Induction of plasma membrane alkaline phosphatase in rat liver

We have determined alkaline phosphatase activity in total liver plasma membrane fractions from rats subjected to a partial hepatectomy and sham operated with or without manipulation of the liver. In all these cases, an increase of the enzyme activity was observed. Kinetic studies of alkaline phosphatase activity performed on plasma membrane fractions from rats subjected to a partial hepatectomy suggest that alkaline phosphatase increase is produced by de novo biosynthesis of enzyme molecules. Determination of alkaline phosphatase activity in purified plasma membrane subfractions corresponding to each of the three functional regions of the hepatocyte surface (blood sinusoidal, lateral and bile canalicular), indicates that the increase of the enzyme activity observed after partial hepatectomy is selectively induced in the bile canalicular domain of the hepatocyte plasma membrane.     

A histochemical study about changes in rat liver plasma membrane enzyme activities after galactosamine administration.

In rats changes in plasma membrane enzyme activities due to Gal-N intoxication were studied by enzymehistochemical methods. The bile canalicular 5'-nucleotidase and nucleoside polyphosphatase activities decreased; the sinusoidal 5'-nucleotidase remained unchanged. The bile canalicular leucyl-beta-naphthyl-amidase showed an increase in activity; the alkaline phosphatase activity remained unchanged. In contrast to the spotty necrosis, changes in plasma membrane enzyme activities were seen in all liver cells, suggesting that changes of these activities, occurring after Gal-N treatment, do not correlate with cell death. The conclusion was drawn that the deviations of the enzyme activities might be due to changes in the lipid environment of the enzyme proteins in the membrane. With the exception of alkaline phosphatase, partial hepatectomy caused the same changes in enzyme activities as did Gal-N intoxication. Nevertheless Gal-N administration to partial hepatectomized rats did not lead to hepatic necrosis. Galactose given simultaneously or within two hours after Gal-N prevented both changes in plasma membrane enzyme activities and hepatocellular damage. This suggests an important role of galactolipids and galactoproteins in the plasma membrane alterations.     

Bile duct ligation-induced redistribution of canalicular antigen in rat hepatocyte plasma membranes demonstrated by immunogold quantitation

Extrahepatic obstructive cholestasis has been demonstrated to induce a redistribution of domain specific membrane proteins in rat hepatocytes reflecting loss or even reversal of cell polarity. In order to further characterize the redistribution of canalicular antigens, we used the Lowicryl K4M immunogold technique for examination of the effects of bile duct ligation (50 h) on the distribution of antigen in rat hepatocytes at the ultrastructural level and quantitated immuno-gold density in the three domains of the plasma membrane. In normal hepatocytes, antigen was localized almost exclusively in the canalicular domain while the sinusoidal and lateral membranes showed only weak immunoreactivity. Other localizations included organelles compatible with known pathways of biosynthesis and degradation. Bile duct ligation markedly reduced immunolabel in the canalicular and increased it slightly in the sinusoidal domain. The number and staining intensity of immunoreactive subcanalicular lysosomes and vesicles probably representing endosomes was augmented. Number of immunogold particles per micron of plasma membrane were 7.86 vs 2.46 (P less than 0.005) in the canalicular, 1.16 vs 1.38 (n.s.) in the sinusoidal, and 1.23 vs 1.08 (n.s.) in the lateral domain resulting in a canalicular decrease by 68.7% and a sinusoidal increase of 19.0%. Overall decrease in total plasma membranes was by 29.7% (P less than 0.05). Thus, our data show that the sinusoidal and lateral domains behave differently. Furthermore, quantitative immunocytochemistry demonstrates a decrease in the canalicular antigen density and suggests a sinusoidal increase. The present data agree with the concept that bile duct ligation results in a loss or even reversal of cell polarity in hepatocytes.     

