The fluid mosaic model of the structure of cell membranes

A fluid mosaic model is presented for the gross organization and structure of the proteins and lipids of biological membranes. The model is consistent with the restrictions imposed by thermodynamics. In this model, the proteins that are integral to the membrane are a heterogeneous set of globular molecules, each arranged in an amphipathic structure, that is, with the ionic and highly polar groups protruding from the membrane into the aqueous phase, and the nonpolar groups largely buried in the hydrophobic interior of the membrane. These globular molecules are partially embedded in a matrix of phospholipid. The bulk of the phospholipid is organized as a discontinuous, fluid bilayer, although a small fraction of the lipid may interact specifically with the membrane proteins. The fluid mosaic structure is therefore formally analogous to a two-dimensional oriented solution of integral proteins (or lipoproteins) in the viscous phospholipid bilayer solvent. Recent experiments with a wide variety of techniqes and several different membrane systems are described, all of which abet consistent with, and add much detail to, the fluid mosaic model. It therefore seems appropriate to suggest possible mechanisms for various membrane functions and membrane-mediated phenomena in the light of the model. As examples, experimentally testable mechanisms are suggested for cell surface changes in malignant transformation, and for cooperative effects exhibited in the interactions of membranes with some specific ligands. Note added in proof: Since this article was written, we have obtained electron microscopic evidence (69) that the concanavalin A binding sites on the membranes of SV40 virus-transformed mouse fibroblasts (3T3 cells) are more clustered than the sites on the membranes of normal cells, as predicted by the hypothesis represented in Fig. 7B. T-here has also appeared a study by Taylor et al. (70) showing the remarkable effects produced on lymphocytes by the addition of antibodies directed to their surface immunoglobulin molecules. The antibodies induce a redistribution and pinocytosis of these surface immunoglobulins, so that within about 30 minutes at 37 degrees C the surface immunoglobulins are completely swept out of the membrane. These effects do not occur, however, if the bivalent antibodies are replaced by their univalent Fab fragments or if the antibody experiments are carried out at 0 degrees C instead of 37 degrees C. These and related results strongly indicate that the bivalent antibodies produce an aggregation of the surface immunoglobulin molecules in the plane of the membrane, which can occur only if the immunoglobulin molecules are free to diffuse in the membrane. This aggregation then appears to trigger off the pinocytosis of the membrane components by some unknown mechanism. Such membrane transformations may be of crucial importance in the induction of an antibody response to an antigen, as well as iv other processes of cell differentiation.

Plant products as antimicrobial agents

The use of and search for drugs and dietary supplements derived from plants have accelerated in recent years. Ethnopharmacologists, botanists, microbiologists, and natural-products chemists are combing the Earth for phytochemicals and "leads" which could be developed for treatment of infectious diseases. While 25 to 50% of current pharmaceuticals are derived from plants, none are used as antimicrobials. Traditional healers have long used plants to prevent or cure infectious conditions; Western medicine is trying to duplicate their successes. Plants are rich in a wide variety of secondary metabolites, such as tannins, terpenoids, alkaloids, and flavonoids, which have been found in vitro to have antimicrobial properties. This review attempts to summarize the current status of botanical screening efforts, as well as in vivo studies of their effectiveness and toxicity. The structure and antimicrobial properties of phytochemicals are also addressed. Since many of these compounds are currently available as unregulated botanical preparations and their use by the public is increasing rapidly, clinicians need to consider the consequences of patients self-medicating with these preparations.

A simple colorimetric method for the measurement of hydrogen peroxide produced by cells in culture

A simple, rapid and inexpensive method for the measurement of hydrogen peroxide (H2O2) produced by cells in culture is described. The assay is based on the horseradish peroxidase (HRPO)-mediated oxidation of phenol red by H2O2 which results in the formation of a compound demonstrating increased absorbance at 610 nm. A linear relationship between absorbance at 610 nm and concentration of H2O2 was found in the 1--60 microM (1--60 nmoles/ml) range. Due to the non-toxic character of phenol red and HRPO, the assay permits measurement of H2O2 production and release by macrophages for time intervals of 5--60 min under regular tissue culture conditions. Using this assay, the ability of a number of agents to induce H2O2 release by guinea pig peritoneal macrophages was demonstrated. These agents were: phorbol myristate acetate (PMA), opsonized zymosan, concanavalin A (Con A), wheat germ agglutinin (WGA), N-formyl-methionyl-leucyl-phenylalanine (FMLP) and A23187.

