Probing the lipid membrane dipole potential by atomic force microscopy

The electrostatic properties of biological membranes can be described by three parameters: the transmembrane potential, the membrane surface potential, and the membrane dipole potential. The first two are well characterized in terms of their magnitudes and biological effects. The dipole potential, however, is not well characterized. Various methods to measure the membrane dipole potential indirectly yield different values, and there is not even agreement on the source of the membrane dipole moment. This ambiguity impedes investigations into the biological effects of the membrane dipole moment, which should be substantial considering the large interfacial fields with which it is associated. Electrostatic analysis of phosphatidylcholine lipid membranes with the atomic force microscope reveals a repulsive force between the negatively charged probe tips and the zwitterionic lipids. This unexpected interaction has been analyzed quantitatively to reveal that the repulsion is due to a weak external field created by the internal membrane dipole potential. The analysis yields a dipole moment of 1.5 Debye per lipid with a dipole potential of +275 mV for supported phosphatidylcholine membranes. This new ability to quantitatively measure the membrane dipole moment in a noninvasive manner with nanometer scale spatial resolution will be useful in identifying the biological effects of the dipole potential.     

Interleukin-2 binding to activated human T lymphocytes triggers generation of cyclic AMP but not of inositol phosphates

Human T lymphocytes stimulated with phytohaemagglutinin undergo a single round of cell division. Further proliferation is dependent on the lymphokine interleukin-2 (IL2) [(1987) Immunology 60, 7-12]. We show here that binding of IL2 to its receptors on the lymphocyte surface triggers the generation of cyclic AMP. In contrast, generation of inositol phosphates from the breakdown of inositol lipids was not detected. We suggest that cyclic AMP may play a role in the transduction of the IL2 proliferative signal in T lymphocytes.     

Higher tyrosine protein kinase activity in resting lymphocytes than in proliferating normal or leukaemic blood cells

We have studied tyrosine phosphorylation in particulate fractions from 11 leukaemic cell lines by using as substrate either a synthetic tyrosine containing peptide or the endogenous proteins. The results were compared with those obtained using similar fractions from normal lymphocytes and bone marrow cells. Particulate fractions from all the leukaemic cell lines and normal bone marrow cells exhibited lower levels of tyrosine protein kinase activity compared to normal lymphocytes. When the phosphorylation of endogenous substrates was assayed, proteins were phosphorylated on tyrosine residues (rather than serine or threonine residues) to a larger extent in normal lymphocytes than in leukaemic cell lines. Separation of labelled endogenous substrates on sodium dodecyl sulfate-polyacrylamide gels showed a number of phosphorylated alkali-resistant bands in the range 14-175kd in the lymphoid cell lines; normal lymphocytes exhibited a smaller number of strongly phosphorylated bands. Normal lymphocytes from different individuals showed reproducible patterns of phosphorylated substrates. Normal bone marrow cells and myeloid leukaemia lines showed weak, if any, phosphorylation. Among the leukaemic cell lines no particular pattern of phosphorylated substrates common to cells of similar phenotype could be detected. We suggest that the level of overall tyrosine protein kinase activity in these fractions reflects their position in the cell cycle rather than their normal or malignant status.     

α-Spinasterol,temperature and moisture content as determining factors in the infestation of Camellia sinensis by Xyleborus fornicatus

Analyses of segments of clones of tea bushes, growing in different climatic conditions, indicated that temperature, moisture content, the amount of available α-spinasterol, and saponin level determined the degree of infestation by the shot-hole borer beetle pest, Xyleborus fornicatus. The principal factors affecting α-spinasterol availability were the concentration of the sterol per se, and the levels of saponins, theanine, arginine, calcium and chebulagic acid. It is proposed that α-spinasterol is converted by X. fornicatus to moulting hormones required for pupation of the beetle larvae, and that this sterol is also necessary for spore formation by the ambrosia fungus, Monacrosporium ambrosium, which is associated with the female adult beetle; tea saponins are inhibitory to the development of both the ambrosia fungus and X. fornicatus. The distribution of amino acids, fiavanols and other polyphenols, saponins, α-spinasterol, α-spinasterol glycoside, β-amyrin epi-friedelinol, friedelin and oleanolic acid throughout the tea bush, at periods of 6–40 months after pruning, is described.