Interaction of non-competitive blockers within the gamma-aminobutyric acid type A chloride channel using chemically reactive probes as chemical sensors for cysteine mutants.

Selected channel-lining cysteine mutants from the M2 segment of rat alpha1 gamma-aminobutyric acid (GABA) type A receptor subunit, at positions 257, 261, 264, and 272 were co-expressed with beta1 and gamma2 subunits in Xenopus oocytes. They generated functional receptors displaying conductance and response to both GABA and picrotoxinin similar to the wild type alpha1beta1gamma2 receptor. Three chemically reactive affinity probes derived from non-competitive blockers were synthesized to react with the engineered cysteines: 1) dithiane bis-sulfone derivative modified by an isothiocyanate function (probe A); 2) fiprole derivatives modified by an alpha-chloroketone (probe B) and alpha-bromoketone (probe C) moiety. These probes blocked the GABA-induced currents on all receptors. This blockade could be fully reversed by a washing procedure on the wild type, the alpha1T261Cbeta1gamma2 and alpha1L264Cbeta1gamma2 mutant receptors. In contrast, an irreversible effect was observed for all three probes on both alpha1V257Cbeta1gamma2 and alpha1S272Cbeta1gamma2 mutant receptors. This effect was probe concentration-dependent and could be abolished by picrotoxinin and/or t-butyl bicyclophosphorothionate. These data indicate a major interaction of non-competitive blockers at position 257 of the presumed M2 segment of rat alpha1 subunit but also suggest an interaction at the more extracellular position 272.     

Substrate specificity and kinetic properties of enzymes belonging to the hormone-sensitive lipase family: comparison with non-lipolytic and lipolytic carboxylesterases

We have studied the kinetics of hydrolysis of triacylglycerols, vinyl esters and p-nitrophenyl butyrate by four carboxylesterases of the HSL family, namely recombinant human hormone-sensitive lipase (HSL), EST2 from Alicyclobacillus acidocaldarius, AFEST from Archeoglobus fulgidus, and protein RV1399C from Mycobacterium tuberculosis. The kinetic properties of enzymes of the HSL family have been compared to those of a series of lipolytic and non-lipolytic carboxylesterases including human pancreatic lipase, guinea pig pancreatic lipase related protein 2, lipases from Mucor miehei and Thermomyces lanuginosus, cutinase from Fusarium solani, LipA from Bacillus subtilis, porcine liver esterase and Esterase A from Aspergilus niger. Results indicate that human HSL, together with other lipolytic carboxylesterases, are active on short chain esters and hydrolyze water insoluble trioctanoin, vinyl laurate and olive oil, whereas the action of EST2, AFEST, protein RV1399C and non-lipolytic carboxylesterases is restricted to solutions of short chain substrates. Lipolytic and non-lipolytic carboxylesterases can be differentiated by their respective value of K(0.5) (apparent K(m)) for the hydrolysis of short chain esters. Among lipolytic enzymes, those possessing a lid domain display higher activity on tributyrin, trioctanoin and olive oil suggesting, then, that the lid structure contributes to enzyme binding to triacylglycerols. Progress reaction curves of the hydrolysis of p-nitrophenyl butyrate by lipolytic carboxylesterases with lid domain show a latency phase which is not observed with human HSL, non-lipolytic carboxylesterases, and lipolytic enzymes devoid of a lid structure as cutinase.     

In Vivo Elevation of Mouse Brain S-Adenosyl-L-Homocysteine after Treatment with L-Homocysteine

Intraperitoneal coadministration of adenosine and L-homocysteine markedly increased S-adenosyl-L-homocysteine in whole mouse brain, but further investigations showed that this elevation could also be produced following administration of L-homocysteine alone. The noted increase was maximal (+1325%) 10 min after treatment, remaining at about this level for 30-40 min before returning to control values after 180 min. Cerebral adenosine levels were decreased after treatment with L-homocysteine, adenosine, or these two substances in combination.     

Evidence for the existence of procolipase in chicken pancreas and pancreatic juice

Purified antibodies raised against chicken colipase were coupled to Sepharose 4B and colipase was isolated in a single step by immunoaffinity chromatography from an extract of chicken pancreas prepared under conditions where trypsin activation is avoided. The purified protein has a single amino terminal residue of alanine and its biochemical properties are similar to those of the precursor form of colipase (procolipase) previously isolated from porcine and equine pancreas or pancreatic juice. Further evidence for the existence of procolipase was obtained from kinetic studies of the hydrolysis of the Intralipid emulsion by untreated and trypsin-treated chicken pancreatic juice.     

Studies on the effect of bile salt and colipase on enzymatic lipolysis. Improved method for the determination of pancreatic lipase and colipase.

The rate of hydrolysis of long chain triglycerides by pure bovine pancreatic lipase has been determined in the presence of variable amounts of bile salts and colipase. Cofactor-free lipase is strongly inhibited by sodium taurodesoxycholate and by mixed bovine bile salts at concentrations higher than the critical micellar concentration. Bile salt inhibited lipase is reactivated by the addition of bovine colipase. Gel filtration of pancreatic juice from several species (Cow, dog, pig) on Sephadex G 100 allows the separation of lipase from colipase. It is found that the enzyme catalyzed hydrolysis of long chain triglycerides by pancreatic lipase from one species is activated by the addition of colipase from other species. Studies on the activation of pancreatic lipase by colipase in the presence of bile salts allowed the re-evaluation of optimal conditions for the determination of lipase and the development of a procedure to assay colipase.     

Characterization of Triton X 100 extracted colipase from porcine pancreas

Colipase was isolated from porcine pancreas homogenate prepared in the presence of detergent (Triton X 100). After precipitation by ammonium sulfate and ethanol, the cofactor was purified by chromatography on SP-Sephadex in the presence of Triton X 100 and on DEAE-cellulose in the absence of detergent. Two molecular forms of porcine colipase were obtained. They represent 80 per cent (colipase A) and 20 per cent (colipase B), respectively, of the total colipase. Valine is the N-terminal residue of both proteins. Their aminoacid composition is similar to that found by Borgstrom for the two forms of porcine colipase. Determination of the sequence of the first sixteen residues at the N-terminal end of colipase A indicates that the cofactor undergoes no proteolytic degradation in this region of the molecule when extraction is carried out in the presence of detergent. The recovery of colipase is about 30 per cent.     

Ovine pancreatic lipase : purification and some properties.

Lipase has been isolated from sheep pancreas. The lipoprotein complex formed in pancreas homogenates by the enzyme and endogenous lipids is split by treatment with acetone. Lipase is further purified by ion-exchange chromatography and gel filtration. The molecular weight and the amino-acid composition of ovine lipase are very similar to that of the porcine and bovine enzymes. As previously found in bovine lipase, no carbohydrate is covalently bound to the polypeptide chain which has a N-terminal residue of lysine. The study of the catalytic properties of ovine pancreatic lipase indicates that the enzyme is fully activated by colipase from various species in the presence of conjugated bile salt micellar solutions.