Biology, immunology, and cariogenicity of Streptococcus mutans

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Potentiation of the T-lymphocyte response to mitogens. I. The responding cell

Human and mouse lymphoid cells, stimulated by phytohemagglutinin (PHA) or lipopolysaccharide W (LPS), release supernatant factor(s) which are mitogenic for mouse thymocytes and which potentiate their responses to PHA or concanavalin A (Con A), The term LAF (lymphocyte-activating factor) is proposed for this activity. LAF not only enhances the mitotic responses of the less dense thymus subpopulations (A, B, and C) separable on discontinuous bovine serum albumin (BSA) gradients but also gives substantial responses in the otherwise inert cells of the denser fractions D and P. LAF does not exert a potentiating stimulatory effect on the responses of unfractionated mouse spleen cells, but does act synergistically with PHA on nonadherent spleen cells and on spleen cells of mice of several strains 5 days after irradiation and injection of thymocytes. Similarly LAF, which has no visible effect on unfractionated human peripheral blood cells, strongly potentiates the PHA response of column-purified lymphocytes, when these are cultured at low concentration. We conclude that LAF stimulates both central and peripheral T lymphocytes and enhances their responses to other stimulants.

Potentiation of the T-lymphocyte response to mitogens. II. The cellular source of potentiating mediator(s)

Effective supernatants (SUP), which potentiate mouse T-cell responses to phytohemagglutin (PHA), are obtained from cells of several species (human, rabbit, rat, mouse) and indeed from syngeneic spleen, thymus, or bone marrow cells. Unstimulated cells release some SUP activity but more is produced after stimulation. Lipopolysaccharide (LPS) produced very active SUP in all cultures tested. PHA was similarly active on human leukocytes only, whereas concanavalin A (Con A) gave highly efficient SUP only with mouse spleen cells. SUP production is not correlated with a mitotic response of the donor cells and is observed in cultures unable to respond mitotically to the stimulant. Adherent mouse spleen cell populations, consisting largely or entirely of macrophages, produce active SUP, while nonadherent cells do not. Similarly, purification of human peripheral leukocytes on nylon columns, with removal of macrophages and other adherent cells, destroys their ability to produce SUP. The importance of indirect effects in stimulating mitotic responses of T cells is emphasized by the fact that the mitotic response of mouse thymocytes to LPS and its ability to potentiate the response of these cells to PHA disappears with removal of adherent cells from the thymocyte population. Conversely the production of SUP from spleen cells stimulated by Con A requires the presence of T cells.

Depressed lymphocyte function after bereavement

During 1975 twenty-six bereaved spouses took part in a detailed prospective investigation of the effects of severe stress on the immune system. T and B cell numbers and function, and hormone concentrations were studied approximately 2 weeks after bereavement and 6 weeks thereafter. The response to phytohaemagglutinin was significantly depressed in the bereaved group on the second occasion, as was the response to concanavalin A at 6 weeks. There was no difference in T and B cell numbers, protein concentrations, the presence of autoantibodies and delayed hypersensitivity, and in cortisol, prolactin, growth hormone, and thyroid hormone assays between the bereaved group and the controls. This is the first time severe psychological stress has been shown to produce a measurable abnormality in immune function which is not obviously caused by hormonal changes.

The effect of in vivo hydrocortisone on subpopulations of human lymphocytes

This study was designed to determine the effect of in vivo hydrocortisone on subpopulations of lymphoid cells in normal humans. Subjects received a single intravenous dose of either 100 mg or 400 mg of hydrocortisone, and blood was drawn at hourly intervals for 6 h, and then again at 10 and 24 h after injection. Profound decreases in absolute numbers of circulating lymphocytes and monocytes occurred at 4-6 h after both 100 mg and 400 mg of hydrocortisone. Counts returned to normal by 24 h. The relative proportion of circulating thymus-derived lymphocytes as measured by the sheep red blood cell rosette assay decreased maximally by 4 h and returned to base line 24 h after hydrocortisone. There was a selective depletion of functional subpopulations of lymphocytes as represented by differential effects on in vitro stimulation with various mitogens and antigens. Phytohaemagglutinin response was relatively unaffected, while responses to concanavalin A were significantly diminished. Responses to pokeweed mitogen were unaffected by 100 mg of hydrocortisone, but greatly diminished by 400 mg of hydrocortisone. In vitro responses to the antigens streptokinase-streptodornase and tetanus toxoid were markedly diminished by in vivo hydrocortisone. Reconstitution of monocyte-depleted cultures with autologous monocytes partially corrected the diminished response to antigens. This transient selective depletion of monocytes and subsets of human lymphocytes by a single dose of hydrocortisone is most compatible with a redistribution of these cells out of the circulation into other body compartments.

