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Biomedical subjects

L Jarett

Publications and source records attributed to L Jarett.

At least 145 records · Page 8Linked to original sources
PubMed1978

Nonuniform distribution and grouping of insulin receptors on the surface of human placental syncytial trophoblast.

These studies were designed to investigate the cytologic localization and topographic distribution of insulin receptors in human placental villi. Biochemical studies showed placental villi to specifically bind 125I-insulin. Radioautographic studies showed the specific binding to be localized to the surface of the syncytial trophoblast. Topographic distribution of insulin binding was determined with ferritin-insulin. Initial studies using ferritin-insulin containing some oligomers of ferritin revealed the insulin receptors to be specifically associated with the glycocalyx region of the surface membranes of microvilli. No insulin receptors were detectable in association with the intermicrovillous plasma membrane even though its glycocalyx is in direct continuity with the glycocalyx of microvilli. Monomeric ferritin-insulin showed the same nonuniform distribution of the insulin receptor, which suggests that there is not complete freedom of lateral mobility of the insulin receptors in the surface membrane of this tissue. The insulin receptors were found to occur as singletons or in groups of two or more. Incubations with monomeric ferritin-insulin at 4 degrees or with tissue prefixed with formaldehyde showed that the groups of insulin receptors were naturally occurring, i.e., they are present prior to and independent of insulin binding and thus not secondary to ligand-induced aggregation. The physiologic meaning of the nonuniform distribution and the groups of insulin receptors is unclear at present.

Cell Membrane↗
PubMed1977

Properties of passive binding of calcium to endoplasmic reticulum from adipocytes.

Calcium binding to isolated adipocyte microsomes enriched in endoplasmic reticulum has been characterized. Binding was concentration-dependent, saturable, and totally dissociable. Steady state was reached within 20 min at all calcium concentrations tested. Three apparent classes of binding sites were identified in kinetic and steady state studies using calcium concentrations from 1 muM to 10 mM. The affinity constants (and maximum binding capacities) as determined by computer analysis for the three classes were 2.1 X 10(5) M-1 (0.28 nmol of calcium/mg of protein), 1.3 X 10(4) M-1 (1.1 nmol/mg), and 1.3 X 10(2) M-1 (35 nmol/mg). The dissociation rate constants for the high and intermediate affinity classes of sites were 1.6 X 10(-3) S-1, respectively, and the association rate constant for the high affinity sites was 8 X 10(2) M-1 S-1. The affinity constant calculated from the rate constants was 5.0 X 10(5) M-1 for the high affinity sites in agreement with the value obtained in studies at steady state. The three classes of binding sites were specific for calcium. Magnesium was a noncompetitive inhibitor of calcium binding to all three classes of sites with a Ki of 9 to 12 mM. Calcium binding at 1 muM calcium was 50% inhibited by 18 muM La3+, 600 muM Sr2+, or 2.7 mM Ba2+. These data represent the first analysis of passive calcium binding to endoplasmic reticulum from nonmuscular cells and the first report of corresponding rate constants for either endoplasmic or sarcoplasmic reticulum. The characteristics of the binding are consistent with the properties of calcium transport by endoplasmic reticulum of adipocytes. The characteristics and specificity of the calcium binding constitute further evidence that endoplasmic reticulum plays an important role in cellular calcium homeostasis.

Adipose Tissue↗
PubMed1976

Energy-dependent calcium transport in endoplasmic reticulum of adipocytes.

