Interdigitation of phosphatidylcholine and phosphatidylethanolamine mixed with complexes of acidic lipids and polymyxin B or polymyxin B nonapeptide

A fatty acid spin label, 16-doxyl-stearic acid, was used to determine the percent interdigitated lipid in mixtures of a neutral phospholipid and an acidic phospholipid. Interdigitation of the acidic lipid was induced with polymyxin B (PMB) at a mole ratio of PMB to acidic lipid of 1:5. This compound does not bind significantly to neutral lipids or induce interdigitation of the neutral lipids by themselves. The neutral lipids used were dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidylcholine (DPPC), or dipalmitoylphosphatidylethanolamine (DPPE), and the acidic lipids were dipalmitoylphosphatidylglycerol (DPPG) or dipalmitoylphosphatidic acid (DPPA). The percent interdigitated lipid was determined from the percent of the spin label which is motionally restricted, assuming that the spin label is homogeneously distributed in the lipid. Assuming further that 100% of the acidic lipid is interdigitated at this saturating concentration of PMB, the percentage of the neutral lipid which can become interdigitated along with it was calculated. The results indicate that about 20 mole % DPPC can be incorporated into and become interdigitated in the interdigitated bilayer of PMB/DPPG at 4 degrees C. As the temperature approaches the phase transition temperature, the lipid becomes progressively less interdigitated; this occurs to a greater degree for the mixtures than for the single acidic lipid. Thus the presence of DPPC promotes transformation of the acidic lipid to a non-interdigitated bilayer at higher temperatures. At the temperature of the lipid phase transition little or none of the lipid in the mixture is interdigitated. Thus the lipid phase transition detected by calorimetry is not that of the interdigitated bilayer. The shorter chain length DMPC can be incorporated to a greater extent than DPPC, 30-50 mol%, in the interdigitated bilayer of PMB-DPPG. This may be a result of reduced exposure of the terminal methyl groups of the shorter myristoyl chains at the polar/apolar interface of the interdigitated bilayer. Less than 29% of the total lipid was interdigitated in a DPPC/DPPA/PMB 1:1:0.2 mixture indicating that none of the DPPC in this mixture becomes interdigitated. This is attributed to the lateral interlipid hydrogen bonding interactions of DPPA which inhibits formation of an interdigitated bilayer. DPPE was found to be incorporated into the interdigitated bilayer of PMB-DPPG to a similar extent as DPPC if the amount of PMB added is sufficient to bind to only the DPPG in the mixture. Differential scanning calorimetry showed that the remaining non-interdigitated DPPE-enriched mixture phase separates into its own domain.(ABSTRACT TRUNCATED AT 400 WORDS)     

Evolutionary conservation of the chromosomal configuration and regulation of amylase genes among eight species of the Drosophila melanogaster species subgroup

Nuclear DNA was extracted from each of the eight species comprising the Drosophila melanogaster species subgroup. Southern hybridization of this DNA by using a molecular probe specific for the alpha-amylase coding region showed that the duplicated structure of the amylase locus, first found in D. melanogaster, is conserved among all species of the melanogaster subgroup. Evidence is also presented for the concerted evolution of the duplicated genes within each species. In addition, it is shown that the glucose repression of amylase gene expression, which has been extensively studied in D. melanogaster, is not confined to this species but occurs in all eight members of the species subgroup. Thus, both the duplicated gene structure and the glucose repression of Drosophila amylase gene activity are stable over extended periods of evolutionary time.      

Purification from human pregnancy serum of a low molecular weight mitogen similar to placental lactogen and growth hormone

Recently, we isolated from the serum of pregnant women a factor that induced rapid proliferation of a lactogen-dependent rat lymphoma cell line (Nb2). This mitogenic factor is reasonably specific to pregnancy, since it was present in serum samples from second trimester as well as term-pregnant women, but not in those of adult men or cycling females. It is unlikely that this mitogenic activity (referred to as pregnancy mitogen [PM]) is due to contamination by classical lactogens, since acetone fractionation of serum yielded a preparation devoid of placental lactogen and prolactin, as determined by radioimmunoassays. Further purification of acetone precipitates from term-pregnant serum by ion exchange chromatography and gel filtration yielded a mitogenic activity with a relative mol wt of approximately 10,000. PM activity in the NB2 cell bioassay was not affected by the presence of prolactin antiserum. However, its activity was immunoneutralized by coincubation with anti-placental lactogen serum and, to a lesser extent, anti-growth hormone serum. It appears that PM was not generated by our extraction procedure, since gel filtration of whole serum also yielded a bioactive fraction of approximately 10 kDa. PM was further purified to homogeneity by high-performance liquid chromatography. Examination of the preliminary amino acid composition of PM revealed differences from that of a bioactive fragment of growth hormone and a corresponding portion of placental lactogen, suggesting that PM could be either a molecular variant of these hormones or a novel protein.     

