblast cell lysates converted hypoxanthine predominantly to inosine. The inosine appeared not to be produced by direct ribosylation of the base, since fibroblast cell lysates had less purine nucleoside phosphorylase activity than epithelial cell lysates. Rather, the inosine produced by fibroblast lysates appeared to be derived from inosine monophosphate through catabolism of the mononucleotide by 5' nucleotidase. An inhibitor of 5' nucleotidase, thymidine triphosphate, reduced the amount of inosine formed.

Effect of surface modifiers on an ectoenzyme: granulocyte 5'-nucleotidase

Several agents that react with plasma membranes, namely the native lectins concanavalin A, Ricinus communis agglutinin, and wheat germ agglutinin, the modified lectin succinyl concanavalin A, and sodium meta-periodate, inhibited the ecto-5'-nucleotidase of intact guinea pig granulocytes. Stimulation of the enzyme was not observed at any lectin concentration. Inhibition by native lectins could be blocked or reversed by appropriate competing hapten sugars. In the case of concanavalin A, reversal could be achieved at 37 degrees C, but not at 5 degrees C. When lectins were used in combination with each other, the effects were found to be largely independent. However, when concanavalin A and R. communis agglutinin were applied together, complications arose because the former lectin binds to the latter as well as to the cell surface. To avoid some of the complexities inherent in studying intact cell 5'-nucleotidase and to gain additional information about the system, two broken cell enzyme preparations were also examined. The enzyme of plasma membrane-enriched fractions was inhibited by all five agents mentioned above. 5'-Nucleotidase solubilized in sodium deoxycholate was inhibited by the four lectins but stimulated by periodate. The effects of the surface modifiers on kinetic data for all three enzyme preparations are consistent with the hypothesis that direct interactions with the enzyme molecule give rise to changes in Vmax; interactions at membrane sites other than 5'-nucleotidase itself could cause increases in apparent Km values. Effects of interactions of ectoenzymes with plant lectins may serve as models for phenomena that result from cell-cell interactions or from interactions of animal cells with lectin-like components of the cellular environment.

Direct evidence for internalization of mitochondrial aspartate aminotransferase into mitoplasts

Mitochondrial aspartate aminotransferase, an enzyme localized on the inner face of the inner mitochondrial membrane, is released into the intermembrane space upon addition of a "movement effector" (succinate, fumarate, pyruvate, or glutamate) [Waksman, A., & Rendon, A. (1974) Biochimie 56, 907-924]. After removal of the movement effector, 90% of the released enzyme rebound to mitoplasts. Lubrol fractionation showed that this bound activity was associated with the inner membrane. Internalization was demonstrated by using both enzymatic and molecular approaches. It was found that 70% of the reassociated enzyme became inaccessible from the outside of the mitoplast either to a nonpermeating substrate (NADH), to mild protease hydrolysis, or to recognition by a specific antibody. In contrast, in inside-out vesicles, the enzyme remained accessible to NADH, protease, and antibodies. Latency measurements performed at different temperatures on whole intact mitochondria confirmed the existence of reversible intermembrane movement of the enzyme in situ.

Transverse organization of phospholipids across the bilayer of plasma-membrane subfractions of rat hepatocytes

Phospholipase C treatment of vesicular subfractions of plasma membranes derived from the three functional domains of rat liver indicated that there is an asymmetric distribution of phospholipids across the bilayer of these membranes. The bile-canalicular and sinusoidal membranes were similar and different from the contiguous membrane.

Passage of serum-destined proteins through the Golgi apparatus of rat liver. An examination of heavy and light Golgi fractions

The participation of hepatic Golgi apparatus in the intracellular transport of blood-destined proteins has been analyzed using Golgi fractions enriched in cis and trans components of the Golgi apparatus. SDS-polyacrylamide gel electrophoresis of the liver Golgi fractions showed several proteins corresponding in relative proportions and mobilities with serum proteins. After a pulse injection of labeled leucine, the secretory content of the cis Golgi fraction was labeled earlier than the trans Golgi fraction. Taken together, the results show the participation of the liver Golgi apparatus in the secretion of most of the serum proteins and provide documentation for a sequential progression of secretory protein through the cis and trans components of the Golgi apparatus.

Membrane changes during cartilage maturation. Increase in 5'-nucleotidase and decrease in adenosine inhibition of adenylate cyclase

To examine the potential participation of the plasma membrane in differentiation, we studied the enzymatic activities of 5'-nucleotidase and adenylate cyclase as a function of chondrocyte maturation. 16-day-old chick embryo tibiae epiphyses were dissected into proliferative, growing and hypertrophying zones. Partially purified membrane fractions prepared by differential centrifugation from the respective tissue segments were assayed for enzymatic activity. Cell suspensions from the same segments were examined cytochemically for the presence of 5'-nucleotidase. The findings show that the 5'-nucleotidase activity of the chick embryo epiphyseal cartilage has the following characteristics: (a) it has a Km of about 25 muM for 5'AMP, and is inhibited by a mixture of 2' and 3'AMP (apparent Ki about 10(-4) M) and by AOPCP; (b) it is predominantly localized at the cell surface but is also detected in the cytoplasm and in association with nuclear heterochromatin; and (c) it increases 10-fold (on a DNA basis) during the maturation of the epiphyseal cartilage cells. The adenylate cyclase activity has these characteristics: (a) it does not change during chondrocyte maturation (on a DNA basis); (b) its susceptibility to adenosine inhibition decreases at least 10-fold. The implication of these findings relative to a possible role of adenosine in cellular communication is discussed.