Preservation of hepatocyte plasma membrane domains during cell division in situ in regenerating rat liver

We have utilized antibodies against five domain-specific integral proteins of the rat hepatocyte plasma membrane to examine the fates of the plasma membrane domains during hepatocyte division in the regenerating rat liver. The proteins were quantified on immunoblots of liver homogenates prepared during the peak of hepatocyte mitotic activity, 28-30 hr after two-thirds hepatectomy. Two sinusoidal/lateral proteins, CE 9 and the asialoglycoprotein receptor, and one bile canalicular protein, dipeptidylpeptidase IV, were not changed significantly in amount; whereas one sinusoidal/lateral protein, the epidermal growth factor receptor, and one bile canalicular protein, HA 4, were reduced to less than or equal to 50% of control levels. Light microscopic examination of plastic sections of regenerating liver tissue revealed that the mitotic hepatocytes generally appeared to retain normal contacts with neighboring interphase hepatocytes. Immunofluorescence was used to localize the domain-specific proteins on mitotic hepatocytes identified in 0.5-micron frozen sections of 28- to 30-hr regenerating liver tissue. Independent of mitotic stage, the hepatocytes retained mutually exclusive bile canalicular and sinusoidal/lateral domains, as defined at the molecular level by the distributions of specific proteins, such as HA 4 and CE 9, respectively.     

Calmodulin may decrease cell surface sialic acid and be involved in the expression of fibronectin during liver regeneration

The decrease of sialic acid in plasma membrane glycoproteins and the expression of cell surface fibronectin were studied during the pre-replicative phase of liver regeneration. The aim of this study was to correlate these cell-surface events to the intracellular surge of calmodulin observed a few hours after partial hepatectomy. The fact that calmodulin decreased the specific activity of UDP-N-acetyl-D-glucosamine 2'-epimerase, a key regulatory enzyme in the biosynthesis of glycoprotein sialic acids, and that trifluoperazine prevented the desialylation indicates that the membrane desialylation is a calmodulin-dependent process. On the other hand, Western blotting using anti-rat fibronectin antibody in trifluoperazine-treated animals suggests that calmodulin may also be involved in the surface expression of fibronectin in regenerating hepatocytes.     

Bile duct ligation-induced redistribution of canalicular antigen in rat hepatocyte plasma membranes demonstrated by immunogold quantitation

Summary Extrahepatic obstructive cholestasis has been demonstrated to induce a redistribution of domain specific membrane proteins in rat hepatocytes reflecting loss or even reversal of cell polarity. In order to further characterize the redistribution of canalicular antigens, we used the Lowicryl K4M immunogold technique for examination of the effects of bile duct ligation (50 h) on the distribution of antigen in rat hepatocytes at the ultrastructural level and quantitated immuno-gold density in the three domains of the plasma membrane. In normal hepatocytes, antigen was localized almost exclusively in the canalicular domain while the sinusoidal and lateral membranes showed only weak immunoreactivity. Other localizations included organelles compatible with known pathways of biosynthesis and degradation. Bile duct ligation markedly reduced immunolabel in the canalicular and increased it slightly in the sinusoidal domain. The number and staining intensity of immunoreactive sub-canalicular lysosomes and vesicles probably representing endosomes was augmented. Number of immunogold particles per m of plasma membrane were 7.86 vs 2.46 (P<0.005) in the canalicular, 1.16 vs 1.38 (n.s.) in the sinusoidal, and 1.23 vs 1.08 (n.s.) in the lateral domain resulting in a canalicular decrease by 68.7% and a sinusoidal increase of 19.0%. Overall decrease in total plasma membranes was by 29.7% (P<0.05). Thus, our data show that the sinusoidal and lateral domains behave differently. Furthermore, quantitative immunocytochemistry demonstrates a decrease in the canalicular antigen density and suggests a sinusoidal increase. The present data agree with the concept that bile duct ligation results in a loss or even reversal of cell polarity in hepatocytes.This study was supported by the Swiss National Science Foundation grants 3.846.0.87 (to L.L.) and 3.992.0.87 (to P.J.M.)     

Quantitative and qualitative changes of plasma membrane glycoproteins in the early period of liver regeneration

Alterations of cell surface glycoconjugates have been observed in many developing systems and may be important in the physiological control of growth and differentiation. Liver regeneration after partial hepatectomy is a suitable model in which to study the regulatory mechanisms of cell proliferation in vivo. We have isolated the sinusoidal plasma membrane of hepatocytes at different times after partial hepatectomy. The sialic acid content and the SDS-polyacrylamide gel electrophoresis pattern of glycoproteins were determined. A decrease of periodic acid-Schiff-profiles, a change in the binding capacities of 125I-concanavalin A, a reduction of the sialic acid content and the appearance and disappearance of specific components have been observed during the pre-replicative phase of liver regeneration. These findings during this early period are consistent with the active involvement of the plasma membrane glycoproteins in the transition of cells to the proliferative state.     

