Preparation of plasma-membrane subfractions from isolated rat hepatocytes

A proteomic study reveals the diversified distribution of plasma membrane-associated proteins in rat hepatocytes

To investigate the heterogeneous protein composition of highly polarized hepatocyte plasma membrane (PM), three PM-associated subfractions were obtained from freshly isolated rat hepatocytes using density gradient centrifugation. The origins of the three subfractions were determined by morphological analysis and western blotting. The proteins were subjected to either one-dimensional (1-D) SDS-PAGE or two-dimensional (2-D) benzyldimethyl-n-hexadecylammonium chloride (BAC)/SDS-PAGE before nano-Liquid Chromatography-Electrospray Ionization--tandem mass spectrometry analysis (LC-ESI-MS/MS). A total of 613 non-redundant proteins were identified, among which 371 (60.5%) proteins were classified as PM or membrane-associated proteins according to GO annotations and the literatures and 32.4% had transmembrane domains. PM proteins from microsomal portion possessed the highest percentage of transmembrane domain, about 46.5% of them containing at least one transmembrane domain. In addition to proteins known to be located at polarized liver PM regions, such as asialoglycoprotein receptor 2, desmoplakin and bile salt export pump, several proteins which had the potential to become novel subfraction-specific proteins were also identified, such as annexin a6, pannexin and radixin. Our analysis also evaluated the application of 1-D SDS-PAGE and 2-D 16-BAC/SDS-PAGE on the separation of integral membrane proteins.

Effect of thyroid hormone binding proteins on insulin receptor binding of B1-thyronine-insulin analogues

Certain thyronine-insulin analogues, which form non-covalent complexes with plasma proteins, have been shown to act preferentially in the liver. We hypothesized that this property may be dependent on the ability of the analogue to bind to the insulin receptor without prior dissociation from the binding protein. NaB1-L-thyroxyl-insulin, NaB1-3,3',5'-triiodothyronine-insulin, NaB1-D-thyroxyl-insulin and NaB1-L-thyroxyl-aminolauroyl-insulin were compared with insulin for their capacity to inhibit the binding of [125I]TyrA14-insulin to rat liver plasma membrane in albumin-free buffer. Effective doses at 50% maximum inhibition of binding (ED50) were calculated with and without addition of the thyroid hormone binding proteins transthyretin, thyroxine binding globulin and human serum albumin. The binding of thyronine-insulin analogues to insulin receptors was inhibited in a dose-dependent manner by the addition of thyroid hormone binding proteins at concentrations in the physiological range. Complexes of thyronine-insulin analogues with thyroid hormone binding proteins exhibit impaired insulin receptor binding affinities compared with those of the analogues in their free form. Hepatoselectivity in vivo may not depend on binding of the intact complexes to hepatocytes. These results have implications for the physiological role of hormone binding proteins and the in vivo properties of other insulin analogues which bind to plasma proteins.

Biochemical analysis of a caveolae-enriched plasma membrane fraction from rat liver

We isolated and characterized a subcellular fraction derived from the blood-sinusoidal plasma membrane of hepatocytes enriched in caveolin and containing several of the molecular components described to be present in caveolae isolated from other cell types. A morphological study by electron microscopy revealed that it was composed of caveolae-attached membrane profiles. Immunoelectron microscopy of isolated fraction showed the specific labeling of internal caveolae membranes with anti-caveolin antibody. Finally, one- and two-dimensional electrophoresis and Western blotting were used for the biochemical analysis of this new rat liver plasma membrane fraction. From the biochemical and the morphological characterization, we conclude that the caveolae-enriched plasma membrane fraction is a plasma membrane fraction, which originates from specialized regions of the sinusoidal plasma membrane, enriched in caveolae.

