Na+-K+-ATPase pumping activity is not directly linked to myo-inositol levels after sorbinil treatment in lenses of diabetic rats.

Changes in tissue levels of sorbitol, myo-inositol, and Na+-K+-ATPase enzyme activity have been implicated in the development of diabetic complications in animal models of the disease and in humans. The ability of the aldose reductase inhibitor sorbinil to reverse the hyperglycemia-induced changes in these lenticular metabolite and enzyme-activity levels in the streptozocin-induced diabetic rat was examined to determine what, if any, relationship exists between these changes. Two weeks of untreated diabetes did not change ouabain-inhibitable ATPase enzyme activity assayed in lens homogenates but did result in a decrease in the Na+-K+-ATPase transport activity as measured by 86Rb uptake in the intact lens. This was accompanied by a 100-fold increase in the levels of sorbitol and significant decreases in the levels of myo-inositol, ATP, and glutathione in the lens. Whereas all of these changes could be reversed by sorbinil treatment, the dose required for restoration of the depleted myo-inositol level (ED50 greater than 20 mg.kg-1.day-1) was much higher than the dose required to reverse the other changes (ED50 range 2-5 mg.kg-1.day-1). These results suggest that the restoration of lenticular Na+ -K+ -ATPase activity is not secondary to a normalization of myo-inositol levels and may provide evidence that the two parameters are not strictly associated in diabetic tissues.

A highly specific aldose reductase inhibitor, ethyl 1-benzyl-3-hydroxy-2(5H)-oxopyrrole-4-carboxylate, and its congeners.

Ethyl 1-benzyl-3-hydroxy-2(5H)-oxopyrrole-4-carboxylate (1, EBPC) is a potent and specific inhibitor of aldose reductase. It was greater than 4000X more potent in its inhibition of rat lens aldose reductase than the closely related rat or pig kidney aldehyde reductase, thus making it the most selective inhibitor of a NADPH-dependent carbonyl reductase identified to date. In agreement with this observation, it was found to be a highly potent inhibitor of aldose reductase from rat sciatic nerve with greater than 98% inhibition at 1 microM, but it was practically devoid of activity against aldehyde reductases from rat liver and brain. Inhibition of aldose reductase was mixed type for glyceraldehyde (Ki = 8.0 x 10(-8) M) and noncompetitive for NADPH (Ki = 1.70 x 10(-8) M). Its potential as an in vitro tool to quantitate monomeric aldo/keto reductase activities in crude tissue extracts is presented. Structure-activity relationships emerging from synthetic modifications of EBPC are discussed. Several modifications were found to be active in vitro against aldose reductase from human placenta and in vivo in a rat model of diabetic complications, but none was more potent than EBPC.

Orally active aldose reductase inhibitors: indazoleacetic, oxopyridazineacetic, and oxopyridopyridazineacetic acid derivatives.

Benzothiazole side chains featured in zopolrestat (1a) and its congeners were incorporated into oxophthalazineacetic acid replacements, including indazole, pyridazinone, and pyridopyridazinone with a pendant acetic acid moiety. Potent aldose reductase inhibition activity among resulting compounds is as widespread as it is in the earlier zopolrestat series, thus lending further support to our hypothesis that there is a binding site on the aldose reductase enzyme with strong affinity for benzothiazoles. Representative new compounds 1-[(5,7-difluoro-2-benzothiazolyl)-methyl]-1H-indazoleacetic acid (62), [6-[[5-(trifluoromethyl)benzothiazol-2-yl]methyl]-8-oxo- 6H-pyrido[2,3-d]-pyridazin-5-yl]acetic acid (70), 3,4-dihydro-4-oxo-5,6-dimethyl-3-[(5,7-difluorobenzothiazol-2-yl) methyl]-1-pyridazineacetic acid (79), and 3,4-dihydro-4-oxo-5,6-cyclohexano-3-[[5-(trifluoromethyl) benzothiazol-2-yl]-methyl]-1-pyridazineacetic acid (82) are potent aldose reductase inhibitors with IC50s of 30, 2.1, 5, and 52.2 nM, respectively. The best of these compounds, 79 and 82, also inhibit accumulation of sorbitol in rat sciatic nerve in a model of diabetic complications, when administered orally at 10 mg/kg. The inhibition values are 76 and 61%, respectively. In addition to benzothiazole, we have examined its surrogates effective in potentiating aldose reductase inhibition activity, including benzoxazole and aryl[1,2,4]oxadiazole. Structure-activity relationships emerging from this program are also discussed.

