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L Lack

Publications and source records attributed to L Lack.

At least 55 records · Page 3Linked to original sources
PubMed1977

The effect of bile salts on the formation and hydrolysis of cholesterol esters by rat liver enzymes.

To determine the effects of different bile salts on the enzymic esterification of cholesterol and the hydrolysis of cholesterol esters rat liver homogenates and rat liver microsomes were incubated with varying amounts of different bile salts. Bile salts inhibited the formation of radioactive cholesterol esters in incubations of either rat liver homogenates or rat liver microsomes containing [14C]cholesterol. Chenodeoxycholate, glycochenodeoxycholate and taurochenodeoxycholate were more potent inhibitors than their comparable cholate analogues. Bile salts stimulated the hydrolysis of cholesterol esters when incubation were carried out with the liver homogenates. The dihydroxy bile salts were again more potent in this regard than the trihydroxylated bile salts. When the effects of bile salts on cholesterol ester hydrolysis were studied in in vitro incubations of hepatic microsomes a biphasic mode of acion was observed. In the absence of Na+ or K+ bile salts stimulated the hydrolysis of cholesterol oleate. However, following the addition of either Na+ or K+ to the microsomal incubations, bile salts caused an inhibition of cholesterol ester hydrolysis. Since cholesterol esterification was also inhibited under these conditions a direct inhibitory effect (not attributable to enhanced hydrolase activity) of the bile salts on the formation of cholesterol esters by the microsomes was established. Furthermore, this inhibition takes place at the transacylation step involving the fatty acyl-CoA ester and the sterol. These results suggest that bile salts can significantly alter the cholesterol-cholesterol ester profile in the liver, and furthermore, that these effects may be influenced by small changes in the intracellular environment in the region where these reactions occur.

Acyltransferases↗
PubMed1977

Interaction of uncharged bile salt derivatives with the ileal bile salt transport system.

Two series of uncharged conjugated bile salt derivatives, N-conjugates of ethanolamine and 3-amino-1,2-propanediol were studied for interaction with the ileal bile salt transport system. Evidence for interaction is threefold. 1) In everted gut sac experiments more material was removed from the mucosal compartment when ileal sacs were used. 2) These derivatives inhibited the in vitro transport of taurocholate. 3) In vivo intestinal perfusion demonstrated greater absorption from ileum than from jejunum. Number three demonstrates that such interactions are followed by transmucosal movement. Their uphill transport was less than taurocholate transport. The Na(+) requirement for cholyl-3-amino-1,2-propanediol interaction with the system was greater than for taurocholate. This observation is similar to that previously observed with taurodehydrocholate, which had a greater Na(+) requirement for transport than taurocholate. Therefore removal of the anionic charge, as well as distortion of steroid shape, increases the Na(+) requirement for substrate interaction with the transport system. These observations support our hypothesis that this interaction involves two recognition components; one includes the steroid moiety, the other a coulombic interaction between the anionic bile salt and a cationic membrane site. Additionally the membrane would have an anionic group to accomodate the Na(+). Both factors (steroidal and coulombic) operate for optimal substrate attachment. Simultaneously the system's affinity for Na(+) increases and active transport then proceeds.

Animals↗
PubMed1976

A sterospecific synthesis of 7alpha-hydroxycholesterol.

The five step synthesis of 7alpha-hydroxycholesterol utilizes the solvolysis of 7alpha-bromocholesterol benzoate with potassium acetate in acetic acid as the key step in controlling the stereospecificity of the reaction sequence. This reaction yields 7alpha-acetoxycholesterol benzoate with retention of configuration at position seven. The diester is readily reduced with lithium aluminum to 7alpha-hydroxycholesterol.

Hydroxycholesterols↗
PubMed1976

The use of [7alpha-3H]- and [7alpha, 7beta-3H]cholesterol in the enzymic assay of cholesterol 7alpha-hydroxylase.