Rearrangement of nuclear calmodulin during proliferative liver cell activation

Calmodulin increases about three-fold in rat liver nuclei after partial hepatectomy. The increase is maximal after 24 hours, when DNA synthesis is also maximal. During the same time re-distribution of calmodulin within the nuclear structure takes place, leading to its association with the nuclear matrix. Incubation of normal rat liver nuclei with Ca2+ induces association of calmodulin with the matrix, indicating that the re-distribution of calmodulin during the replicative period is related to the increase in nuclear Ca2+. The nuclear matrix contains several calmodulin binding proteins of which one, having Mr of 130 kDa, has been identified as myosin light chain kinase (MLCK). Three acceptor proteins, having Mr of 120, 65, and 60 kDa decrease 24 hours after partial hepatectomy, MLCK and a protein of Mr 150 kDa instead increase.     

Changes in sialic acid content of the plasma membrane in hepatocellular proliferation

The content of sialic acid bound to the sinusoidal region of plasma membrane during the prereplicative phase after the intravenous injection of a solution containing triiodothyronine, amino acids, glucagon and heparin (T.A.G.H. solution) has been measured. The results obtained show that an important decrease in sialic acid content is produced as it occurs in the hepatic cells of hepatectomized animals. In order to know if sialidase activity is involved in the decrease of sialic acid content during liver regeneration, the activity of sinusoidal plasma membrane sialidases during the prereplicative phase after the partial hepatectomy has been studied. No modifications of sialidase activity were detected during this period of time indicating that this decrease in sialic acid content has to be produced by other mechanisms such as diminution in the synthesis of precursor molecules. On the other hand due to the importance of Ca2+-calmodulin complexes in the activation of the hepatic cell proliferation the possible implication of this complex on the loss of sialic acid, observing the effect of trifluoperazine (inhibitor of Ca2+-calmodulin complexes) during the prereplicative phase of liver regeneration has been studied. The results show a delay in the decrease of the amount of sugar studied from 10 to 12 hours compared to the results obtained with the hepatectomized rats that have not received trifluoperazine.     

Analysis of hepatocyte plasma membrane domains during rat development using monoclonal antibodies

The plasma membrane of adult rat hepatocyte consists of three domains, which have been identified by the monoclonal antibodies A39 and A59 as markers of the sinusoidal domain, B1 of the lateral, and B10 of the canalicular domains (Eur J Cell Biol 39:122, 1985). These monoclonal antibodies were used to study, by indirect immunocytochemistry, formation of the hepatocyte plasma membrane domains during development, from day 15 of gestation to day 35 post partum. The antigens defined by A39, B1, and B10 were detected, from day 15, over the major part of the hepatocyte plasma membrane except for the membranes of newly formed bile canaliculi, which were not labeled by B1 and only poorly labeled, if at all, by A39 and B10. As soon as fetuses were 16 days old, B1 labeled predominantly the lateral domain, as in the adult. Labeling with B10 progressively intensified on the membranes of bile canaliculi, but localization was not exclusively canalicular until day 21 post partum. A39 intensely labeled the canalicular membranes at 19-21 days of gestation, while at 35 days post partum it exhibited the predominantly sinusoidal labeling observed in adult hepatocytes. The antigen defined by A59 was not detected before birth and was found exclusively on the sinusoidal domain, as in the adult. These results show that the patterns of antigen distribution on different plasma membrane domains establish themselves at different rates. The marked differences observed between fetal or neonatal and adult hepatocytes might be responsible for immaturity of liver functions in the neonate.     

Identification of rat hepatocyte plasma membrane proteins using monoclonal antibodies

We have localized and identified five rat hepatocyte plasma membrane proteins using hybridoma technology in combination with morphological and biochemical methods. Three different membrane preparations were used as immunogens: isolated hepatocytes, a preparation of plasma membrane sheets that contained all three recognizable surface domains of the intact hepatocyte (sinusoidal, lateral, and bile canalicular), and a glycoprotein subfraction of that plasma membrane preparation. We selected monoclonal IgGs that were hepatocyte specific and localized them using both immunofluorescence on 0.5-micron sections of frozen liver and immunoperoxidase at the ultrastructural level. One antigen (HA 4) was localized predominantly to the bile canalicular surface, whereas three (CE 9, HA 21, and HA 116) were localized predominantly to the lateral and sinusoidal surfaces. One antigen (HA 16) was present in all three domains. Only one antigen (HA 116) could be detected in intracellular structures both in the periphery of the cell and in the Golgi region. The antigens were all integral membrane proteins as judged by their stability to alkaline extraction and solubility in detergents. The apparent molecular weights of the antigens were established by immunoprecipitation and/or immunoblotting. In a related study (Bartles, J.R., L.T. Braiterman, and A.L. Hubbard, 1985, J. Cell. Biol., 100:1126-1138), we present biochemical confirmation of the domain-specific localizations for two of the antigens, HA 4 and CE 9, and demonstrate their suitability as endogenous domain markers for monitoring the separation of bile canalicular and sinusoidal lateral membrane on sucrose density gradients.     