Hydantoin bioisosteres. In vivo active spiro hydroxy acetic acid aldose reductase inhibitors.

The hypothesis that clinical side effects of the aldose reductase inhibitor (ARI) sorbinil were related to its hydantoin ring led to a bioisosteric analysis and replacement of the hydantoin by a spiro hydroxy acetic acid moiety as in 40. These hydroxy acids, compared to hydantoins, showed a similar potency increase on chroman 2-methyl substitution, a similar orthogonal relationship of acidic to aromatic moieties, and similar ARI enantioselectivity. In this series the six-membered spiro hydroxy acetic acid anion array is a bioisostere for a spiro hydantoin anion and leads to ARIs with excellent in vivo activity. In vitro and in vivo activity was improved over 40 by chroman cis 2-methylation as in 4 and by aromatic 6,7-halogen substitution. Compounds with the best acute in vivo activity in rats were compared for chronic in vivo activity. The highest tissue levels and best chronic in vivo activities were found in the racemic 6,7-dichloro and 6-fluoro-7-chloro analogues 18 and 23. ARI activity was enantioselective for 58 and 60, the 2R,4R-enantiomers of 18 and 23. 7-Chloro-6-fluoro-cis-4-hydroxy-2(R)-methyl-chroman-4-acetic acid (60) was selected for phase 1 clinical trials and did not exhibit sorbinil-like hypersensitivity side effects.

Potent, orally active aldose reductase inhibitors related to zopolrestat: surrogates for benzothiazole side chain.

A broad structure-activity program was undertaken in search of effective surrogates for the key benzothiazole side chain of the potent aldose reductase inhibitor, zopolrestat (1). A structure-driven approach was pursued, which spanned exploration of three areas: (1) 5/6 fused heterocycles such as benzoxazole, benzothiophene, benzofuran, and imidazopyridine; (2) 5-membered heterocycles, including oxadiazole, oxazole, thiazole, and thiadiazole, with pendant aryl groups, and (3) thioanilide as a formal equivalent of benzothiazole. Several benzoxazole- and 1,2,4-oxadiazole-derived analogues were found to be potent inhibitors of aldose reductase from human placenta and were orally active in preventing sorbitol accumulation in rat sciatic nerve, in an acute test of diabetic complications. 3,4-Dihydro-4-oxo-3-[(5,7-difluoro-2-benzoxazolyl)methyl]-1- phthalazineacetic acid (124) was the best of the benzoxazole series (IC50 = 3.2 x 10(-9) M); it suppressed accumulation of sorbitol in rat sciatic nerve by 78% at an oral dose of 10 mg/kg. Compound 139, 3,4-dihydro-4-oxo-3-[[(2-fluorophenyl)-1,2,4- oxadiazol-5-yl]methyl]-1-phthalazineacetic acid, with IC50 less than 1.0 x 10(-8) M, caused a 69% reduction in sorbitol accumulation in rat sciatic nerve at an oral dose of 25 mg/kg. The thioanilide side chain featured in 3-[2-[[3-(trifluoromethyl)phenyl]amino]-2-thioxoethyl]-3,4-dihydro - 4-oxo-1-phthalazineacetic acid (195) proved to be an effective surrogate for benzothiazole. Compound 195 was highly potent in vitro (IC50 = 5.2 x 10(-8) M) but did not show oral activity when tested at 100 mg/kg. Additional structure-activity relationships encompassing a variety of heterocyclic side chains are discussed.

Novel, potent aldose reductase inhibitors: 3,4-dihydro-4-oxo-3-[[5-(trifluoromethyl)-2-benzothiazolyl] methyl]-1-phthalazineacetic acid (zopolrestat) and congeners.