Prostaglandin-producing suppressor cells in Hodgkin's disease

We examined the role of a prostaglandin-producing suppressor cell in the hyporesponsiveness to phytohemagglutinin seen in Hodgkin's disease. Addition of indomethacin to phytohemagglutinin cultures of lymphocytes from six patients with Hodgkin's disease resulted in an increase of 182 +/- 60 per cent in 3H-thymidine incorporation versus a 44 +/- 18% increase in 29 controls (mean +/- S.D., P less than 0.001). Without indomethacin the mean response of the lymphocytes in Hodgkin's disease was 48% of that of control. With indomethacin it was 94% of the control value. Phytohemagglutinin cultures of Hodgkin-disease lymphocytes produced approximately fourfold more prostaglandin E2 after 48 hours than did normal lymphocytes (P less than 0.02). Removal of glass-adherent cells markedly decreased the enhancement seen with indomethacin; it reduced prostaglandin E2 production by more than 80% and eliminated the differences in response to phytohemagglutinin between Hodgkin-disease and normal lymphocytes. Thus, a glass-adherent, prostaglandin-producing suppressor cell is responsible for the hyporesponsiveness to phytohemagglutinin seen with Hodgkin-disease lymphocytes.

Inhibition of in vitro nuclear transport by a lectin that binds to nuclear pores

Selective transport of proteins is a major mechanism by which biochemical differences are maintained between the cytoplasm and nucleus. To begin to investigate the molecular mechanism of nuclear transport, we used an in vitro transport system composed of a Xenopus egg extract, rat liver nuclei, and a fluorescently labeled nuclear protein, nucleoplasmin. With this system, we screened for inhibitors of transport. We found that the lectin, wheat germ agglutinin (WGA), completely inhibits the nuclear transport of fluorescently labeled nucleoplasmin. No other lectin tested affected nuclear transport. The inhibition by WGA was not seen when N-acetylglucosamine was present and was reversible by subsequent addition of sugar. When rat liver nuclei that had been incubated with ferritin-labeled WGA were examined by electron microscopy, multiple molecules of WGA were found bound to the cytoplasmic face of each nuclear pore. Gel electrophoresis and nitrocellulose transfer identified one major and several minor nuclear protein bands as binding 125I-labeled WGA. The most abundant protein of these, a 63-65-kD glycoprotein, is a candidate for the inhibitory site of action of WGA on nuclear protein transport. WGA is the first identified inhibitor of nuclear protein transport and interacts directly with the nuclear pore.

Bone resorbing activity in supernatant fluid from cultured human peripheral blood leukocytes

A new soluble mediator was found in supernatant fluid from cultures of human peripheral blood leukocytes that were stimulated by phytohemagglutinin, or by antigenic material present in human dental plaque deposits. This soluble Jactor produced bone resorption in organ cultures of fetal rat bones as measured by increased release of calcium-45, and also increased the number of active osteoclasts.

The identification in adult bone marrow of pluripotent and restricted stem cells of the myeloid and lymphoid systems

The precise relationship between the stem cells for the lymphoid system and those for the blood-forming system is unclear. While it is generally assumed that the hemopoietic stem cell, the spleen colony-forming unit (CFU-S), is also the stem cell for the lymphoid system, there is little evidence for this hypothesis. To investigate the stem cells in these two systems, we irradiated bone marrow cells to induce unique chromosome aberrations in the stem cell population and injected them at limiting dilution into stem cell-deficient recipients. Several months (between 3 and 11) were allowed for the injected cells to repopulate the hemopoietic system. At that time, the bone marrow, spleen, and thymus were examined for a high frequency of cells having the same unique chromosome aberration. The presence of such markers shows that the marker was induced in a cell with extensive proliferative capacity, i.e., a stem cell. In addition, the splenic lymphocytes were stimulated with phytohemagglutinin (PHA) or lipopolysaccharide (LPS) to search for unique chromosomes in dividing T and B cells, respectively. Finally, bone marrow cells were injected into secondary irradiated recipients to determine if the marker occurred in CFU-S and to determine whether or not the same tissue distributions of marked cells could be propogated by bone marrow cells in a second recipient. After examination of 28 primary recipients, it was possible to identify three unique patterns of stem cell regeneration. In one set of mice, a unique chromosome marker was observed in CFU-S and in PHA- and LPS-stimulated cultures. These mice provide direct evidence for a pluripotent stem cell in bone marrow. In addition, two restricted stem cells were identified by this analysis. In three recipients, abnormal karyotypes were found only in myeloid cells and not in B and T lymphocytes. These mice presumably received a marked stem cell restricted to differentiate only into myeloid progeny. In three other recipients, chromosome aberrations were found only in PHA-stimulated cells; CFU-S and cells from LPS cultures did not have cells with the unique chromosome. This pattern suggests that bone marrow contains cells committed to differentiation only into T lymphocytes. For each of the three types of stem cells, secondary recipients had the same cellular distribution of marked cells as the primary recipients. This observation provides further evidence that unique markers can be induced in both pluripotent and restricted stem cells.