Identification and distribution of proteins in isolated endosomal fractions of rat liver: involvement in endocytosis, recycling and transcytosis

1. The polypeptides of the three endosomal fractions isolated from livers of oestradiol-treated rats were analysed by two-dimensional gel electrophoresis. Silver-stained gels revealed that although the three endosomal fractions shared a generally similar pattern of approx. 120 components, qualitative and quantitative differences between the three endocytic fractions could be demonstrated. 2. The 'early' endosomes [compartment of uncoupling of receptors and ligands (CURL)] comprised the most complex fraction and contained most of the polypeptides found in the 'late' endosomes [multivesicular bodies (MVBs)] and the receptor recycling compartment (RRC). When CURL was analysed by two-dimensional gel electrophoresis after partition with Triton X-114, it showed the largest number of integral membrane polypeptides. 3. Some of the major receptors (polymeric immunoglobulin receptor, transferrin receptor, low-density lipoprotein receptor, asialoglycoprotein receptor, beta1-integrin, mannose 6-phosphate receptor, epidermal growth factor receptor and AGp110) and internalized ligands (IgA, IgG, albumin, haptoglobin, transferrin and alpha2-macroglobulin) were further studied by Western blotting. 4. The distribution of the identified receptors and ligands among the three endosomal fractions was in agreement with their expected functionalities. 5. The polypeptide composition of the bile was also examined and compared with ligands and proteins identified in the different endocytic fractions. 6. Finally, an electron microscopy study confirms the distinctive physical and ultrastructural features of the three isolated endosomal fractions.

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.

Isolation and culture of hepatocytes from the cynomolgus monkey (Macaca fascicularis)

Isolation and culture techniques for hepatocytes from whole livers of the cynomolgus monkey, Macaca fascicularis, are described. Hepatocytes were isolated by two-step perfusion of livers, using collagenase with hyaluronidase; fructose and trypsin inhibitor were included to reduce cell loss. Yields from a single liver average 4 X 10(9) cells with viabilities of 90.8 +/- 5.7%. Cells, plated on collagen substrates, were assessed for changes in morphology and various marker enzyme activities over a period of 7 d in culture. Cells exhibited a morphology similar to that observed for this species in vivo; little change in attached and spread cells was observed over the length of time monitored. Enzyme activities for catalase, succinate dehydrogenase, and tyrosine aminotransferase were observed to decrease significantly (though considerable activity remained), whereas acid phosphatase and 5'-nucleotide phosphodiesterase remained unchanged. Activity of cytochrome P-450 reductase was observed to increase slightly for the first 2 d, then decrease to about 60% of initial levels. Activity of alpha-mannosidase was stable for 4 d but was observed to be increased at Day 7. Cells were observed to retain metabolic responsiveness, demonstrated by glucose production by both gluconeogenesis and glycogenolysis in response to glucagon stimulation. The monkey hepatocytes obtained by methods described here thus retain hepatocellular morphology and activity through at least 1 wk in culture without medium or culture modification.

Plasma membrane fluidity in isolated rat hepatocytes: comparative study using DPH and TMA-DPH as fluorescent probes

The study of membrane fluidity is a rapidly expanding field of research and its interest in hepatology has been stressed recently. The present study is the first report concerned with the determination of membrane fluidity of isolated rat hepatocytes. The data have been compared with those obtained in plasma membrane fractions and subfractions (basolateral or canalicular) derived from homogenates. The fluorescent probes used to measure the fluidity were diphenylhexatriene (DPH) a 'classical' probe, and its derivative trimethylammoniodiphenylhexatriene (TMA-DPH) at 25 degrees C and at 37 degrees C. The values obtained with DPH were lower than those with TMA-DPH, probably due to the localization of the probes in different regions of the phospholipid bilayer. In addition, DPH revealed significant differences in the fluorescence polarization values obtained in isolated hepatocytes compared with membrane fractions, which was in contrast to TMA-DPH, where the respective values were of the same order of magnitude. This behaviour is probably due to the mobility of DPH in the membrane core and its rapid internalization into the cell, whereas TMA-DPH remains anchored for a long time on the cell surface. These findings suggest that TMA-DPH is a better probe than DPH for measuring the fluorescence polarization of whole isolated hepatocytes and that the use of different probes might be of help in exploring different zones of the membrane bilayer.

