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)     

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.     

Phylogenetic diversification of immunoglobulin genes and the antibody repertoire

Immunoglobulins are encoded by a large multigene system that undergoes somatic rearrangement and additional genetic change during the development of immunoglobulin-producing cells. Inducible antibody and antibody-like responses are found in all vertebrates. However, immunoglobulin possessing disulfide-bonded heavy and light chains and domain-type organization has been described only in representatives of the jawed vertebrates. High degrees of nucleotide and predicted amino acid sequence identity are evident when the segmental elements that constitute the immunoglobulin gene loci in phylogenetically divergent vertebrates are compared. However, the organization of gene loci and the manner in which the independent elements recombine (and diversify) vary markedly among different taxa. One striking pattern of gene organization is the "cluster type" that appears to be restricted to the chondrichthyes (cartilaginous fishes) and limits segmental rearrangement to closely linked elements. This type of gene organization is associated with both heavy- and light-chain gene loci. In some cases, the clusters are "joined" or "partially joined" in the germ line, in effect predetermining or partially predetermining, respectively, the encoded specificities (the assumption being that these are expressed) of the individual loci. By relating the sequences of transcribed gene products to their respective germ-line genes, it is evident that, in some cases, joined-type genes are expressed. This raises a question about the existence and/or nature of allelic exclusion in these species. The extensive variation in gene organization found throughout the vertebrate species may relate directly to the role of intersegmental (V<==>D<==>J) distances in the commitment of the individual antibody-producing cell to a particular genetic specificity. Thus, the evolution of this locus, perhaps more so than that of others, may reflect the interrelationships between genetic organization and function.      

Effect of pH and fatty acid chain length on the interaction of myelin basic protein with phosphatidylglycerol

The basic protein of myelin binds electrostatically to acidic lipids but has several hydrophobic segments which may penetrate into the lipid bilayer. Calorimetric and spin-label evidence suggests that below the phase transition temperature, Tc, several phase states occur in the complex of phosphatidylglycerol with basic protein, possibly due to differences in the degree of penetration of the protein and/or interdigitation of the lipid acyl chains. One of these states is a metastable state which starts to melt 10 degrees C below the Tc of the pure lipid and then refreezes, with release of heat, into a stable state. The stable state melts near the Tc of the pure lipid but restricts the motion of fatty acid spin-labeled near the terminal methyl much more than does the pure lipid. The relationship between the rate of conversion to the stable state and the degree of penetration of the protein at varying pH, in the range 4--8, and the lipid acyl chain length, in the range 14 to 18 carbons, was investigated. Altering the pH in this range affects protonation of the histidines of the protein but has no effect on the lipid at pH 4 and above. The rate of conversion of the sample to both the metastable state and the stable state decreased with increase in pH for phosphatidylglycerol with all lipid chain lengths. It also decreased with decreasing chain length at constant pH. This suggested that the lipid could refreeze into the stable state more readily if a smaller proportion of the total bilayer thickness was occupied by the hydrophobic segments of the protein. The consistency of these results with the concept of penetration of portions of the protein partway into the bilayer lends support to this hypothesis.