Phospholipid methyltransferase activity in pancreatic islets: activation by calcium

Pancreatic islet homogenates contain a Mg2+-requiring phospholipid methyltransferase activity, the activity of which was doubled by calcium (K0.5 less than 5 microM). Other divalent metal ions stimulated the activity from 11 to 35%, but zinc and strontium were inhibitory. Cyclic AMP had no effect on the enzyme activity and cyclic GMP inhibited it slightly. Calcium increased the Vmax of the enzyme without affecting its Km with respect to S-adenosylmethionine (6 microM). Chlorpromazine, trifluoperazine, and dibucaine inhibited the calcium-stimulatable activity without affecting the activity in the absence of calcium. Phosphatidylserine stimulated, and arachidonic acid and palmitic acid inhibited, the basal enzyme activity. The methylated products were found to be primarily mono- and dimethylphosphatidylethanolamine (30%) and phosphatidylcholine (43%) and an, as yet unidentified, nonpolar lipid fraction (27%), as judged by thin-layer chromatography. In the presence of calcium, incorporation of methyl groups into phosphatidylcholine, mono- and dimethylphosphatidylethanolamine, and nonpolar lipids was increased by 131, 60, and 46%, respectively. Based on the localization of the enzyme activity in the insulin secretory granule fraction, it is proposed that phospholipid methylation plays a role in coupling the stimulus to the initial events in insulin secretion, leading to the exocytosis of insulin.     

Distribution and specificity of nucleotide-reactive autoantibodies in human SLE

An enzyme-linked immunosorbent assay (ELISA) was utilized to characterize nucleotide-reactive antibodies present in the sera of 67 human subjects: 27 active SLE, 20 inactive SLE, and 20 asymptomatic controls. This assay consisted of measuring the quantity of antibodies retained by a panel of immobilized 5'-nucleotide-BSA conjugates (AMP-, GMP-, CMP-, UMP-, and TMP-BSA) together with ssDNA and dsDNA antigens. Although the relative distribution of antibodies binding to nucleotide-BSA antigens (i.e., anti-GMP greater than anti-AMP greater than or equal to anti-TMP greater than anti-UMP greater than or equal to anti-CMP antibodies) was independent of clinical status, the sera of active SLE patients possessed three- and five-fold higher concentrations of these antibodies relative to those present in inactive SLE and control subjects, respectively. Affinity purification of the most dominant of these antibody populations with DNA- and GMP-agarose adsorbents suggested that the majority of anti-GMP antibodies were monospecific with respect to the guanine base moiety. For example, antibodies retained by GMP-agarose reacted with GMP-BSA and ssDNA but not with other nucleotide-BSA or dsDNA antigens. However, ELISA competition-inhibition studies with affinity-purified anti-GMP antibodies indicated that although the guanine base represents an important determinant, guanine-enriched oligo- and polynucleotides were preferred substrates (i.e., guanine-dependent, oligonucleotide specificity). This was exemplified by the finding that a 500- and 50-fold molar excess of dGMP and d(G)4 were required to achieve the same degree of inhibition as that observed with d(G)8. Finally, and as evaluated by indirect immunofluorescence with fixed HEp-2 cells, affinity-purified anti-GMP antibodies reacted with antigens restricted to nucleolar organelles.     

Lipid bilayer permeation by neutral aluminum citrate and by three alpha-hydroxy carboxylic acids

Several groups have proposed that aluminum (Al) may permeate biological membranes as a neutral complex with citrate. We tested this hypothesis by measuring aluminum citrate flux across unilamellar phospholipid vesicles (liposomes). Results from two independent procedures show that lipid bilayer permeation by the neutral aluminum-citrate complex is slow (P approximately equal to 1 x 10(-11) cm.s-1). We then compared aluminum-citrate permeation with permeation by a series of alpha-hydroxy carboxylic acids and by trimethylcitrate. This comparison showed that the aluminum-citrate flux is limited by diffusion across the water/lipid interface. This is due to hydrogen bonding between water and the citrate carboxyl groups, and by hydration of the bound metal in the aqueous phase. By analogy with citric acid, steric hindrance of diffusion within the bilayer does not affect the permeation rate of aluminum citrate. Elevated tissue levels of Al in subjects fed a diet supplemented with citric acid and Al(OH)3 cannot be explained by lipid bilayer permeation of the neutral complex.     