A localized surface protein of guinea pig sperm exhibits free diffusion in its domain

Using the technique of fluorescence redistribution after photobleaching, we are studying the cellular mechanisms involved in localizing surface molecules to particular domains. A number of antigens localized to discrete surface regions have been identified with monoclonal antibodies on guinea pig sperm cells ( Primakoff , P., and D. G. Myles , 1983, Dev. Biol., 98:417-428). One of these monoclonal antibodies, PT-1, binds exclusively to the posterior tail region of the sperm cell surface. PT-1 recognizes an integral membrane protein that in complex with n-octyl-beta-D-glucopyranoside has a sedimentation coefficient of 6.8S in sucrose density gradients. Fluorescence redistribution after photobleaching measurements reveal that within its surface domain the PT-1 antigen diffuses rapidly (D = 2.5 X 10(-9) cm2/s) and completely (greater than 90% recovery after bleaching). These results rule out for this membrane protein all models that invoke immobilization as a mechanism for maintaining localization. We propose that the mechanism for localization of the PT-1 antigen may be a barrier to diffusion at the domain boundary.

Inhibition by sporidesmin of hepatocyte bile acid transport

Exposure of isolated rat hepatocytes (approx. 2 x 10(7)--5 x 10(7) cells/10ml of incubation mixture) to 0.5 mg of the mycotoxin sporidesmin for 30--60 min at 37 degrees C produced loss of plasma-membrane microvilli with some disruption of organelle distribution in the sub-surface region. There was accompanying inhibition of [14C]cholate and [14C]taurocholate transport, but bile acid conjugation was not altered. Inhibition of cholate uptake was maximal after exposure of hepatocytes to sporidesmin for 1 min, and was not reversed by washing cells free of extracellular sporidesmin. N-Ethylmaleimide (0.1 mM) or dithiothreitol (1 mM) partially protected hepatocytes from sporidesmin inhibition of bile acid uptake. Significant protection was not given by other thiols or by zinc sulphate, cholesterol, ascorbate or alpha-tocopherol. The results are discussed in terms of sporidesmin action on cell membranes and the toxin's effect on bile secretion.

Modulation of a glycoprotein recognition system on rat hepatic endothelial cells by glucose and diabetes mellitus

The cellular location and carbohydrate specificities of a glycoprotein recognition system on rat hepatic sinusoidal cells have been determined. Purified preparations of endothelial, Kupffer, and parenchymal cells were prepared by collagenase liver perfusion, centrifugation on Percoll gradients, and centrifugal elutriation. (125)I-labeled agalactoorosomucoid, an N-acetylglucosamine-terminated glycoprotein, was selectively taken up in vitro by endothelial cells. Uptake was shown to be protein dependent, calcium ion dependent, and saturable, and could be described by Michaelis-Menten kinetics (apparent K(m) 0.29 muM; apparent maximum velocity 4.8 pmol/h per 5 x 10(6) cells). Uptake was inhibited not only by N-acetylglucosamine, mannose, and mannan but also by glucose, fructose, and a glucose-albumin conjugate. Inhibition by glucose was competitive over a wide range of concentrations and was almost 100% at a glucose concentration of 56 mM. Fasting and the induction of diabetes mellitus prior to isolation of cells was associated with 60% reductions in the recovery of endothelial cells. Uptake by cells isolated from fasted rats was enhanced (apparent maximum velocity 14.3 pmol/h per 5 x 10(6) cells without change in the apparent K(m)). These observations suggest that fasting is associated with a marked increase in the mean number of glycoprotein receptors per endothelial cell isolated from normal rats. This effect of fasting could be due to upregulation of glycoprotein receptors on endothelial cells or to the selective isolation of a subpopulation of endothelial cells from fasted animals that bears more glycoprotein receptors per cell than does another subpopulation of these cells. In addition, in vivo studies of the fate of intravenously administered (125)I-agalactoorosomucoid indicated that its rate of disappearance from plasma, hepatic accumulation, and catabolism were slower in diabetic than in normal rats. The results suggest that modulation of a carbohydrate-mediated glycoprotein recognition system located on hepatic endothelial cells can be induced by glucose and glucose-conjugated proteins and by fasting and diabetes mellitus. The findings in this study suggest a mechanism for abnormal glycoprotein metabolism in diabetes mellitus.

