Recombinant human apolipoprotein E3 (apoE), purified from E. coli, inhibited the proliferation of several cell types, including endothelial cells and tumor cells in a dose- and time-dependent manner. ApoE inhibited both de novo DNA synthesis and proliferation as assessed by an increase in cell number. Maximal inhibition of cell growth by apoE was achieved under conditions where proliferation was dependent on heparin-binding growth factors. Thus, at low serum concentrations (0-2.5%) basic fibroblast growth factor (bFGF) stimulated the proliferation of bovine aortic endothelial (BAE) cells severalfold. The bFGF-dependent proliferation was dramatically inhibited by apoE with an IC50 approximately 50 nM. Under conditions where cell proliferation was mainly serum-dependent, apoE also suppressed growth but required higher concentrations to be effective (IC50 approximately 500 nM). ApoE also inhibited growth of bovine corneal endothelial cells, human melanoma cells, and human breast carcinoma cells. The IC50 values obtained with these cells were generally 3-5 times higher than with BAE cells. Inhibition of cell proliferation by apoE was reversible and dependent on the time of apoE addition to the culture. In addition, apoE inhibited the chemotactic response of endothelial cells that were induced to migrate by a gradient of soluble bFGF. Inhibition of cell proliferation by apoE may be mediated both by competition for growth factor binding to proteoglycans and by an antiadhesive activity of apoE. The present results demonstrate that apoE is a potent inhibitor of proliferation of several cell types and suggest that apoE may be effective in modulating angiogenesis, tumor cell growth, and metastasis.
Two Spirulina platensis strains, SP-G and SP-RB, resistant and sensitive to photoinhibition of photosynthesis, respectively, were grown outdoors in dense cultures and under different photon fluxes provided by shading. Cultures of both strains grown under full sunlight were more resistant to photoinhibition than those grown under nets with 15-50% decreases in the incident photon flux. Cultures grown outdoors were more resistant to photoinhibition than the laboratory ones. At noon, the photosynthetic activity, as expressed by O2 evolution, was higher for cultures grown under 50% shade, as compared with unshaded cultures. Productivity of the shaded cultures, in terms of biomass produced per day, was always higher when the cultures were protected from photoinhibition.
The photoaffinity herbicide azidoatrazine (2-azido-4-ethylamino-6-isopropylamino-s-triazine) selectively labels the L subunit of the reaction center of the photosynthetic bacterium Rhodopseudomonas sphaeroides. Herbicide-resistant mutants retain the L subunit and have altered binding properties for methylthio-and chloro-substituted triazines as well as altered equilibrium constants for electron transfer between primary and secondary electron acceptors. We suggest that a subtle alteration in the L subunit is responsible for herbicide resistance and that the L subunit is the functional analog of the 32-kDa QB protein of chloroplast membranes.The purple, nonsulfur bacterium Rhodopseudomonas sphaeroides contains an intracytoplasmic membrane system that is derived from invaginations of the cytoplasmic membrane (1, 2). These membranes are designated as chromatophores because they are the site of localization of bacteriochlorophyll (BChl) and other pigments that function to harvest the light energy that drives photosynthesis. The bacterial reaction center (RC) is the site at which the absorbed radiant energy is converted into chemical energy by a charge separation across the photosynthetic membrane (3). The RC of R. sphaeroides is mechanistically an analog of photosystem II (PSII) in higher plant chloroplasts (4,5). In both PSII and the bacterial RC, a photon of light causes the oxidation of a special RC chlorophyll, with the resultant transfer of an electron to a tightly bound quinone (QA), which is then oxidized by a secondary quinone (QB) to form a stable semiquinone (Q-). A second photoreduction results in a second electron transfer to QB, resulting in production of the fully reduced quinol, which diffuses away from the RC and is cycled through the quinone pool to donate electrons to the remaining components of the electron transport chain (2-5).The RC of R. sphaeroides contains three polypeptides with estimated molecular weights of 28, 24, and 22 kDa, which are designated the H, M, and L subunits, respectively. In addition, purified RCs contain four BChls, two bacteriopheophytins, two ubiquinones, and one iron (2). Studies with purified antibodies suggest that the H and M subunits transverse the membrane, while the L subunit is minimally exposed on the periplasmic surface (6). The M and L subunits are resistant to protease treatments, indicating that they are integral membrane polypeptides (7, 8). The function of the H subunit is unknown, since the M-L complex still catalyzes charge separations even after removal of the H subunit (9). A possible structural role for the coordination of RC and light-harvesting chlorophyll interactions has been proposed for the H subunit in the RC of Rhodopseudomonas capsulata (10,11). The M subunit binds azidoanthraquinone and is believed to be the location of the primary quinone (QA) binding site (12). The secondary quinone (QB) binding site has not been unequivocally established, although antibodies prepared against the M subunit prevent electron flow from QA to QB, sugges...
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