Constrained C-terminal hexapeptide neurotensin analogues containing a 3-oxoindolizidine skeleton

Summary In order to enforce different spatial orientations in the C-terminal hexapeptide of neurotensin (NT_8–13) and to gain information about the importance of the 10–11 peptide bond for binding to NT receptors, the Pro¹⁰-Tyr¹¹ fragment has been replaced with (2R,8S,8aR)-, (2S,8S,8aR)-, (2S,8S,8aS)-, (2S,8R,8aS)- and (2R,8R,8aS)-8-amino-2-benzyl-3-oxoindolizidine-2-carboxylic acid. Molecular dynamics calculations and energy minimization studies have shown that, contrarily to the Pro-Tyr moiety, none of these indolizidines display a tendency to adopt type I and III -turns, but those having (8S,8aR) or (8R,8aS) stereochemistry essentially adopt extended conformations and the (8S,8aS) stereoisomer prefers a nonstandard folding. The four diastereomeric NT_8–13 analogues incorporating (8S,8aR) or (8R,8aS) indolizidines displayed binding affinities for the brain NT receptor similar to that of [Ala¹¹]-NT_8–13 and only five- to ninefold lower than that of the corresponding analogue, [Phe¹¹]NT_8–13. Although this slight decrease could be attributed to differences in conformational behavior between these constrained NT_8–13 analogues and [Phe¹¹]NT_8–13 or NT_8–13, it is not clear whether the -turn around Pro¹⁰-AA¹¹ (AA=Phe, Tyr) is conserved upon receptor binding. An excessive restriction in the motions of the aromatic side chain, imposed by the highly steric constraint of the indolizidine moiety, emerges as an alternative explanation. The findings reported here demonstrate the possibility of replacing the Pro¹⁰-Tyr¹¹ dipeptide in NT_8–13 with a non-peptide residue without affecting considerably the affinity for brain NT receptors.     

Cooperativity and effects of ionic strength on the binding of sodium n-dodecyl sulphate to lysozyme

The binding of sodium n-dodecyl sulphate to lysozyme has been measured by equilibrium dialysis at 25°C and pH 3.2 over a range of ionic strengts from 0.0119 to 0.2119. Binding isotherms in the region corresponding to ionic binding between the surfactant anions and cationic amino acid residues on the protein have been interpreted in terms of the Hill equation and exhibit positive cooperativity with Hill coefficients in the region of 7–11. The Gibbs energies of binding have been calculated from the Hill binding constants and from the Wyman binding potentials. The stability of the surfactant-protein complexes is discussed in relation to the stability of surfactant micelles. Ionic binding of the surfactant is weakened and hydrophobic binding strengthened by increasing ionic strength.     

Vegetation changes of a Danish mire 1957–1981

A Danish mire influenced by culture until ca. 100 years ago and since then with incipient growth of Betula pubescens , was analyzed in 1957 and 1981 with regard to height of the Betula trees and cover percentage of plant species in the field layer. In 1957, every m2 along a 110 m transect was examined; in 1981 only 56 m2 representatively selected from the transect were examined. Betula showed considerable changes with a net intake of 10 trees (23%) and a net loss of 23 trees (52%), i.e. a net loss of 13 trees (30%). The mean height of the trees has, however, increased by 80 cm and the sum of the height of the trees per m2 by 29 cm. An increased total cover was found for Em–petrum nigrum, Molinia coerulea , and Erica tetralix , and a decreased total cover for Calluna, Sphagnum magellanicum, S. nemoreum, S. rubellum, Hypnum cupressi–forme, Pleurozium schreberi , and Aulacomnium palustre. An almost unchanged total cover was found for Eriophorum vaginatum, E. angustifolium, Sphagnum recurvum, Andromeda polifolia, Drosera rotundifolia , and Oxycoccus palustris. On the basis of quantitative changes in the individual plots, a specific index of change is calculated, decreasing in the order Empetrum > Sphagnum magellanicum > S. recurvum = Molinia > Calluna > Hypnum cupressiforme > Pleurozium schreberi> Oxycoccus palustre> Sphagnum nemoreum = S. rubellum = Eriophorum vaginatum = E. angustifolium > Aulacomnium palustre > Erica> Drosera > Andromeda. Analyses from 1981 demonstrate that usually the light conditions and/or the water content and rarely the bulk density of the soil is correlated with the change in cover of the individual species. For some species it is also shown that the change is correlated with the strongly increased cover of Empetrum in particular. The changes are finally illustrated by showing the position of the species along ecological gradients in 1981.     