Correlations between enzymatic and immunologic properties of human peripheral blood mononuclear cells. I. Ectoenzymes of normal and immunodeficient peripheral blood mononuclear cells

The activity of plasma membrane marker enzymes which are involved in purine metabolism (5'-nucleotidase, alkaline 5'-nucleotide phosphodiesterase), in active ion transport (Na-K-Mg-adenosine triphosphatase, ouabain-sensitive Na-K-adenosine triphosphatase), in aminoacid transport (gamma-glutamyltranspeptidase), and in basic physiologic functions (alkaline phosphomonoesterase) were assayed in mononuclear cells isolated from peripheral blood of normal donors and of patients with primary immunodeficiency. Irrespective of the clinical classification of the immunodeficiency, the cells of patients were characterized by significantly diminished 5'-nucleotidase and to a certain extent by lower alkaline phosphomonoesterase activities. Average activity levels of other enzymes were similar in cells of patients and controls, but scattering was more pronounced in the first group. Determination of substrate affinity revealed different kinetic properties of 5'-nucleotidase in cells from patients and normal donors; however, the extent of inhibition by beta-glycerophosphate or alpha, beta-adenosine-methylene diphosphate was comparable for both types of cells. The presence of inhibitory compounds in patients' serum was excluded by mixing experiments. When activities of the various plasma-membrane-associated enzymes were compared with each other, significant correlations emerged in normal lymphocytes. Most of these correlations were absent in cell membranes of immunodeficient patients. The findings indicate that the plasma membrane of lymphocytes from patients with immunodeficiency may be characterized by an altered distribution of enzymatic constituents.

Partial purification and characterization of oestrogen receptors in subfractions of hepatocyte plasma membranes

To assess the subcellular distribution of oestrogen-binding components in their native state, plasma membrane and other cell fractions were prepared from hepatocytes in the absence of [(3)H]oestradiol-17beta. Cells from livers of ovariectomized rats were disrupted, with submaximal homogenization in buffered isotonic sucrose with CaCl(2) and proteinase inhibitor, and fractionated by using isotonic media. Fractions were characterized by determinations of enzyme activities, biochemical constituents and ligand binding. Specific binding of 2nm-[(3)H]oestradiol-17beta to intact cells and their fractions was detemined after equilibration for 1.5h at 4 degrees C. More than 92% of the radioactivity from representative preparations was verified as authentic oestradiol by thin-layer chromatography. Activities of plasma-membrane marker enzymes as well as binding sites for oestrogen and for wheat germ agglutinin were present principally in particulate fractions, rather than in 105000g-supernatant fractions. However, by using alternative homogenization procedures (i.e. hypotonic media), known to fragment and strip structural components, oestradiol-binding sites and activities of plasma-membrane marker enzymes were distributed predominantly into cytosol. By using the more conservative procedures, plasma membranes of low (rho=1.13-1.16) and high (rho=1.16-1.18) density were purified from crude nuclear fractions. A second low-density subfraction of plasma membrane was prepared from microsome-rich fractions. Activities of plasma-membrane marker enzymes were enriched to about 28 and four times that of the homogenate in plasma membranes of low and high density respectively. Binding sites for wheat germ agglutinin and oestradiol were concentrated in low-density plasma membranes to 46-63 times that of the homogenate. Specific binding of oestrogen in low-density plasma membranes purified from crude nuclei was saturable, with an apparent association constant of 3.5nm. At saturation, such oestradiol receptors corresponded to 526fmol/mg of membrane protein. A Hill plot showed a moderate degree of positive co-operativity in the interaction of hormone with plasma membranes. Specific binding of [(3)H]oestradiol-17beta was reduced by a 200-fold molar excess of unlabelled oestradiol-17beta, oestriol or diethylstilbestrol, but not by oestradiol-17alpha, cortisol, testosterone or progesterone. Binding was also blocked by prior exposure of membranes to trypsin or to 60 degrees C, but remained essentially undiminished by extraction of membranes with either hypotonic or high-salt buffers. Extraction with 0.1% (v/v) Triton X-100 partially solubilized the oestrogen-binding component(s) of plasma membranes. Particle-free extracts were resolved on 5-20% (w/v) sucrose density gradients with either 0.01m- or 0.4m-KCl, and the fractions were analysed by adsorption to hydroxyapatite. In low-salt gradients macromolecule-bound oestrogen sedimented at predominantly 7.4S and binding was 1560 times that of the homogenate. Under high-salt conditions oestradiol-binding activity occurred at both 3.6S and 4.9S.

