Asymmetric Trans-esterification of meso-2,5-Dibromoadipate and Synthesis of Optically Active cis-2,5-Disubstituted Pyrrolidines

Asymmetric trans-esterification of meso-2,5-dibromoadipate to (–)-benzyl methyl 2,5-dibromoadipate by lipase with subsequent chemical reactions afforded optically active cis-2,5-disubstituted pyrrolidines. An equivalent asymmetric transformation was performed by selectively hydrolyzing a cis-2,5-disubstituted pyrrolidine having a chiral N-substituent.     

Anomer specificity of the 14 kDa galactose-binding lectin: A reappraisal

A β-anomer preference among galactosides has been attributed to the S-type 14 kDa galactose binding lectin. Here the anomeric preference of this lectin from bovine brain (BBL) is reexamined using inhibition of lectin-mediated haemagglutination, binding of the lectin to dot-blotted glycoproteins and affinity electrophoresis of the lectin through polysaccharide-containing gels. 1.0-methyl α-D-galactoside was 8 times better inhibitor of BBL than the corresponding ß-anomer. The terminal galactose in bovine thyroglobulin (exclusively. α-linked) were also nearly 8 times more inhibitory than those in asialofetuin (exclusively ß-linked). The terminal α-galactose-containing endogenous glycoproteins of bovine brain were nearly 4 times better inhibitors of BBL than laminin. Removal of terminal α-galactose units by α-galactosidase fully abolished the BBL binding of thyroglobulin and endogenous glycoproteins. BBL was also sugar-specifically retarded by polyacrylamide gel containing guar galactommannan which bears only α-linked galactose. Data indicated that α-galactosides were sometimes better than their β-anomers in binding to BBL. The significance of this observation to the physiological role of galactose-binding lectins is discussed.     

大鼠红细胞葡萄糖代谢的异头物选择性

为研究大鼠红细胞对葡萄糖利用的异头物选择性及其作用机制,应用大鼠红细胞,对葡萄糖的两种异头物作了异构化速率、乳酸生成量、内流速度和大鼠红细胞已糖激酶作用下的磷酸化速度等进行了测定．结果指出,３７℃时大鼠红细胞的Ｄ－葡萄糖β－异头物和α－异头物代谢成乳酸的速度分别是０．２７μｍｏｌ／ｇＨｂ（３ｍｉｎ）和０．２１μｍｏｌ／ｇＨｂ（３ｍｉｎ）,即前者快于后者３０％．同时β－Ｄ－葡萄糖向红细胞内转运速度也快于后者：分别是５．０和３．５μｍｏｌ／ｇＨｂ（３ｍｉｎ）．大鼠红细胞已糖激酶的葡萄糖磷酸化速率实验结果指出：β－异头物比α－异头物快３０％;对于该两种异头物已糖激酶的Ｋｍ值均为５３μｍｏｌ／Ｌ．红细胞与α－和β－Ｄ－葡萄糖保温１ｍｉｎ后,其葡萄糖浓度均达到１ｍｍｏｌ／Ｌ左右,说明至少在１ｍｉｎ内对于已糖激酶的磷酸化此两种异头物的葡萄糖浓度均已饱和．这些结果提示,大鼠红细胞葡萄糖利用的β－异头物优选性主要与其磷酸化速度有关,而与其转运速度关系不大．     

Ligand specificity and ligand-induced conformational change in gal repressor

Gal repressor (GalR) binds D-galactose, which is responsible for lifting of repression of the gal operon. Proton T1 measurements of alpha- and beta-anomers of galactose as a function of gal repressor show preferential binding of the beta-anomer. The beta-anomer was isolated by high-performance liquid chromatography and was shown to bind tightly to GalR. Calorimetry was used to determine enthalpy changes at several temperatures. Heat capacity change was found to be positive, indicating that a significant amount of hydrophobic surface area was exposed upon galactose binding. Bis-ANS binding to GalR is significantly enhanced in the presence of a saturating amount of galactose, indicating additional exposure of hydrophobic surfaces. We propose that the galactose-induced conformational change involves the opening of the two subdomains, which may disrupt protein-protein interactions responsible for repression.     

Anomeric preference of glucose utilization in rat erythrocytes

The -anomer of glucose relative to the -anomer was more rapidly metabolized into lactate by rat erythrocytes at 37° C (/ ratio = ca. 1.3): the amounts of - and -D-glucose metabolized into lactate during 3 min were 0.21 and 0.27 mol/gHb, respectively. Also, the transport of -D-glucose into erythrocytes was more rapid than that of -D-glucose: the amounts of - and -D-glucose transported into erythrocytes during 3 min were approximately 3.5 and 5.0 mol/gHb, respectively. Glucose phosphorylation by rat erythrocyte hexokinase (i.e., a possible rate-limiting step in glycolysis) occurred at higher velocities with the -anomer than with the a-anomer (/ ratio = 1.28). The Km value of hexokinase for either anomer of glucose was 53 M. The glucose concentrations in erythrocytes incubated with - and -D-glucose reached about 1 mM in 1 min, indicating that hexokinase is almost completely saturated with glucose within less than 1 min. The results suggest that glucose phosphorylation and glucose transport are major and minor determinants, respectively, for the anomeric preference of glucose utilization in rat erythrocytes.