Leu-enkephalin-stimulated growth hormone and prolactin release in the rat: comparison with the effect of morphine

The effect of Leu⁵-enkephalin on growth hormone (GH) and prolactin (PRL) release was studied $\text{in vivo}$ in the infant rat and compared to that of morphine. In 10 day-old pups, intracerebroventricular injection of Leu⁵-enkephalin (50, 75 and 100 μg) resulted in a dose-related increase in plasma GH; morphine was active as GH releaser at the dose of 5 and 10 μg, but not at 2.5 μg. Pretreatment with naloxone (2 mg/kg ip) suppressed the GH-releasing effect of either Leu⁵-enkephalin (100 μg) or morphine (10 μg). Leu⁵-enkephalin (75 and 100 μg) induced a rise in plasma PRL which was neither dose-related nor antagonized by naloxone; morphine (5 and 10 μg) was active as PRL releaser and its effect was antagonized by naloxone. These results indicate that: 1) Leu⁵-enkephalin stimulates both GH and PRL release; 2) the release of GH by Leu⁵-enkephalin but likely not that of PRL involves specific opiate receptors; 3) morphine releases GH and PRL through specific opiate receptors.     

Effect of chloride withdrawal on the geometry of the T-tubules in amphibian and mammalian muscle

Summary The relative chloride permeabilities of the T-tubule membranes in mammalian (rat sternomastoid) and amphibian (toad sartorius) skeletal muscle fibers have been assessed from the change in volume of the T-tubules during chloride withdrawal from fibers exposed to low extracellular chloride concentrations. A 3.5- to 4.2-fold increase in T-tubule volume was found in mammalian fibers, and this was shown to be independent of the composition of the low chloride solution or the nature of the fixative used in preparation for electron microscopy. The increase in T-tubule volume was transient and was inhibited by factors which block chloride conductance, i.e., low pH, 2,4-dichlorophenoxyacetic acid, and nitrate ions. A small increase (1.48-fold) in T-tubule volume was seen in amphibian fibers when chloride ions were replaced by sulphate ions. No increase in volume was observed in amphibian T-tubules when methyl sulphate ions replaced chloride ions. The results support the idea that the chloride permeability of the T-tubule membrane is significantly higher in mammalian fibers than in amphibian fibers.     

Secretion of proteoglycans by chondrocytes. Influence of colchicine, cytochalasin B, and beta-D-xyloside.

Chondrocytes obtained from epiphyseal cartilage of fetal guinea pigs or ear cartilage of young rabbits were cultured in monolayer. The influence of colchicine, cytochalasin B, and p-nitrophenyl-β-d-xylopyranoside on secretion of proteoglycans was investigated. Radioactive sulfate was used as a precursor. As observed previously in other systems, β-d-xylosides initiated the synthesis of free chondroitin sulfate chains, competing with the endogenous proteoglycan core protein acceptor. The molecular weights of the chondroitin sulfate chains synthesized both on the xyloside and on the core-protein acceptor in maximally stimulated cells were similar and significantly lower than in proteoglycans synthesized in the absence of xyloside. The size of the chondroitin sulfate chains synthesized on the xyloside was inversely related to the concentration of this compound. This finding suggests that the chain length is dependent on the ratio between available acceptor and chain-lengthening enzymes or precursors. Cytochalasin B, a microfilament-modifying agent, inhibited proteoglycan synthesis, without any effect on secretion. Cells treated with cytochalasin B could be stimulated with β-d-xyloside to synthesize free chondroitin sulfate chains to the same relative degree as cells with intact microfilaments. Colchicine, an antimicrotubular agent, partially inhibited synthesis and secretion of proteoglycan. However, cells treated with colchicine could be stimulated with β-d-xyloside to synthesize and secrete free chondroitin sulfate chains to about the same relative degree as cells with intact microtubules. The data suggest that microtubules may have a facilitatory rather than an obligatory role in the secretion of proteoglycans and that at least part of the effect of colchicine is located at or after the site of glycosaminoglycan synthesis.     

Bovine protothecosis. A brief report of ten cases

Prototheca zopfii was isolated repeatedly from milk samples from ten cows (of a herd of 192 dairy cows) with reduced milk yield and indurated mammary glands. The strain was moderately sensitive to streptomycin, polymyxin and gentamycin, but resistant or relatively resistant to other antibiotics and antimycotics commonly used in clinical practice. An attempt to treat the infection with Ethidium bromide, which was found effective in vitro, did not succeed. The number of Prototheca excreted decreased, but a complete cure was not obtained. In histological sections of the udder, Prototheca cells were demonstrated both intracellularly and interstitially.