Membrane characteristics of adult rat liver parenchymal cells in primary monolayer culture

Parenchymal cells, isolated from normal adult rat liver (3 x 10(7) cells/g liver) by collagenase perfusion and maintained in nondividing monolayer culture, were employed to investigate cell surface properties of hepatocytes. Membrane transport systems for asialoorosomucoid (A-OM) and methotrexate (MTX) were lost rapidly in culture, whereas induction of tyrosine aminotransferase and transport of alpha-aminoisobutyrate actually increased during the first 3 days. Alterations in the membrane transport systems for A-OM and MTX reflected more generalized modifications of cell surface components induced during primary culture. Thus, the binding of concanavalin A(Con A) and wheat germ agglutinin (WGA) to cultured hepatocytes increased approximately 2-fold between 24 and 96 hr, and the incorportion of radioactive mannose and glucosamine into trichloroacetic acid-insoluble proteins increased 13-fold and 4-fold, respectively. Plasma membranes were isolated from cultured hepatocytes and the major structural proteins and glycoproteins were analyzed by SDS-polyacrylamide gel electrophoresis. Membrane instability between 24 and 96 hr of culture was characterized by time-dependent alterations in specific polypeptides and extensive changes in Con A- and WGA-binding glycoproteins. Although addition of a complex hormone supplement to the medium increased the number of viable cells and sustained A-OM and MTX transport systems for 24 hr, it had no influence on the altered membrane protein and glycoprotein profiles observed in its absence.

Redistribution of membrane proteins in isolated mouse intestinal epithelial cells

Single mouse intestinal epithelial cells (IEC) may be isolated by the use of a combination of methods used for the isolation of IEC from other species. Isolated cells remain viable for several hours. The membrane integral enzymes alkaline phosphatase and leucine aminopeptidase of isolated IEC are localized to the brush borders of IEC in tissue and in most newly isolated IEC. With time, both enzymes are found distributed over the entire cell surface. Redistribution appears to occur by diffusion in the plane of the membrane. It is slowed, but not blocked, if cells are maintained at 0 degrees C instead of at 37 degrees C, and it is not blocked by fixation in 0.5-3% paraformaldehyde. Drugs that alter cell membrane potential or that affect cell levels of ATP enhance the rate of redistribution of the enzymes.

Heterogeneous distribution of the sodium-dependent alanine transport activity in the rat hepatocyte plasma membrane

The localization of the sodium-dependent alanine uptake activity in rat liver cells was studied. Fractions representative of the canalicular, the contiguous (lateral) and the blood-sinusoidal surface of the hepatocyte were isolated by means of centrifugal fractionation and density gradient centrifugation. The distribution of various marker-enzyme activities in conjunction with the occurrence of alanine transport activity was studied both in fractions obtained after zonal density gradient centrifugation, and in the subcellular fractions mentioned above. It is concluded that the sodium-dependent alanine transport activity is primarily located in the blood-sinusoidal plasma membrane of the hepatocyte.

