Chiral samarium iodo and terbutylate derivatives as catalysts for MPV ketone reductions

Samarium iodo and terbutylate derivatives coordinated by various chiral ligands have been prepared and used in situ as new catalysts for MPV reductions of aromatic ketones. The most active catalyst is samarium terbutylate binaphtolate albeit with moderate enantioselectivity.     

Opioid receptor antagonist affinity ligands: 6β-bromoacetamido-6-desoxynaltrexone and 6β-thioglycolamido-6-desoxynaltrexone

The present study, utilizing thioglycolamido as the reactive group, describes the synthesis and pharmacology of a new opioid antagonist affinity ligand, 6-thioglycolamido-6-desoxynaltrexone (TAN) and compares TAN with a related known compound, 6-bromoacetamido-6-desoxynaltrexone (BAN). Both compounds were tested for their reversible and irreversible inhibition of [³H]naloxone binding to calf brain membranes. Reversible binding of BAN and TAN had K_i values of 1×10^–9 and 1×10^–10 M, respectively as determined by log probit plots. Irreversible binding was determined after extensive washing to remove all non-covalently bound ligand. At a concentration of 5×10^–8 and 1×10^–8 M for BAN and TAN irreversible binding was inhibited 50% of the maximum value. A study of the time course of irreversible inhibition of [³H]naloxone binding revealed that maximal inhibition occurred within 5 min with a concentration of 1×10^–7 M of either agent. TAN but not BAN when administered systematically to mice produced an antinociceptive effect as measured by the writhing test. When administered intracerebraventricularly BAN did not block morphine-induced analgesia for more than 2 hr; whereas, with a single ED₅₀ dose of 20 nmoles of TAN i.c.v. morphine-induced analgesia was almost completely blocked for a period of over 24 hr, as determined by the tail flick test. Although the SH group of TAN were required for the covalent interaction with opioid receptors, the site of TAN's interaction appears to involve other than protein SH groups.     

Cationic probes: Specificity of distribution of metal-binding sites in bovine sperm

Salts of transition elements that alter the rate of sperm cell movement act at or near calcium-binding sites. After living bull sperm cells had been preincubated in VO₄^3?, Ni²⁺, Zn²⁺, Mn²⁺, and also La³⁺, they were then fixed. Crisply defined organelles and the absence of particulate deposits in the morphological controls contrasted sharply with the treated specimens; the latter contained regions of increased electron density, the nature and distribution of which depended on the test substance, reflecting the differential affinities of the specific ions. La³⁺ formed fine dense areas, mainly at the exocytic surface of the plasma membrane. VO₄^3? marks the cell surface but also left particulate densities within the cell. Ni²⁺ caused a nearly uniformly dense deposit at the surface and on the satellite fibers and axonemal microtubules. Zn²⁺ formed less uniform but coarser deposits, while in Mn²⁺ the distribution was similar to that in Zn²⁺ but much denser in the axonemal matrix and on the satellite fibers. Verapamil restricted the size and number of the opacities, while procaine permitted a similar distribution of slightly larger size reaction product. The differences in size and distribution of the enhanced densities were consistent and replicable for the individual assay substances. Vanadate, which specifically inhibits Na, K-ATPase, bound to ouabain-sensitive enzyme loci, however, completely disrupting the axonemal complex. This suggests that an important role of dynein in flagellar motion may relate to intracellular transport of Ca²⁺.     

Zinc is a constituent of bovine heart cytochrome c oxidase preparations

Cytochrome c oxidase preparations from bovine heart muscle contain 1 zinc per 2 irons. Metal contents of nine preparations determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) show that Cu, Fe and Zn are the only metals present in significant amounts with average Cu/Fe, Fe/Zn, and Cu/Zn atom ratios of 1.3, 2.1 and 2.8, respectively. Removal of adventitious copper results in a Cu:Fe:Zn stoichiometry of 2:2:1. The zinc is tightly bound. Dialysis against a solution of 1,10-phenanthroline at pH 7.4 or an acidic buffer (pH 4.4) does not remove Zn. Dialysis against 0.8 M KCN at pH 10 causes partial loss of both Cu and Zn. This is the first evidence for the presence of Zn in a cytochrome c oxidase.     

Purification and characterization of a benzodiazepine-like substance from mammalian brain

An endogenous brain ligand which competes with [³H]-flunitrazepam for the binding to benzodiazepine receptor has been isolated and purified to homogeneity. The purification procedures involve the extraction of the endogenous ligand by homogenizing the brain tissue in water containing various protease inhibitors followed by filtration through a PM 10 membrane (exclusion limit: 10,000-dalton), column chromatographies on Sephadex G-50, Bio-Rad P2 and a series of C₁₈ reverse phase HPLC columns. The purified endogenous ligand was eluted as a single and symmetrical peak monitored at either 220 or 280 nm. Furthermore, the ligand activity coincided with the absorption peak. The purified endogenous ligand is thermostable, insensitive to various peptidases and proteolytic enzymes, resistant to DNAse, RNAse, and carbohydrate enzyme e.g. neuraminidase (EC 3.2.1.18) and acid treatment. It has a major absorption peak at 220 nm and a minor one at 313 nm. The endogenous ligand appears to be quite specific since it only inhibits the binding of ligand to the central type benzodiazepine receptor but not to other receptors, e.g. peripheral type benzodiazepine receptor, ₁-adrenoceptor, ₂-adrenoceptor, -adrenoceptor and muscarinic cholinergic receptor. Furthermore, the inhibition of the receptor binding by the endogenous ligand is enhanced by GABA suggesting that the endogenous ligand is a benzodiazepine receptor agonist. The structure of the endogenous ligand is unknown.Special issue dedicated to Dr. Elling Kvamme     