Redistribution and fate of colchicine-induced alkaline phosphatase in rat hepatocytes: possible formation of autophagosomes whose membrane is derived from excess plasma membrane

The redistribution and fate of colchicine-induced alkaline phosphatase (ALPase) in rat hepatocytes were investigated by electron microscopic enzyme cytochemistry and biochemistry. ALPase activity markedly increased in rat hepatocytes after colchicine treatment (2.0 mg/kg body weight, intraperitoneal injection). At 20–24 h after colchicine treatment, the liver showed the highest activity of ALPase. Thereafter, ALPase activity decreased and returned to normal levels at 48 h. In normal hepatocytes from control rats, ALPase activity was seen only on the bile canalicular membrane. However, at 20–24 h after colchicine treatment, colchicine-induced ALPase was redistributed in the sinusoidal and lateral (basolateral) membranes as well as in the bile canalicular membrane. At 30–36 h after colchicine treatment, ALPase activity on the basolateral membrane gradually decreased. In contrast, ALPase in the bile canalicular membrane increased along with the enlargement of bile canaliculi, suggesting that ALPase in the basolateral membrane had been transported to the bile canalicular membrane. Furthermore, ALPase-positive vesicles, cisternae and autophagosome-like structures were frequently seen in the cytoplasm. ALPase was also positive in some lysosomal membranes. ALPase in hepatocytes at 48 h after colchicine treatment returned to almost the same location as in control hepatocytes. Altogether, it is suggested that excessively induced ALPase is at least partially retrieved by invagination of the bile canalicular membrane and then transported to lysosomes for degradation. In addition, this study indicates that excess plasma membrane might be a possible origin of autophagosomal membrane.     

Isolation of rat hepatocyte plasma membranes. I. Presence of the three major domains

A rat liver plasma membrane preparation was isolated and characterized both biochemically and morphologically. The isolation procedure was rapid, simple and effective in producing a membrane fraction with the following biochemical characteristics: approximately 40-fold enrichment in three plasma membrane markers, 5'-nucleotidase, alkaline phosphodiesterase I (both putative bile canalicular membrane enzymes), and the asialo-glycoprotein (ASGP) receptor (a membrane glycoprotein present along the sinusoidal front of hepatocytes); a yield of each of these plasma membrane markers that averaged approximately 16%; and minimal contamination by lysosomes, nuclei, and mitochondria, but persistent contamination by elements of the endoplasmic reticulum. Morphological analysis of the preparation revealed that all three major domains of the hepatocyte plasma membrane (sinusoidal, lateral, and bile canalicular) were present in substantial amounts. The identification of sinusoidal membrane was further confirmed when ASGP binding sites were localized predominantly to this membrane in the isolated PM using electron microscope autoradiography. By morphometry, the sinusoidal front membrane accounted for 47% of the total membrane in the preparation, whereas the lateral surface and bile canalicular membrane accounted for 6.8% and 23% respectively. This is the first report of such a large fraction of sinusoidal membrane in a liver plasma membrane preparation.     

A two-dimensional electrophoretic analysis of the proteins and glycoproteins of liver plasma membrane domains and endosomes. Implications for endocytosis and transcytosis.