The endoplasmic reticulum from isolated rat adipocytes has the ability to actively accumulate calcium. The calcium uptake was characterized using the 20,000 X g supernatant (S1 fraction) of total cellular homogenate. Endoplasmic reticulum vesicles isolated from the S1 fraction as a 160,000 X g microsomal pellet prior to testing demonstrated little ability to accumulate calcium. The calcium uptake in the S1 fraction was localized to the endoplasmic reticulum vesicles by morphologic appearance, by the use of selective inhibitors of calcium uptake, and by high speed sedimentation of the accumulated calcium. The uptake was MgATP- and temperature-dependent and was sustained by the oxalate used as the intravesicular trapping agent. Uptake was linear with time for at least 30 min at all calcium concentrations tested (3 to 100 muM) and exhibited a pH optimum of approximately 7.0. The sulfhydryl inhibitor p-chloromercuribenzene sulfonate produced a dose-dependent inhibition of calcium uptake with total inhibition at 0.07 mumol/mg protein. Ruthenium red and sodium azide inhibited less than 5% of the uptake at concentrations (5 muM and 10 mM, respectively) which completely blocked calcium uptake by mitochondria isolated from the same cells. The Km for calcium uptake was 10 muM total calcium which corresponded to approximately 3.6 muM ionized calcium in the assay system. The maximum velocity of the uptake was 5.0 nmol (mg of microsomal protein)-1 (min)-1 at 24 degrees under the assay conditions used and exhibited a Q10 of 1.8. The uptake activity of the endoplasmic reticulum vesicles in the S1 fraction exhibited a marked time- and temperature-dependent lability which might account in part for the lack of uptake in the isolated microsomal fraction. This energy-dependent calcium uptake system would appear to be of physiologic importance to the regulation of intracellular calcium.

4-Chloromercuribenzenesulfonate↗
PubMed1976

Characterization of calcium binding to adipocyte plasma membranes.

Calcium binding to adipocyte plasma membranes has been assessed by equilibrium dialysis and by membrane filtration techniques. Calcium binding was specific and saturable, displaying two distinct classes of binding sites. The affinity constants and maximum binding capacities in the presence of 0.1 M KCl were 4.5 X 10(4) M-1 and 1.8 nmol/mg of protein and 2.0 X 10(3) M-1 and 13.7 nmol/mg for the high and low affinity sites, respectively. Bound calcium was totally dissociated in the presence of excess calcium within 11.0 min in two distinct phases corresponding to the two classes of sites. Association and dissociation rate constants for the high affinity sites were 7.7 X 10(2) M-1S-1 and 9.2 X 10(-3S-1 respectively. Free energy changes at 24 degrees were +6.4 kcal mol-1 for the high affinity sites and +4.5 kcal mol-1 for the low affinity sites. The high affinity sites demonstrated a pH optimum of 7.0 whereas the binding to the low affinity sites progressively increased between pH 6.0 and 9.0. Low concentrations of MgCl2 (less than 300 muM) enhanced calcium binding slightly, whereas high concentrations of KCl and MgCl2 were noncompetitive inhibitors of calcium binding. Procaine and ruthenium red had no effect on calcium binding and lanthanum was a poor inhibitor of calcium binding. This represents the first report of calcium binding to adipocyte plasma membranes and the first kinetic analysis of calcium binding to biological membranes. The specificity of this calcium-binding system in adipocyte plasma membranes suggests its importance in cellular bioregulation.

Adipose Tissue↗
PubMed1976

Adipocyte beta-adrenergic receptors. Identification and subcellular localization by (-)-[3H]dihydroalprenolol binding.