Effects of dietary energy level on luteinizing hormone and adrenal function in the post partum beef cow

The effects of dietary energy and suckling on adrenal function and luteinizing hormone (LH) concentrations were investigated in primiparous postpartum cows. Ten heifers were assigned at calving to either high (22.8 Mcal/day) or low (15.2 Mcal/day) energy diets. Blood samples were collected every 15 minutes for 8 hours on 28, 42, and 56 days post partum. Calves were allowed to suckle ad libitum during sampling periods. Serum samples were analyzed by radioimmunoassay for LH and cortisol. Concentrations of catecholamines were quantified by reverse-phase HPLC. Body weights were decreased (P<0.01) by low energy intake. In addition, low energy diet cows had lower mean LH concentrations (0.97 +/- 0.09 vs 1.57 +/- 0.07 ng/ml), P<0.05) than high energy diet cows. Luteinizing hormone concentrations in high energy diet cows increased with days post partum, resulting in a treatment-by-time interaction (P<0.005). Treatment did not affect mean cortisol concentrations. However, within 15 minutes of suckling cortisol release was significantly above baseline in 77% of the observed suckling events. Dihydroxyphenylalanine (DOPA) increased in high energy diet cows compared with that of low energy diet cows (2,833 +/- 243 vs 1,294 +/- 243 pg/ml, P<0.01). Norepinephrine (NE) and 3,4-dihydroxyphenylacetic acid (DOPAC) were not influenced by treatment. Plasma NE decreased during the postpartum interval (P<0.005). These data suggest that reduced energy intake may prevent the increase in LH associated with increasing days post partum and alter adrenal function. In addition, spontaneous suckling events elicit a release of cortisol.     

Predation at a snail's pace: what's time to a gastropod?

Summary Predation by naticid gastropods shows evidence of adaptation to maximize the rate of energy intake. The predation rate of Polinices duplicatus feeding on artificially altered, thin-shelled Mercenaria mercenaria was faster than the predation rate on normal Mercenaria. The rate of energy intake was limited by handling time. The time saved by predation on thin-shelled prey was used to forage. Thus time was shown to be valuable to P. duplicatus, and cost-benefit functions using time and energy as currencies are appropriate for estimating dietary efficiency and predicting prey choice.Despite the clear superiority of thin-shelled prey, P. duplicatus did not learn to prefer this novel prey type, suggesting that predator choices are sterotyped, reflecting optima selected over evolutionary time.     

Effect of lipid structural modifications on their intermolecular hydrogen bonding interactions and membrane functions

The large number of different membrane lipids with various structural modifications and properties and the characteristic lipid composition of different types of membranes suggest that different lipids have specific functions in the membrane. Many of the varying properties of lipids with different polar head groups and in different ionization states can be attributed to the presence of interactive or repulsive forces between the head groups in the bilayer. The interactive forces are hydrogen bonds between hydrogen bond donating groups such as --P--OH,--OH, and--NH3+ and hydrogen bond accepting groups such as --P--O- and --COO-. These interactions increase the lipid phase transition temperature and can account for the tendency of certain lipids to go into the hexagonal phase and the dependence of this tendency on the pH and ionization state of the lipid. The presence or absence of these interactions can also affect the penetration of hydrophobic substances into the bilayer, including hydrophobic residues of membrane proteins. Evidence for this suggestion has been gathered from studies of the myelin basic protein, a water-soluble protein with a number of hydrophobic residues. In this way the lipid composition can affect the conformation and activity of membrane proteins. Since hydrogen-bonding interactions depend on the ionization state of the lipid, they can be altered by changes in the environment which affect the pK of the ionizable groups. The formation of the hexagonal phase or inverted micelles, the conformation and activity of membrane proteins, and other functions mediated by lipids could thus be regulated in this way.     

Modulation of bovine milk galactosyltransferase activity by lipids

The effect of lipids singly and in combination on the ability of galactosyltransferase to transfer galactose to N-acetyl-D-glucosamine-forming lactosamine and to glucose forming lactose has been studied. Lecithins, as egg phosphatidylcholine (PC), or saturated as dimyristoylphosphatidylcholine and dipalmitoylphosphatidylcholine stimulated the activity of the enzyme to form lactosamine to different extents. Egg PC produced the greatest stimulation of all the lecithins tested. Egg phosphatidic acid (PA) inhibited the activity of the enzyme at very low concentrations of lipid. In mixed vesicles of gel phase or liquid crystalline phase lecithins and egg PA, the acidic lipid was able to overcome the stimulation produced by the lecithins. The dominant effect of the head group was demonstrated by the effects of gel phase dimyristoylphosphatidic acid (DMPA). In mixtures with PC, DMPA also was able to inhibit the enzyme for lactosamine synthesis but higher concentrations of the gel phase DMPA were required for inhibition compared to the liquid crystalline PA. Although the head group appeared to dominate the inhibition, the nature of the acyl chains of the lipid played a secondary role at least. Other acid lipids, phosphatidylserine (PS) and phosphatidylinositol (PI) were much less effective than PA. PS alone inhibited the activity of the enzyme. However, in mixed lipids (PS and egg PC), PS was unable to reverse the stimulation produced by PC while PC was able to reverse the inhibition produced by PS. PI alone had no effect on the enzyme activity. In mixtures with egg PC, the stimulating effect of PC was dominant. In the lactose synthetase reaction, the effect of lipids was similar to that of the lactosamine synthetase, i.e. PC stimulated and PA inhibited activity and in mixtures of PC and PA, the inhibitory effect of PA was dominant.     

Growth of Phytomonas serpens in a Defined Medium; Nutritional Requirements

ABSTRACT. Three strains of Phytomonas serpens two from tomatoes, Lycopersicon esculentum one from the insect Phtia picta (Hemiptera, Coreidae), were cultivated in a chemically defined medium developed from a defined medium for cultivating insect flagellates. Besides organic growth factors required by other insect trypanosomatids this flagellate requires, serine and inositol. Glutamine stimulates growth, and, surprisingly, does not require heme.