Implications of antinutritional components in soybean foods

There are a number of components present in soybeans that exert a negative impact on the nutritional quality of the protein. Among those factors that are destroyed by heat treatment are the protease inhibitors and lectins. Protease inhibitors exert their antinutritional effect by causing pancreatic hypertrophy/hyperplasia, which ultimately results in an inhibition of growth. The lectin, by virtue of its ability to bind to glycoprotein receptors on the epithelial cells lining the intestinal mucosa, inhibits growth by interfering with the absorption of nutrients. Of lesser significance are the antinutritional effects produced by relatively heat stable factors, such as goitrogens, tannins, phytoestrogens, flatus-producing oligosaccharides, phytate, and saponins. Other diverse but ill-defined factors appear to increase the requirements for vitamins A, B12, D, and E. The processing of soybeans under severe alkaline conditions leads to the formation of lysinoalanine, which has been shown to damage the kidneys of rats. This is not generally true, however, for edible soy protein that has been produced under milder alkaline conditions. Also meriting consideration is the allergenic response that may sometimes occur in humans, as well as calves and piglets, on dietary exposure to soybeans.

Human lymphocytic metabolism. Effects of cyclic and noncyclic nucleotides on stimulation by phytohemagglutinin

The effects of extracellular nucleotides and agents which elevate intracellular cyclic adenosine 3',5'-monophosphate (cyclic AMP) concentrations on human lymphocyte metabolism have been studied. Aminophylline, isoproterenol, and prostaglandins, all of which elevate lymphocyte cyclic AMP levels, inhibited incorporation of (3)H-labeled thymidine, uridine, and leucine into the DNA, RNA, and protein of phytohemagglutinin (PHA)-stimulated lymphocytes. Aminophylline inhibition was maximal only when the inhibitor was added within 1 hr after exposure of cells to PHA, suggesting that a relatively early step in the lymphocyte transformation process may be affected. The addition of various nucleotides to the culture medium also inhibited incorporation of labeled precursors. The best inhibitor, dibutyryl cyclic AMP (DU cyclic AMP), produced maximal inhibition only if present during the 1st hr after initial exposure to PHA. Among the various cyclic nucleotides derivatives of guanosine and adenine were the most effective inhibitors (substantial inhibition at 0.1 mM concentrations). However, the inhibition was not specific for nucleotides containing the cyclic phosphodiester moiety since the tri-, di-, and monophosphates of adenosine and guanosine were equally effective in diminishing thymidine uptake. The above inhibitions were not due to secondary effects of the inhibitors on the interaction of PHA with lymphocytes as judged by (125)I-labeled PHA binding studies.Low concentrations (1-10 mumoles/liter) of cyclic AMP produced slight stimulation of thymidine-(3)H uptake in resting lymphocytes (lymphocytes not stimulated with PHA). However, the effects were quite small as compared with those produced by PHA itself. Attempts to demonstrate increased thymidine uptake 48 hr after pulsing lymphocytes with aminophylline or isoproterenol were unsuccessful. The relationship of these observations to a possible regulatory role for cyclic AMP in PHA-stimulated lymphocytes is discussed.

Guanosine 3':5'-cyclic monophosphate: a possible intracellular mediator of mitogenic influences in lymphocytes

Adenosine 3':5'-cyclic monophosphate (cyclic AMP) has been shown to have an antimitotic role in various cell types, and it has been hypothesized that a decrease of cyclic AMP concentration in the cell initiates or permits cell division. This hypothesis has been evaluated with respect to clonal proliferation of lymphocytes. Two potent mitogenic agents, phytohemagglutinin and concanavalin A, which induce thymic-dependent lymphocytes to undergo clonal proliferation, were examined for their ability to initiate proliferation and to alter the concentrations of cyclic AMP and guanosine 3':5'-cyclic monophosphate (cyclic GMP) in purified human peripheral blood-lymphocytes. Optimal mitogenic concentrations of phytohemagglutinin and concanavalin A produced 10- to 50-fold increases in the concentration of lymphocyte cyclic GMP within the first 20 min of exposure to the mitogens. No changes were seen in the concentration of cyclic AMP after stimulation with either mitogen in purified form. Increases of less than 2-fold in the concentration of lymphocyte cyclic AMP observed with a less purified preparation of phytohemagglutinin could be attributed to the agglutinating rather than the mitogenic properties of the mitogen. A revised hypothesis is presented in which a temporally discrete rise in lymphocyte cyclic GMP concentration is viewed as an active signal to induce proliferation, while the elevation of cyclic AMP concentration in these cells is viewed as a regulatory influence that limits or inhibits mitogenic action.