(--)-[3H]Dihydroalprenolol, a potent beta-adrenergic antagonist, was used to identify binding sites which have the characteristics of beta-adrenergic receptors in membranes from rat adipocytes. The subcellular distribution of the (--)-[3H]Dihydroalprenolol binding sites was examined. The binding sites were predominantly in the plasma membrane fraction, consistent with the proposal that the physiologically significant beta-adrenergic receptors are localized in the adipocyte plasma membrane. Binding of (--)-[3H]dihydroalprenolol to unfractionated adipose membranes was saturable with 0.24 pmol bound/mg of protein at saturation. Half-maximal saturation occurred at 15 nM providing an estimate of the equilibrium dissociation constant, KD, for the interaction of (--)-[3H]dihydroalprenolol with its adipocyte receptor. Kinetic analysis of (--)-[3H]dihydroalprenolol binding provided a value of 2.4 X 10(7) M-1 min-1 for the forward bimolecular rate constant, k1. Dissociation of (--)-[3H]dihydroalprenolol was a first order reaction with a rate constant, k2, of 2.94 X 10(-1) min-1. The ratio k2/k1 = 12 nM provides an independent measurement of the KD for the interaction of (--)-[3H]dihydroalprenolol with its receptor which is in good agreement with the values obtained by steady state analysis (12 to 15 nM). Beta-Adrenergic agonists and antagonists competed for the binding sites in unfractionated adipocyte membranes with a typical beta1-adrenergic specificity. The order of potency of agonists was (--)-isoproterenol greater than (--)-norepinephrine congruent to (--)-epinephrine. The beta-adrenergic antagonist, (--)-propranolol, potently competed for the binding sites with a KD of 17 nM. Compounds such as dihydroxyphenylaline, dihydroxymandelic acid, normetanephrine, pyrocatechol, and phentolamine which are structurally related to beta-adrenergic agents, but are devoid of beta-adrenergic physiologicl effects in adipocytes, did not compete for the binding sites. Binding was highly stereospecific, the (+) isomers of adrenergic agonists and antagonists requiring 23- to 330-fold higher concentrations to half-maximally inhibit binding than the corresponding (--) stereoisomers. (--)-[3H]Dihydroalprenolol binding was examined highly enriched plasma membrane, mitochondrial, and microsomal (endoplasmic reticulum) fractions of adipocytes. In the presence of 12 nM (--)-[3H]dihydroalprenolol, the specific activity of binding in the plasma membrane fraction was 5-fold higher than that of the mitochondrial fraction and 8-fold higher than that of the microsomal (endoplasmic reticulum) fraction. The specificity and affinity characteristics of the plasma membrane binding sites were found to be virtually identical with those of the unfractionated adipocyte membranes. The observation that (--)-[3H]dihydroalprenolol binding sites are predominantly localized in the plasma membrane fraction suggests the potential usefulness of this ligand as a marker for adipocyte plasma membranes.

Adenylyl Cyclases↗
PubMed1976

Ability of insulin to increase calcium binding by adipocyte plasma membranes.

Calcium specifically binds to adipocyte plasma membranes, demonstrating two classes of binding sites having affinity constants of 4.5 x 10(4) M-1 and 2.0 x 10(3) M-1. Insulin (100 microunits/ml) added directly to the isolated plasma membranes caused no alteration in calcium binding, whereas incubation of the adipocytes with 100 microunits/ml of insulin resulted in a 2.50 +/- 1.6% increase in calcium binding to the subsequently isolated plasma membranes. The increase in calcium binding produced by insulin resulted from an increase in the maximum binding capacities of both classes of binding sites without alteration in their affinity constants. Additionally, a second pool of calcium in adipocyte plasma membranes has been identified by atomic absorption analysis; it was more than two times larger than the maximum binding capacity of the calcium binding system. This pool of calcium was stable, did not participate in the 45Ca2+ exchange, and was unaltered by insulin treatment. A simialr stable pool of magnesium exists in plasma membranes and was also unaffected by insulin treatment. The increased capacity of the isolated plasma membranes to bind calcium after insulin treatment of the cells may represent an important bioregulating mechanism and supports the concept that calcium may play an important role in the effector system for insulin.

Adipose Tissue↗
PubMed1975

Ultrastructural localization of insulin receptors on adipocytes.