Subcellular distribution and movement of 5'-nucleotidase in rat cells

1. Cell-surface 5'-nucleotidase was assayed by incubating whole-cell suspensions with 5'[3H]-AMP in iso-osmotic buffer and measuring [3H]adenosine production. The activity of cell-surface 5'-nucleotidase in hepatocytes, adipocytes and lymphocytes isolated from the rat was 15.0, 0.5 and 0.8pmol/min per cell at 37 degrees C respectively. 2. Disruption of the cells by vigorous mechanical homogenization or detergent treatment exposed additional 5'-nucleotidase activity, which represented 52%, 25% and 21% of the total activity in the three cell types respectively. This increase in 5'-nucleotidase activity which occurred when the cells were homogenized was due to a second pool of 5'-nucleotidase within the cell, rather than activation of the cell-surface enzyme. 3. In hepatocytes the intracellular 5'-nucleotidase activity was membrane-bound, indistinguishable from cell-surface 5'-nucleotidase in its inhibition by rabbit anti-(rat liver 5'-nucleotidase) serum and its kinetics with AMP, and was located on the extracytoplasmic face of vesicles within the cell. 4. The cell-surface 5'-nucleotidase of rat hepatocytes was rapidly inhibited when rabbit anti-(rat liver 5'-nucleotidase) serum or concanavalin A was added to the medium at 37 degrees C. Incubation with antiserum for 5 min at 37 degrees C inhibited 83 +/- 3% of the cell-surface enzyme. 5. Incubation of hepatocytes with exogenous antiserum or concanavalin A for 30 min at 37 degrees C resulted in over 50% inhibition of the intracellular enzyme. This inhibition was not prevented by disruption of the cytoskeleton or by ATP depletion. 6. Incubation of hepatocytes with exogenous antiserum or concanavalin A for up to 2h at 0 degrees C caused little or no inhibition of the intracellular enzyme, but over 75% inhibition of the cell-surface enzyme. 7. When surface-inhibited hepatocytes were washed and resuspended in buffer at 37 degrees C, 5'-nucleotidase was observed to redistribute from the intracellular pool to the cell surface.

Lipoprotein lipase activity of rat cardiac muscle. The intracellular distribution of the enzyme between fractions prepared from cardiac muscle and cells isolated from the hearts of fed and starved animals

1. Subcellular fractions, characterized by using morphological, compositional and enzymic markers, were prepared from rat heart tissue and cells isolated from the hearts of fed and 24 h-starved rats. 2. The lipoprotein lipase activity of fractions from whole tissue and isolated cells was determined in either fresh fractions or in acetone/diethyl ether powders of the fractions. 3. Lipoprotein lipase activity was present in all the fractions from tissue and cells, but was found to be of highest relative specific activity in the microsomal () fractions. 4. In fractions prepared from the isolated cells of hearts from starved rats the proportion of the total lipoprotein lipase present and its relative specific activity in the microsomal fraction were greater than in the equivalent fractions from fed animals. 5. The enhancement of lipoprotein lipase activity as a result of the acetone/diethyl ether powder preparation of fractions was most extensive in the microsomal fractions. 6. Investigation of the microsomal fraction showed that the lipoprotein lipase activity present was in two pools, one of which was within endoplasmic-reticulum vesicles. 7. The observations were consistent with the possibility that the cardiac-muscle cell could be the origin of the lipoprotein lipase activity functional in triacylglycerol uptake by the heart.

Investigation of the subcellular distribution of cyclic-AMP phosphodiesterase in rat hepatocytes, using a rapid immunological procedure for the isolation of plasma membrane

