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R D Howells

Publications and source records attributed to R D Howells.

60 records · Page 4Linked to original sources
PubMed1981

Characterization and partial purification of solubilized active opiate receptors from toad brain.

Opiate receptors have been solubilized from toad brain membranes in active form by using digitonin. Between 40% and 50% of the stereospecific binding activity present in toad brain membranes is recoverable in the ultracentrifugal supernatant of digitonin extracts. Binding of opiates to the solubilized receptor is enhanced 4- to 5-fold by decreasing digitonin concentration to 0.1% or less prior to binding. The solubilized receptor is similar to the membrane-bound receptor in its affinity for various ligands and its sensitivity to heat, trypsin, and N-ethylmaleimide. Moreover, the sodium effect seen in membrane-bound receptor is retained in the solubilized preparation. Both membrane-bound and soluble toad receptors show weak binding of enkephalins, suggesting that they are predominantly of the mu type. The solubilized opiate receptor has an approximate molecular weight of 350,000-400,000. Purification of up to 20-fold has been achieved by gel filtration on Sepharose CL-6B.

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PubMed1980

Benzodiazepine binding in chicken retina and its interaction with gamma-aminobutyric acid.

Chicken retina contains saturable, stereoselective high affinity binding sites for benzodiazepines. The density of binding sites in chicken retina exceeds that found in rat and cow retina. GABA and muscimol increase specific 3H-flunitrazepam binding in the chicken retina in a dose-dependent manner. This effect is reversed by bicuculline methiodide. The enhancement of specific binding caused by GABA was shown to be due to an increase in the affinity of flunitrazepam, while the number of binding sites remained unchanged.

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PubMed1980

Opiate binding sites in the retina: properties and distribution.

The presence of opiate binding sites in rat retina was confirmed and their properties and distribution were investigated. Etorphine, naloxone and methionine enkephalin exhibit high affinity, saturable binding to retinal membrane preparations. Scatchard analysis of the binding data yielded linear plots for each ligand. The P1 subcellular fraction contains the greatest density of sites per milligram of protein. The response of the binding of all ligands to sodium ions was similar in the retina to that observed in the brain. Competition experiments using naloxone and methionine enkephalin indicated that a homogeneous class of binding sites exists in the rat retina. Opiate binding sites were also demonstrated in the retina of cow, toad and skate. Methionine enkephalin-like immunoreactivity was not clearly detectable in rat retina but was present in toad retina. The physiological role of the opiate binding sites in the retina remains to be elucidated.

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PubMedDate not supplied

Solubilization in good yield of active opiate binding sites from mammalian brain.

Active opiate binding sites have been solubilized from mammalian brain cell membranes. The presence of 0.5-0.1 M NaCl during treatment of membranes from rat brain, human frontal cortex, and bovine corpus striatum with glycodeoxycholate or digitonin resulted in the extraction of active opiate binding sites in yields ranging up to 43%. The criteria for solubility of the sites were their inability to sediment at 10(5) x g after 2 hr and their apparent molecular weight of 3- 4 x 10(5) as determined by gel filtration. The receptors in solution resemble the membrane-bound sites with respect to saturability, stereo-specificity, sensitivity to heat and reagents, and high affinity for opioid ligands. The interaction of solubilized sites with immobilized lectins was used to demonstrate the glycoprotein nature of the opiate receptor. Soluble receptors from all species studied were retained by wheat germ agglutinin(WGA)-agarose and could be specifically eluted with N-acetylglucosamine. No retention of solubilized material was observed with eight other lectins examined, including horseshoe crab lectin, a sialic acid specific agglutinin. The receptor protein eluted from WGA columns was enriched 25-50-fold over the crude soluble fraction.

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