1. Polypeptides of liver plasma membrane fractions enriched in three surface domains of hepatocytes, blood-sinusoidal, lateral and bile canalicular, were analysed by isoelectric focusing (IEF) and non-equilibrium pH gel electrophoresis (NEPHGE) across a wide pH range, followed by SDS/PAGE. The overall Coomassie Blue-stained polypeptide patterns in the fractions were different. lateral plasma membrane fractions contained a characteristically higher number of polypeptides focusing at the basic pH range, whereas few basic polypeptides were present in sinusoidal plasma membrane fractions. The glycoproteins in these plasma membrane fractions stained by a lectin overlay technique with radio-iodinated concanavalin A, wheat-germ agglutinin and a slug lectin, were also different. 2. The polypeptides and glycoproteins of 'early' and 'late' endosome fractions were also compared by two-dimensional electrophoresis. Their composition was shown by Coomassie Blue staining, lectin overlay staining and in membranes metabolically labelled with [35S]methionine to be generally similar. The glycoproteins of sinusoidal plasma membranes and early and late endosomes were generally similar, but major differences in polypeptides of molecular mass 20-50 kDa, pI 7.5-8.5, in plasma membranes and endosomes were demonstrated, with a specific population of basic (pI 8-9) low-molecular-mass polypeptides being present at highest levels in 'late' endosomal fractions (shown by Coomassie Blue staining). 3. Analysis of the distribution of three specific membrane glycoproteins identified by using immunoblotting techniques showed that the asialoglycoprotein and the divalent-cation-sensitive mannose 6-phosphate receptors were present in sinusoidal plasma membrane and in early and late endocytic fractions: they were not detected in canalicular plasma membrane fractions. In contrast, 5'-nucleotidase was detected in all fractions examined. The role of the endocytic compartment in regulating trafficking pathways between the plasma membrane domains of the hepatocyte is discussed.     

A histochemical study about changes in rat liver plasma membrane enzyme activities after galactosamine administration

Summary In rats changes in plasma membrane enzyme activities due to Gal-N intoxication were studied by enzymehistochemical methods. The bile canalicular 5-nucleotidase and nucleoside polyphosphatase activities decreased; the sinusoidal 5-nucleotidase remained unchanged. The bile canalicular leucyl--naphthyl-amindase showed an increase in activity; the alkaline phosphatase activity remained unchanged. In contrast to the spotty necrosis, changes in plasma membrane enzyme activities were seen in all liver cells, suggesting that changes of these activities, occurring after Gal-N treatment, do not correlate with cell death. The conclusion was drawn that the deviations of the enzyme activities might be due to changes in the lipid environment of the enzyme proteins in the membrane.With the exception of alkaline phosphatase, partial hepatectomy caused the same changes in enzyme activities as did Gal-N intoxication. Nevertheless Gal-N administration to partial hepatectomized rats did not lead to hepatic necrosis. Galactose given simultaneously or within two hours after Gal-N prevented both changes in plasma membrane enzyme activities and hepatocellular damage. This suggests an important role of galactolipids and galactoproteins in the plasma membrane alterations.Dedicated to Prof. Dr. E. Havinga on the occasion of his 70th birthday     

Separation of hepatocyte plasma membrane domains by free flow electrophoresis

We have applied free flow electrophoresis to separate the canalicular and basolateral (sinusoidal and lateral) domains of rat hepatocyte plasma membranes. Hepatocyte plasma membranes were prepurified by rat zonal and discontinous sucrose gradient centrifugation. In electrophoretic separation, the canalicular membranes were more deflected toward the anode than the basolateral membranes. Na+-dependent taurocholate uptake could be measured in both membrane fractions, transport activity being highest in fractions containing the highest specific activity in the basolateral marker enzyme Na+-K+-ATPase. Thus, differences in electrophoretic mobility permit the separation of functional intact plasma membrane vesicles derived from basolateral and canalicular plasma membrane domains of rat hepatocyte.     

Lipopolysaccharide alters ecto-ATP-diphosphohydrolase and causes relocation of its reaction product in experimental intrahepatic cholestasis

Lipopolysaccharide (LPS), a bacterial endotoxin, exerts profound inflammatory actions toward various tissues and cells. We induced intrahepatic cholestasis in rats by administration of LPS and followed ecto-ATP-diphosphohydrolase (ecto-apyrase) activity in the liver. The activity of the enzyme had decreased to 77% 2 h after injection compared with the activity in control animals. The maximum decrease was detected 24 h after administration. The activity was found to have partially recovered 1 week after injection, but had yet to reach control levels. In contrast to the decrease in ecto-apyrase activity, there were increases in alkaline phosphatase activity and bilirubin concentration, markers of cholestasis. In response to LPS, the reaction product of ecto-apyrase was found to relocate from the canalicular domain of the plasma membrane of hepatocytes, its predominant localization in the liver of intact animals, to the basolateral and sinusoidal domains. The pattern of histochemical reaction indicated modulation of the enzyme activity and changes in trafficking of intracellular proteins. Taken together, our findings showed that LPS administration alters ecto-apyrase and causes relocation of its reaction product from the canalicular domain of the plasma membrane of hepatocytes in the rat. It is suggested that relocation of the reaction product may be a protective mechanism to enable the hepatocytes to withstand the cytokine-induced metabolic perturbations.     