The distribution of cyclic-AMP phosphodiesterase was investigated in subcellular fractions prepared from homogenates of rat liver or isolated hepatocytes. When measured at 1 mM or 1 microM substrate concentration, approx. 35% or 50%, respectively, of enzyme activity was particulate. The soluble activity appeared to be predominantly a 'high Km' form, whereas the particulate activity had both 'high Km' and 'low Km' components. The recovery of cyclic-AMP phosphodiesterase was measured using 1 microM substrate concentraiton, in plasma membrane-containing fractions prepared either by centrifugation or by the use of specific immunoadsorbents. The recovery of phosphodiesterase was lower than that of marker enzymes for plasma membrane, and comparable with the recovery of markers for intracellular membranes. It was concluded that regulation of both 'high Km' and 'low Km' phosphodiesterase could potentially make a significant contribution to the control of cyclic AMP concentration, even at microM levels, in the liver. the 'low Km' enzyme, for which activation by hormones has been previously described, appears to be located predominantly in intracellular membranes in hepatocytes. The immunological procedure for membrane isolation allowed the rapid preparation of plasma membranes in high yield. Liver cells were incubated with rabbit anti-(rat erythrocyte) serum and homogenized. The antibody-coated membrane fragments were then extracted onto an immunoadsorbent consisting of sheep anti-(rabbit IgG) immunoglobulin covalently bound to aminocellulose. Plasma membrane was obtained in approx. 40% yield within 50 min of homogenizing cells.

Transport and metabolism of bromosulfophthalein by isolated rat liver cells

Kinetics of transport and metabolism of bromosulfophthalein have been studied in isolated liver cells in a dose-dependent manner obtaining the following results. The disposition of bromosulfophthalein in suspensions of isolated liver cells is similar to the turnover in the whole liver. The initial maximal rate of uptake of bromosulfophthalein is 2--3 times faster than intracellular conjugation with glutathione. Conjugation proceeds to an equilibrium between intracellular substrate (bromosulfophthalein) and product (bromosulfophthalein-glutathione conjugate) which are both transiently accumulated in the cell. Formation of bromosulfophthalein-glutathione is accompanied by an equimolar decrease of glutathione. The bromosulfophthalein-glutathione conjugate is slowly released from the cells in an energy-dependent and saturable transport process. The maximal velocity of excretion amounts to only 6% of the maximal velocity of uptake and to 20% of the maximal velocity of conjugation. Excretion, therefore, represents the slowest step in the overall turnover.

Responsiveness to glucagon by isolated rat hepatocytes controlled by the redox state of the cytosolic nicotinamide--adenine dinucleotide couple acting on adenosine 3':5'-cyclic monophosphate phosphodiesterase

1. The effects of changes in the cytoplasmic [NADH]/[NAD+] ratio on the efficacy of glucagon to alter rates of metabolism in isolated rat hepatocytes were examined. 2. Under reduced conditions (with 10mM-lactate), 10nM-glucagon stimulated both gluconeogenesis and urea synthesis in isolated hepatocytes from 48h-starved rats; under oxidized conditions (with 10mM-pyruvate), 10nM-glucagon had no effect on either of these rates. 3. The ability of glucagon to alter the concentration of 3':5'-cyclic AMP and the rates of glucose output, glycogen breakdown and glycolysis in cells from fed rats were each affected by a change in the extracellular [lactate]/[pyruvate] ratio; minimal effects of glucagon occurred at low [lactate]/[pyruvate] ratios. 4. Dose-response curves for glucagon-mediated changes in cyclic AMP concentration and glucose output indicated that under oxidized conditions the ability of glucagon to alter each parameter was decreased without affecting the concentration of hormone at which half-maximal effects occurred. 5. The phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (0.05 mM) significantly reversed the inhibitory effects of pyruvate on glucagon-stimulated glucose output. 6. For exogenously added cyclic [3H]AMP(0.1 mM), oxidized conditions decreased the stimulatory effect on glucose output as well as the intracellular concentration of cyclic AMP attained, but did not alter the amount of cyclic [3H]AMP taken up. 7. The effects of lactate, pyruvate, NAD+ and NADH on cyclic AMP phosphodiesterase activities of rat hepatocytes were examined. 8. NADH (0.01--1 MM) inhibited the low-Km enzyme, particularly that which was associated with the plasma membrane. 9. The inhibition of membrane-bound cyclic AMP phosphodiesterase by NADH was specific, reversible and resulted in a decrease in the maximal velocity of the enzyme. 10. It is proposed that regulation of the membrane-bound low-Km cyclic AMP phosphodiesterase by nicotinamide nucleotides provides the molecular basis for the effect of redox state on the hormonal control of hepatocyte metabolism by glucagon.

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.