The method for preparing a stable, biologically active, covalently linked ferritin--insulin complex has been modified to provide a 25-fold increase in yield compared to the original procedure while reducing the molar fatio of ferritin to insulin to 1:1 from 40:1. Ultrastructural studies of isolated adipocytes revealed specific binding of ferritin--insulin to the cell surface in irregular clusters associated with the glycocalyx coating. The number of ferritin--insulin molecules observed was consistent with the number of sulin molecules observed was consistent with the number of receptors calculated from 125I-labeled insulin binding studies. The ferritin--insulin was not observed in the cytoplasm of the cell but was found on the convave side of surface connected vesicles. These surface connected vesicles were part of an alveolar-like system of plasma membrane invaginations which project in various directions in the cytoplasm and by thin sectioning can appear as pinocytotic-like microvesicles. The morphological observations on ferritin--insulin binding were supported by the finding that 125I-labeled insulin binding was almost exclusively localized to highly purified plasma membranes isolated by fractionation of adipocytes after incubation with 125I-labeled insulin. These data supported the theory that insulin did not need to enter a cell to cause biological effects and was consistent with the negative cooperativity concept of insulin binding to cell receptors.

Adipose Tissue↗
PubMed1975

Pitfalls in the use of lead nitrate for the histochemical demonstration of adenylate cyclase activity.

The biochemistry of the lead histochemical technique for demonstrating adenylate cyclase was studied. The enzyme activity of fat cell plasma membranes, using 5'-adenylyl-imidodiphosphate (AMP-PNP) as substrate, was completely inhibited at 1 times 10- minus 4 M Pb(NO3)2 and yet at 4 times 10- minus 3 M Pb(NO3)2 precipitate could be demonstrated by electron microscopy on both sides of plasma membrane vesicles. No lead-diphosphoimide or lead-phosphate precipitate could be visualized by electron microscopy when the lead was reduced to a level (2 times 10- minus 5 M) which caused only 50% inhibition of the enzyme. A solubility product coefficient of 1 times 10- minus 10 M was found necessary to allow precipitation of lead-phosphate complex in the adenylate cyclase medium. Varying the ratio of substrate or dextran relative to the lead failed to protect the inhibition of the enzyme. Increasing concentrations of beta-mercaptoethanol restored the basal and stimulated activity of adenylate cyclase but also prevented the precipitation reaction. Lead at 2 times 10- minus 3 M caused the nonenzymatic hydrolysis of AMP-PNP, resulting in the production of small but significant quantities of cyclic AMP and substantial amounts of AMP. This hydrolysis was inhibited by alloxan but unaffected by dextran of NaF. The adenylate cyclase activity of pancreatic islet homogenates and of fat pad capillaries was completely inhibited by lead concentrations equal to or less than those used in histochemical studies (Howell, S. L., and M. Whitfield. 1972. J. Histochem. Cytochem. 20:873-879. and Wagner, R. C., P. Kreiner, R. J. Barrnett, and M. W. Bitensky. 1972. Proc. Natl. Acad. Sci. U.S.A. 69:3175-3179.). The present study shows that the lead histochemical method cannot be used for localization of adenylate cyclase because of the inhibition of the enzyme and artifacts produced by high lead concentrations and the inability to produce a visible precipitate at low lead concentrations which only partially inhibit the enzyme.

Adenosine Monophosphate↗
PubMed1975

Insulin degradation by isolated fat cells and their subcellular fractions.

Isolated frt cells and purified subcellular fractions of fat cells have been shown to degrade insulin to biologically inactive trichloroacetic-acid-soluble fragments. Further study of this activity has revealed the following characteristics: 1 Most of the insulin-degrading enzymes are intracellular, inaccessible to insulin or trypsin when fat cells are intact. More that 90 per cent of the recovered activity is found in the high-speed supernatant (cytosol) when cell fractionation studies are performed. 2. The plasma membrane contains significant insulin-degradative capacity, as shown by tryptic digestion of intact cells and cell fractionation. 3. The pH optimum of the cell-membrane insulin-degrading site is more acid than that of the cytosol activity, but the tow enzyme systems are similar with regard to substrate specificity, response to metabolic inhibitors, and elution volume of degradation products on gel filtration. 4. The plasma-membrane-degrading activity differs from the specific insulin-binding site with regard to saturation kinetics, optimum temperature, substrate specificity, sensitivity to sulfhydryl-blocking agents, and trypsin snesitivity.

Adipose Tissue↗