Localization of the ecto-ATPase (ecto-nucleotidase) in the rat hepatocyte plasma membrane. Implications for the functions of the ecto-ATPase

The surface distribution of the plasma membrane Ca2+ (Mg2+)-ATPase (ecto-ATPase) in rat hepatocytes was determined by several methods. 1) Two polyclonal antibodies specific for the ecto-ATPase were used to examine the distribution of the enzyme in frozen sections of rat liver by immunofluorescence. Fluorescent staining was observed at the bile canalicular region of hepatocytes. 2) Plasma membranes were isolated from the canalicular and sinusoidal regions of rat liver. The specific activity of ecto-ATPase in the canalicular membranes was 22 times higher than that of sinusoidal membranes. The enrichment of the ecto-ATPase activity in the canalicular membrane is closely parallel to that of two other canalicular membrane markers, gamma-glutamyltranspeptidase and leucine aminopeptidase. 3) By immunoblots with polyclonal antibodies against the ecto-ATPase and the Na+,K+-ATPase, it was found that the ecto-ATPase protein was only detected in canalicular membranes and not in sinusoidal membranes, while the Na+,K+-ATPase protein was only detected in sinusoidal membranes and not in canalicular membranes. These results indicate that the ecto-ATPase is enriched in the canalicular membranes of rat hepatocytes.     

Characterization of rat hepatocyte plasma membrane domains by monoclonal antibodies

The hepatocyte plasma membrane consists of three morphologically and functionally distinct domains, the sinusoidal, the lateral and the canalicular. To study the distribution of antigenic determinants among these domains, we prepared monoclonal antibodies by immunizing mice with a crude, plasma membrane-enriched liver fraction. Four monoclonal antibodies were obtained that recognized various parts of the rat hepatocyte plasma membrane when tested by indirect immunofluorescence and immunoperoxidase assay performed on formaldehyde-fixed liver tissue. Each antibody gave a different staining pattern when analyzed by light and electron microscopy. A59 exclusively labelled the part of the sinusoidal membrane facing the sinusoids. A39 mainly labelled the sinusoidal membrane. B1 mainly labelled the lateral membrane, while the labelling by B10 was almost completely limited to the canalicular membrane. Immunoblotting showed that the antibody B1 recognized an antigen of approximately 100 kilodaltons and that B10 recognized an antigen of approximately 125 to 130 kilodaltons. These antibodies allow us to distinguish the three domains of the hepatocyte plasma membrane.     

Cytophotometric analysis of alkaline phosphatase and 5'-nucleotidase activity in regenerating rat liver after partial hepatectomy

Alkaline phosphatase and 5'-nucleotidase activities were analysed cytophotometrically in cryostat sections of female rat liver after partial hepatectomy. Alkaline phosphatase activity increased rapidly after operation up to a maximum seven-fold rise at 24 h in comparison with sham operated or control rats. There was no indication of preferential localization of alkaline phosphatase activity in either periportal or pericentral areas at any time point in control rats, sham operated rats or hepatectomized rats. Microscopical observation revealed that (a) all alkaline phosphatase activity was present at the bile canalicular surface of hepatocytes and (b) hepatocytes in mitosis did not show any increase in activity. These findings indicate that the high alkaline phosphatase activity after partial hepatectomy is not involved primarily in proliferation processes because cell division mainly takes place periportally. It may be needed for enhanced bile secretion by conversion of intracellular phosphorylcholine into choline which can be transported into the bile. The intracellular phosphorylcholine level is high after operation due to changes in phospholipid metabolism. 5'-Nucleotidase appeared to be three times higher pericentrally than periportally under normal conditions. Partial hepatectomy caused a 40 per cent decrease in activity in pericentral areas and only a small decrease periportally. It has been suggested that 5'-nucleotidase plays a role in breakdown of messenger RNA and its activity in control liver could be considerably lower periportally because plasma protein synthesis mainly takes place in this area.(ABSTRACT TRUNCATED AT 250 WORDS)