A new working hypothesis that there is a hitherto unrecognized binding site on the aldose reductase (AR) enzyme with strong affinity for benzothiazoles was pursued for the design of novel, potent aldose reductase inhibitors (ARIs). The first application of this hypothesis led to a novel series of 3,4-dihydro-4-oxo-3-(benzothiazolylmethyl)-1-phthalazineacetic+ + + acids. The parent of this series (207) was a potent inhibitor of AR from human placenta (IC50 = 1.9 x 10(-8) M) and was orally active in preventing sorbitol accumulation in rat sciatic nerve, in an acute test of diabetic complications (ED50 = 18.5 mg/kg). Optimization of this lead through medicinal chemical rationale, including analogy from other drug series, led to more potent congeners of 207 and culminated in the design of 3,4-dihydro-4-oxo-3-[[5-(trifluoromethyl)-2-benzothiazolyl] methyl]-1-phthalazineacetic acid (216, CP-73,850, zopolrestat). Zopolrestat was found to be more potent than 207, both in vitro and in vivo. Its IC50 against AR and ED50 in the acute test were 3.1 x 10(-9)M and 3.6 mg/kg, respectively. Its ED50s in reversing already elevated sorbitol accumulation in rat sciatic nerve, retina, and lens in a chronic test were 1.9, 17.6, and 18.4 mg/kg, respectively. It was well absorbed in diabetic patients, resulting in high blood level, showed a highly favorable plasma half-life (27.5 h), and is undergoing further clinical evaluation. An assortment of synthetic methods used for the construction of benzothiazoles, including an efficient synthesis of zopolrestat, is described. Structure-activity relationships in the new series are discussed.

Spiro hydantoin aldose reductase inhibitors derived from 8-aza-4-chromanones.

A series of spiro hydantoins derived from 8-azachromanones (2,3-dihydro-4H-pyrano[2,3-b]pyridin-4-ones) has been prepared and tested for aldose reductase inhibitory activity. The standard Bucherer-Bergs conditions had to be drastically modified to increase yields from less than 1% to an acceptable 50% range. One of the most potent compounds was cis-6'-chloro-2',3'-dihydro-2'-methylspiro[imidazolidine-4,4'-4'H- pyrano[2,3-b]pyridine]-2,5-dione; resolution of this compound showed that the 2'R,4'S enantiomer 16 was the most active spiro hydantoin in this series with an IC50 of 7.5 x 10(-9) against human placenta aldose reductase.

Effects of the aldose reductase inhibitor sorbinil on the isolated cultured rat lens.

The isolated cultured rat lens has been used to examine the effects of the aldose reductase inhibitor sorbinil on lenticular polyol accumulation and sugar cataract formation. Lenses incubated in medium containing 35 mmol/L glucose accumulated sorbitol over a seven-day period without the appearance of overt opacities. Sorbitol accumulation was inhibited in a dose response fashion by sorbinil with an IC50 of 3.1 X 10(-6) mol/L. In lenses incubated in the presence of 29.5 mmol/L xylose, xylitol accumulation was accompanied by an increase in the water content of the lens and the development of a classical sugar cataract. All of these effects could be prevented by the addition of sorbinil to the culture medium. Complete inhibition of cataract formation required greater than an 80% inhibition of the xylitol accumulation. Reversal of a preformed xylose cataract by sorbinil could be achieved if the inhibitor was added at the stage of cortical opacities (20 h). Cataract progression proceeded normally over the next 48 hours and then the lens slowly began to clear. The rate of the reversal was dependent on the dose of sorbinil.

Massive Dracunculus insignis infection in a dog.

An 11-year-old 13-kg (28.6-lb) spayed female Cocker Spaniel was examined because of subcutaneous nodules on the hind limbs and ventral aspects of the thorax and abdomen. Focal areas of erythema and pyoderma were associated with the nodules, and purulent exudate could be expressed from a fistula in the nodules. A nematode approximately 20.5 cm in length was isolated from a draining fistula in 1 nodule and identified as Dracunculus insignis. The dog was treated with ivermectin, fenbendazole, and metronidazole, but the owner was still able to recover worms from multiple nodules for the next year.

Enzymatic properties of the beta-galactoside alpha 1 leads to 2 fucosyltransferase from porcine submaxillary gland.