Effects of CaSO4 and NaCl on growth and nitrogen fixation ofLeucaena leucocephala

Effects and interactions of varying CaSO₄ and NaCl levels on growth and nitrogen fixation ofLeucaena leucocephala K8 were examined. Leucaena was grown in nutrient solution at four levels of CaSO₄ (0.5, 1.0, 2.5 and 5.0 mM) and NaCl (1, 25, 50 and 100 mM) in randomized blocks with five replications. While NaCl significantly reduced plant growth, additions of CaSO₄ increased plant height, leaf number, and biomass of salt treated plants. For the nonsaline treatments, high CaSO₄ levels slightly depressed growth, which contradicts suggestions that Leucaena has a high calcium requirement. A significant calcium/sodium interaction was not seen for nodule number or weight. Nodule number was significantly depressed by 100 mM NaCl and nodule weight of the salt stressed plants significantly increased as CaSO₄ concentration increased from 0.5 to 2.5 mM. Effects of NaCl and CaSO₄ on nitrogen content of plant parts were inconclusive. The promotion of Leucaena salinity tolerance by addition of CaSO₄ may be attributed to the effect of calcium in maintaing the selective permeability of membranes.     

Cell kinetics of vocal fold epithelium in rats.

In order to investigate the kinetics of vocal fold epithelium a bromodeoxyuridine-anti bromodeoxyuridine method has been applied in vivo at both light and electron microscopy level. This method is able to define the length of both epithelium turnover and cell-cycle in basal elements, as well as the existence of a higher proliferation rate during night time in comparison with day time. Moreover distinct labeling patterns observed in incorporating cells allow us to define the precise localization in S-phase of cycling elements.     

A study of photoreceptor-retinal pigment epithelium complex: age-related changes in monkeys

Retinas of 4-, 10-, and 20-year-old monkeys were studied by light microscopy, electron microscopy, and scanning electron microscopy. Sections from the midperipheral region of every retina were selected for comparison. Although no significant differences were found between 4- and 10-year-old retinas, four major changes were found in 20-year-old monkey retinas: (i) increased number of displaced photoreceptor cells (DPC), (ii) increased number of macrophages of different morphology in subretinal space, (iii) increase in pigment granules in retinal pigment epithelium (RPE) cells, and (iv) altered morphology of Muller cells. DPC included both rods and cones. Their location and morphology depended on the stage of their displacement. These cells were usually oval or rounded in shape and were found either among the outer segments of other photoreceptor cells, having stalks extending into the outer nuclear layer, or were located in the subretinal space and had no stalk. A narrow space around the DPC stalks, indicating a change in the intercellular connection between photoreceptor cells and Muller cells, was observed. Furthermore, the Muller cells related to DPC had shortened and markedly reduced microvilli. Two types of macrophages were found in the subretinal space of aged monkey retinas. One type was similar in morphology to RPE cells. Some of these cells were noticed detaching from RPE. Other types of macrophages were nonpigmented. The modifications in RPE were closely related to the changes in the associated neuroretina. The RPE cells in aged retina were devoid of microvilli or had a few thin microvilli. The pleomorphic pigment granules were dispersed throughout the cytoplasm. These cells varied in their size, shape, and surface features. These changes could significantly alter the retinal metabolic equilibrium and may be indicative of age related degenerative processes.     

A quantitative cytochemical study of determination for xylem-element formation in response to wounding in roots of Pisum sativum L.

Quantitative cytochemical studies of cortical parenchyma cells of roots of Pisum sativum in which the central vascular bundle is severed, showed esterase activity to be an early marker of the determination of cells to form a vascular bridge. Explantation, onto a basal culture medium, of wound segments taken from roots at different times after severing the stele showed the irreversibility of the esterase activity on removal from the inducing environment, so confirming this as a marker of cell determination. A general determination for the stele was shown to occur by 8–10 h after wounding, but information relating to tracheid secondary-cell-wall formation was not apparently available until 18–20 h after wounding. Determination appeared to occur well before mitosis. The timings of the differentiation steps indicate a simple diffusion model to explain the mechanism of arrival of the initiating molecules.