5'-Nucleotidase activity of mouse peritoneal macrophages. II. Cellular distribution and effects of endocytosis

The diazonium salt of sulfanilic acid (DASA) can inactivate about 80% of the total 5'-nucleotidase of viable macrophages. The remaining 20% can be inactivated if the cells are first lysed in detergent, and presumably represents an intracellular pool of 5'-nucleotidase. The bulk of this pool may represent cytoplasmic vesicles derived from plasma membrane by endocytosis. This internal compartment is expanded up to threefold immediately after the cells have ingested a large latex load. This is consistent with previous observations on the internalization of 5'-nucleotidase in latex phagosomes. In latex-filled cells this intracellular pool of enzyme is inactivated over a few hours, and the cells then slowly increase their enzyme activity to nearly normal levels. However, 24 h after latex ingestion the metabolism of 5'-nucleotidase in these recovered cells is abnormal, as the rate of enzyme degradation is about twice the normal rate, and the DASA-insensitive enzyme pool in these cells is strikingly diminished. This may reflect effects of the accumulated indigestible particles on the fate of incoming pinocytic vesicles or on newly synthesized plasma membrane precursor. Another endocytic stimulus, concanavalin A, also reduces the total cell 5'-nucleotidase activity. This effect, which is time and temperature dependent, can be prevented by the competitive sugar alpha-methyl mannose. The concanavalin A inhibition can be reversed in the absence of new protein synthesis or in cells cultivated in serum-free conditions. It is not known whether the effect of concanavalin A on 5'-nucleotidase depends upon the interiorizaiton of plasma membrane or is strictly associated with events at the cell surface.

Association of a protease (plasminogen activator) with a specific membrane fraction isolated from transformed cells

The intracellular distribution of specific protease, plasminogen activator (PA), has been examined in Rous sarcoma virus-transformed chick embryo fibroblasts (RSV-CEF). Cellular homogenates were fractionated by differential centrifugation followed by sucrose gradient centrifugation. The activities and the percent distribution of a series of marker enzymes, specific for different subcellular organelles, were compared to those of PA. Normal CEF have been similarly fractionated and the relatively low amount of PA activity present in these cells has been analyzed in terms of its subcellular distribution. A membrane fraction was isolated from the RSV-CEF that contained the bulk of the PA activity and less than 8% of the total cellular protein. The specific activity of the PA in this fraction is 40-fold higher than that of a comparable fraction isolated from companion cultures of normal cells. This fraction contains little or no nuclear and cytoplasmic material and is contaminated only to a relatively small degree with mitochondria, lysosomes, endoplasmic reticulum. Significant amounts of a putative Golgi membrane marker are present in this fraction. The relatively high specific activities of Na+,K+-ATPase, 5'-nucleotidase, and [3H]fucose indicate that the fraction is enriched in surface membrane. Further purification of the fraction by equilibrium centrifugation on shallow sucrose gradients reduces further the contaminating activities and results in a PA distribution that closely parallels the distribution of the membrane enzyme, 5'-nucleotidase. PA was not released from its membrane association by hypotonic and hypertonic extraction and ultrasonication, while granule-bound enzymes were released by these treatments. The PA activity from hamster SV40 cells fractionated the same way as that of RSV-CEF. These results suggest that a protease that is dramatically enhanced upon malignant transformation is associated with "plasma membrane-like" elements of the cell and may serve as an intrinsic modifier of cell surface proteins after malignant transformation.

Subcellular distribution and response to gastroinetstinal hormones of adenylate cyclase in the rat pancreas. Partial purification of a stable plasma membrane preparation

1. The subcellular distribution of adenylate cyclase activity in rat pancreatic homogenates was examined after differential centrifugation. Divalent cations exerted significant effects on this distribution. In addition, the ratio of adenylate cyclase activities in the presence of the C-terminal octapeptide of cholecystokinin-pancreozymin and secretin was lower in the crude 'mitochondrial' fraction than in 'microsomal' fractions. This difference was due to the lability of cholecystokinin-pancreozymin receptors compared to secretin receptors. The Km,app of activation was affected more than the V by this lability. Such a degradation of cholecystokinin-pancreozymin receptors was markedly delayed by isolation and storage in the presence of a phospholipid mixture. 2. A simple, reasonably rapid (6 h), and easily reproducible method was developed to prepare a stable semi-purified plasma membrane fraction, characterized by a 10-fold increase in the specific activity of adenylate cyclase with respect to the whole homogenate. At variance with data obtained on crude subcellular fractions, the V of adenylate cyclase activity observed in this preparation, under maximal concentration of the C-terminal octapeptide of cholecystokinin-pancreozymin, was higher than that obtained with secretin or the vasoactive intestinal polypeptide.