Kinetics of ligand binding to a cluster of membrane-associated receptors

The process of ligand binding to a cluster of membrane-associated receptors is examined theoretically. The theoretical model proposed involves the diffusion of ligands from the solution to the disc-like cluster of receptors on the surface of the spherical cell. When the ligand hits the internal part of the disc-like cluster, it begins to move laterally until it leaves the disc through its outer surface or is bound by one of the receptors inside the disc. If the ligand leaves the cluster, it returns to the solution and hits the disc again after a certain period, etc. According to our model the transition from a diffusion-limited to a reaction-limited process of binding is determined by the dimensionless parameter Dt _c/a ², where D is the lateral diffusion coefficient,t _c is the characteristic time of reaction, anda is the radius of the disc-like cluster. The forward rate constantk _f turns out to be a function of . Comparing the results of our calculations ofk _f with some experimental data we found that agreement is achieved at high , i.e. the process of ligand binding by clustered receptors is predominantly reaction-limited.     

The sigma compounds 1,3-di-o-tolylguanidine and N-allylnormetazocine inhibit agonist-stimulated inositol phospholipid metabolism in bovine adrenal medullary cells

Muscarine stimulated a concentration-dependent accumulation of [³H]inositol phosphates in bovine adrenal medullary cells preloaded with [³H]inositol. This muscarinic activation of inositol phospholipid metabolism was fully inhibited by the -ligand 1,3-di-o-tolylguanidine (DTG) with an IC₅₀ of approximately 45 M. Higher concentrations (100 M) of (+) N-allylnormetazocine (SKF-10047) also partially inhibited this response. A concentration of DTG sufficient to fully inhibit the muscarinic response also produced a significant partial inhibition of [³H]inositol phosphate accumulation in response to histamine but not to angiotensin II. These data demonstrate that -compounds inhibit agonist-stimulated inositol phospholipid metabolism in bovine adrenal medullary cells, with a degree of selectivity towards the muscarinic response.     

Chemical tools targeting readers of lysine methylation

Reader domains that recognize methylated lysine and arginine residues on histones play a role in the recruitment, stabilization, and regulation of chromatin regulatory proteins. Targeting reader proteins with small molecule and peptidomimetic inhibitors has enabled the elucidation of the structure and function of specific domains and uncovered their role in diseases. Recent progress towards chemical probes that target readers of lysine methylation, including the Royal family and plant homeodomains (PHD), is discussed here. We highlight recently developed covalent cyclic peptide inhibitors of a plant homeodomain. Additionally, inhibitors targeting previously untargeted Tudor domains and chromodomains are discussed.     

Nω-Hydroxyamino-α-amino acids as a new class of very strong inhibitors of arginases

 The effects of various compounds bearing an N-OH group such as N-hydroxy-guanidines, amidoximes, and hydroxylamines, on bovine and rat liver arginases was studied. Some of these compounds with an l-α-amino acid function at an appropriate distance from the N-OH group acted as strong competitive liver arginase inhibitors, displaying Ki values between 4 and 150 μM. Two compounds, N ^ε-hydroxy-l-lysine and N ^ω-hydroxy-d,l-indospicine, which exhibited Ki values of 4 and 20 μM (at pH 7.4), were the most potent inhibitors of arginase described to date. The distance between the α-amino acid and N-OH functions appeared to be crucial for potent inhibition of arginase, as N ^δ-hydroxy-l-ornithine, which has one -CH₂ group less than N ^ε-hydroxy-l-lysine, exhibited a 37-fold higher Ki value than N ^ε-hydroxy-l-lysine. Based on these results, a model for the interaction of N ^ω-hydroxyamino-l-α-amino acids with the arginase active site is proposed. This model involves the binding of the N-OH group of the inhibitors to the arginase Mn(II) center and suggests that N ^ε-hydroxy-l-lysine is a good transition state analog of arginase.     

Effects of sorbitol addition on the action of free and immobilized hydrolytic enzymes in organic media

The effect of the addition of sorbitol on the activity and stability of enzymes was examined by monitoring transesterification reactions performed in organic media at various water activities (a(w) = 0.08 to 0.97). Lipases from Chromobacterium viscosum and Candida rugosa immobilized on celite, and chymotrypsin, free or immobilized on celite, were used. When the sorbitol-containing enzymes were employed, higher reaction rates and less hydrolysis were observed. Immobilization of chymotrypsin resulted in high activity and operational stability, while the nonimmobilized enzyme was stable only in the presence of sorbitol. The activity of all preparations diminished after washing them with pyridine to remove sorbitol. Furthermore, severe stability problems occurred in the preparations lacking sorbitol. Sorbitol treatment, even after removal of the sorbitol itself, improved the activity of nonimmobilized chymotrypsin relative to the washed control. On the other hand, washing to remove sorbitol had a negative effect on the activity of both coimmobilized lipase and coimmobilized chymotrypsin. Addition of a substrate analogue, N-acetyl-L-phenylalanine, to chymotrypsin yielded a preparation that exhibited higher activity than both the control and its sorbitol-containing counterpart. Differential scanning calorimetry measurements revealed that the chymotrypsin-sorbitol complex was stable against thermal denaturation, undergoing transition at a high temperature (89 degrees C). The transition temperatures of the substrate-containing chymotrypsin and of the control were identical (72 degrees C). (c) 1995 John Wiley & Sons, Inc.