The acceptor substrate specificity and kinetic properties of the purified porcine submaxillary beta-galactoside alpha 1 leads to 2 fucosyltransferase have been examined. The transferase forms the Fuc alpha 1 leads to 2Gal linkage with oligosaccharides, glycoproteins, and glycolipids which contain nonreducing terminal galactose residues and shows no absolute specificity for a particular penultimate residue or for the linkage between the galactose and the penultimate residue. The fucosyltransferase is active in the absence of divalent metal ions, but it is stimulated upon addition of Mn2+, Mg2+, Ca2+, or Co2+. Kinetic analysis indicates an increase in the Km for both donor and acceptor substrates and in the Vmax in the presence of Mn2+. Initial rate studies and inhibition patterns suggest that the transferase has either a rapid equilibrium random kinetic mechanism or a steady state ordered mechanism with GDP-fucose binding first. Human "Bombay" erythrocytes which lack cell surface Fuc alpha 1 leads to 2Gal structures are fucosylated by the transferase, but expression of H blood group activity is dependent on treatment of the cells with neuraminidase. After neuraminidase digestion, the fucosylated cells are serologically identical to native O-type cells. Analysis of the fucosylated material in the erythrocyte membrane on sodium dodecyl sulfate-polyacrylamide gel electrophoresis suggests that fucose is incorporated primarily into glycoprotein acceptors.

Affinity chromatography of glycosyltransferases.

This review summarizes the use of biospecific chromatography techniques in the purification of mammalian glycosyltransferases. Ligands that are analogues of donor or acceptor substrates have been linked to cyanogen bromide-activated agarose for use as affinity adsorbents. Immobilized lectins have been employed to recognize the carbohydrate moieties of glycosyltransferase and remove them from complex mixtures. The application of these methods has permitted extensive purification of many membrane-bound glycosyltransferases, some to homogeneity.

Purification to homogeneity of H blood group beta-galactoside alpha 1 leads to 2 fucosyltransferase from porcine submaxillary gland.

A beta-galactoside alpha 1 leads to 2 fucosyltransferase has been solubilized from porcine submaxillary glands and purified 124,000-fold to homogeneity by repeated affinity chromatography on GDP-hexanolamine agarose. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of the purified enzyme revealed two electrophoretic species with apparent Mr = 60,000 and 55,000. The two enzyme species have not been completely resolved, but both appear to be active forms of the fucosyltransferase with approximately equal specific activities. Glycosidase digestion of the fucosylated products with the alpha 1 leads to 2-specific fucosidase from Clostridium perfringens and the alpha 1 leads to 3/alpha 1 leads to 4-specific fucosidase from almond emulsin indicates that the enzyme forms exclusively the Fuc alpha 1 leads to 2Gal linkage with a variety of acceptor substrates. A GDP-fucose hydrolase activity co-purifies with the fucosyltransferase. Identical rates of thermal inactivation and co-migration on gel electrophoresis under nondenaturing conditions suggest that the two activities are due to a single enzyme species.

The kinetic mechansim of bovine milk galactosyltransferase. The role of alpha-lactalbumin.

Initial rate parameters obtained with bovine galactosyltransferase at saturating Mn2+ concentrations, and a variety of acceptors including N-acetylglucosamine, glucose, ovalbumin, and di-N-acetylglucosamine are inconsistent with an ordered addition of UDP-galactose and acceptor substrates to the enzyme-Mn2+ complex. Inhibition patterns with N-acetylglucosamine or UDP-glucose as inhibitors of the galactosylation of ovalbumin indicated that either UDP-galactose or N-acetylglucosamine can bind to an enzyme-Mn2+ complex by a random equilibrium mechanism. Initial rate studies also indicate that alpha-lactalbumin may bind to either an enzyme-Mn2+-acceptor complex or an enzyme-Mn2+-UDP-galactose complex, suggesting that lactose synthesis also proceeds by a random equilibrium addition of substrates and alpha-lactalbumin. From the initial rate data assuming the random equilibrium mechanism, the dissociation constants for UDP-galactose, acceptor substrates, and alpha-lactalbumin from the appropriate complexes have been calculated. These values are in good agreement with those obtained independently by nonkinetic methods, providing additional support for the proposed random equilibrium mechanism. From similar studies with a cross-linked complex of alpha-lactalbumin and transferase, dissociation constants for UDP-galactose and acceptor substrates from the enzyme-Mn2+-alpha-lactalbumin complex were calculated. Comparison of each of the dissociation constants in the substrate addition phase shows that the binding of acceptor substrates and alpha-lactalbumin to enzyme-Mn2+ complexes is highly synergistic; the affinity of alpha-lactalbumin for the enzyme-Mn2+ acceptor complex is about 2 orders of magnitude greater than for the enzyme-Mn2+ complex. Similarly, the affinity of the acceptor for the enzyme-Mn2+-alpha-lactalbumin complex is about 2 orders of magnitude greater than the enzyme-Mn2+ complex. Synergism is also observed between alpha-lactalbumin and UDP-galactose binding but the synergism is much less than that observed with acceptor substrates and alpha-lactalbumin. Thus, the large decrease in the Michaelis constant for glucose in the presence of alpha-lactalbumin, which is observed for lactose synthesis by the galactosyltranferase, is primarily the result of the high degree of synergism in the binding of alpha-lactalbumin and glucose to enzyme-Mn2+ complexes. This synergism also accounts for the activation of N-acetyllactosamine synthesis by alpha-lactalbumin at low concentrations (less than 2 mM) of N-acetylglucosamine. An abortive enzyme-Mn2+-UDP-acceptor complex in the product release phase of the reaction appears to account for the inhibition of either lactose, or N-acetyllactosamine synthesis at a high concentration of either N-acetylglucosamine or glucose. This abortive complex is further stabilized by alpha-lactalbumin, thus the resulting substrate inhibition is observed at much lower acceptor concentrations in the presence of alpha-lactalbumin.