A tritium release method is described for following the enzymic conversion of cholesterol to 7alpha-hydroxycholesterol. Incubations of rat liver subcellular preparations (containing microsomes) with [7alpha-3H]cholesterol or [7alpha,7beta-3H]cholesterol release the labeled hydrogen in the 7alpha position as 3H2O which, after counting, allows for the determination of the fraction of exogenous cholesterol converted to 7alpha-hydroxycholesterol. These findings document those recently reported by Van Cantfort, Renson, and Gielen (1975. Eur J. Biochem. 55:23). Analysis of incubation mixtures containing both [4-14C]cholesterol and either [7alpha-3H] or [7alpha,7beta-3H]cholesterol demonstrate that one atom of hydrogen (from the 7alpha position) is incorporated into H2O for every molecule of exogenous cholesterol that is converted to 7alpha-hydroxycholesterol. In the case of [7alpha-3H]cholesterol no label is retained by the product. With [7alpha,7beta-3H]cholesterol, one atom is released as 3H2O and one is retained by the product in the 7beta position. Microsomal incubations with [7alpha,7beta-3H]cholesterol were performed, followed by the acetylation of the steroid fractions with [14C]acetic anhydride. If intermixing of exogenous with endogenous cholesterol were complete during the enzymic reaction, one would expect the 3H: 14C ratio of the isolated cholesterol acetate to be four times that observed in the 7alpha-acetoxycholesterol acetate. Average values of 4.23 in one series and 4.03 in a second series indicate that intermixing was sufficiently complete to use the tritium release method as an indicator of mass conversion.

Animals↗
PubMed1976

Ionic requirements for the active ileal bile salt transport system.

Taurocholate transport by everted ileal gut sacs was studied in physiological media containing graded amounts of sodium ions. Significant uphill transport of taurocholate was observed when the bulk of NaCl was replaced by osmotic equivalents of mannitol or choline chloride. Seventy-seven percent of control transport activity was observed when 36 milliequivalents per liter of Na+ were present in the incubation medium with mannitol acting as the isosmotic replacement, and 74% of the control transport was retained when 31 milliequivalents per liter of Na+ were present in the incubation medium with choline chloride acting as the osmotic replacement. Lowering the Na+ concentration to 19 milliequivalents per liter (i.e., 84% replacement of Na+) still allowed for 69% of the uphill transport observed in the control incubations. Taurodehydrocholate transport by ileal everted sacs was more sensitive to decreased Na+ concentrations; 29% of control transport was observed at 31 milliequivalents per liter of Na+. A kinetic analysis comparing the transport of taurocholate with taurodehydrocholate, the triketo analogue, at different concentrations of Na+ indicated that the apparent affinity of the transport system for Na+ is greater in the presence of taurocholate than in the presence of taurodehydrocholate. The ability of taurodehydrocholate to depress taurocholate transport is less in media of low Na+ concentration. Finally, in vivo intestinal perfusion studies demonstrated that the depression of taurocholate absorption, following Na+ removal, is reversible. These results are in agreement with the idea that Na+ has a physiological role in intestinal bile salt transport, and that the affinities of the anionic bile salt and the sodium cation for the transport system appear to be cooperative in that one enhances the binding of the other.

Animals↗
PubMed1975

Interactions of cationic bile salt derivatives with the ileal bile salt transport system.

Previous structure-activity studies of the active ileal bile salt transport system have demonstrated that a single negative charge on the side chain is essential for active transport. Furthermore, mutual inhibition studies between different pairs of bile salt substrates indicated that dihydroxy bile salts had a greater apparent affinity for the transport system than the trihydroxylated compounds and triketo bile salts had the least such affinity. In this study, a series of cationic bile salt derivatives (cholamine conjugates) were prepared with one, two, and three alpha-hydroxyl groups on the steroid moiety. Based on the previous observations one would expect (1) no active transport of any of the cholamine conjugates by the ileal transport system; (2) interaction of these compounds with the transport system in such a way as to inhibit the transport of bile salts, with inhibition potency of the transport of any single bile salt inversely related to the number of hydroxyl groups present on the cholamine conjugate; and (3) transport of triketo anionic bile salts to be most readily inhibited, trihydroxy compounds less readily inhibited, and dihydroxy bile salts least inhibited. Using everted gut sac preparations it was demonstrated that all three aforementioned expectations did occur. Furthermore, reversible inhibition of ileal absorption of taurocholate and the bile salt derivative taurodehydrocholate could be demonstrated in vivo. The dihydroxy cholamine conjugates were better inhibitors than the trihydroxy compound. Relative specificity for the bile salt system of these cationic bile salt derivatives was demonstrated in the in vivo preparation by comparing its inhibition of taurodehydrocholate absorption with their lesser capacity to inhibit glucose transport.

Animals↗