Co-purification of the Lewis blood group N-acetylglucosaminide alpha 1 goes to 4 fucosyltransferase and an N-acetylglucosaminide alpha 1 goes to 3 fucosyltransferase from human milk.

The Lewis blood group-specified N-acetylglucosaminide alpha 1 goes to 4 fucosyltransferase and an N-acetylglucosaminide alpha 1 goes to 3 fucosyltransferase have been copurified over 500,000-fold from human milk by affinity chromatography on GDP-hexanolamine agarose. The purified enzyme preparation migrates as two major bands on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with apparent Mr = 53,000 and 51,000. Analysis of the acceptor substrate specificity of the transferase(s) and structural characterization of the reaction products indicate that the enzyme(s) forms the Fuc alpha 1 goes to 4GlcNAc, Fuc alpha 1 goes to 3GlcNAc, and Fuc alpha 1 goes to 3Glc linkages with oligosaccharide acceptors containing the nonreducing terminal sequences Gal beta 1 goes to 3GlcNAc, Gal beta 1 goes to 4GlcNAc, and Gal beta 1 goes to 4Glc, respectively. The two fucosyltransferase activities are activated to the same extent by a variety of divalent metal ions, inactivated at identical rates by thermal denaturation or reaction with N-ethylmaleimide, and inhibited to the same extent by rabbit antiserum prepared against the purified fucosyltransferase(s). In addition, kinetic analysis of the initial rate data obtained using acceptors for one of the fucosyltransferase activities as an inhibitor of the second suggests that acceptors for both fucosyltransferase activities bind at a single active site.

The lactose synthase acceptor site: a structural map derived from acceptor studies.

A pictorial map of the lactose synthase (galactosyl transferase) acceptor binding site has been formulated from this and published studies on substrate analogs and inhibitors. The basic requirements are a pyranose, thiopyranose or inositol ring structure and equatorial substituents (if any) at C-2, C-3, C-4, and C-5. The aglycone (at C-1) may be either alpha or beta-, but alpha- is somewhat preferred. In the absence of alpha-lactalbumin galactosyl transferase will accept long chain 2-N-acyl substituents on the glucosamine (GlcNH2) structure. An equatorial amino or N-acetyl substituent (e.g. mannosamine, N-acetylmannosamine) is also a suitable acceptor in the absence of alpha-lactalbumin since both N-acetylglucosamine and N-acetylmannosamine have complementary binding loci for the N-acyl moiety. The aglycone moiety must be equatorial (beta-configuration). However, upon alpha-lactalbumin binding the aglycone specificity allows for axial (alpha-configuration) as well as equatorial substituents. Furthermore, the 2-N-acyl substituent binding locus is blocked beyond a 2-N-hexanoyl group. It is suggested that alpha-lactalbumin binds to a hydrophobic site some distance from the C-2 group.