Concanavalin A binding to membranes of the Golgi apparatus and resultant modification of galactosyltransferase activity

Chemoproteomic-enabled characterization of small GTPase Rab1a as a target of an N-arylbenzimidazole ligand's rescue of Parkinson's-associated cell toxicity

The development of phenotypic models of Parkinson's disease (PD) has enabled screening and identification of phenotypically active small molecules that restore complex biological pathways affected by PD toxicity.

Characterization of α-mannosidase from Erythrina indica seeds and influence of endogenous lectin on its activity

alpha-mannosidase from Erythrina indica seeds is a Zn(2+) dependent glycoprotein with 8.6% carbohydrate. The enzyme has a temperature optimum of 50 degrees C and energy of activation calculated from Arrhenius plot was found to be 23 kJ mol(-1). N-terminal sequence up to five amino acid residues was found to be DTQEN (Asp, Thr, Gln, Glu, and Asn). In chemical modification studies treatment of the enzyme with NBS led to total loss of enzyme activity and modification of a single tryptophan residue led to inactivation. Fluorescence studies over a pH range of 3-8 have shown tryptophan residue to be in highly hydrophobic environment and pH change did not bring about any appreciable change in its environment. Far-UV CD spectrum indicated predominance of alpha-helical structure in the enzyme. alpha-Mannosidase from E indica exhibits immunological identity with alpha-mannosidase from Canavalia ensiformis but not with the same enzyme from Glycine max and Cicer arietinum. Incubation of E. indica seed lectin with alpha-mannosidase resulted in 35% increase in its activity, while no such activation was observed for acid phosphatase from E. indica. Lectin induced activation of alpha-mannosidase could be completely abolished in presence of lactose, a sugar specific for lectin.

The hyperinsulinemia produced by concanavalin A in rats is opioid-dependent and hormonally regulated

The present study examines the effect of concanavalin A (Con A) on the blood insulin and glucose levels of rats. Male and female rats treated with Con A (62.5-500 micrograms/kg) for three days showed a dose- and time-dependent hyperinsulinemia that lasted more than 48 h. Male rats were more sensitive to Con A. Thus, 6 h after treatment with Con A the circulating insulin levels in male rats had increased by 85% (control: 10.2 +/- 0.9 mU/l and Con A-treated: 18.8 +/- 1 mU/l) compared to only 38% (control: 7.5 +/- 0.2 mU/l; Con A-treated: 10.3 +/- mU/l) in females. An identical response was seen after 12 h. Con A (250 micrograms/kg) produced time-dependent hypoglycemia in both sexes but more pronounced in males. There was no correlation between the hypoglycemia and hyperinsulinemia described above. The Con A-induced hyperinsulinemia in rats of both sexes was abolished in gonadectomized animals (intact males: +101 +/- 17% vs orchiectomized males: -5 +/- 3%; intact females: +86 +/- 23% vs ovariectomized females: -18 +/- 7.2%). Pretreating intact male and female rats with human chorionic gonadotropin also significantly inhibited the Con A-induced hyperinsulinemia. Estradiol (10 micrograms/kg,i.m.) significantly blocked the Con A-induced increase in circulating insulin in male rats (101 +/- 17% for controls vs 32 +/- 5.3% for estradiol-treated animals, P < 0.05) while testosterone (10 mg/kg, i.m.) had no similar effect on intact female rats. Pretreating Con A-injected rats with opioid antagonists such as naloxone (1 mg/kg, s.c.) and naltrexone (5 mg/kg, s.c.) blocked the hyperinsulinemia produced by the lectin in males (control: +101 +/- 17% vs naloxone-treated: +5 +/- 14%, or naltrexone-treated: -23 +/- 4.5%) and females (control: +86 +/- 23% vs naloxone-treated: +21 +/- 20%, or naltrexone-treated: -18 +/- 11%). These results demonstrate that Con A increases the levels of circulating insulin in rats and that this response is opioid-dependent and hormonally regulated.

Concanavalin A induced inhibition of 5'-nucleotidase from guinea pig skeletal muscle and bull seminal plasma: a comparative study

Both purified and membrane-bound 5'-nucleotidases (EC 3.1,3.5) from guinea pig skeletal muscle and bull seminal plasma are inhibited by Concanavalin A (Con A). 5'-Nucleotidase purified from skeletal muscle is inhibited by Con A by an apparent uncompetitive process (K'i = 160 nM), while the lectin inhibits the particulate enzyme by an apparent non-competitive process (Ki = K'i = 50 nM). 5'-Nucleotidase purified from bull seminal plasma is inhibited by Con A by an apparent non-competitive process (K'i = Ki = 270 nM), while the membrane-bound enzyme is subjected to a mixed type inhibition by the lectin (K'i greater than Ki; 30 and 14 nM, respectively). The enzyme purified from skeletal muscle exhibits a significant cooperativity in the interaction with Con A. The inhibition of bull seminal plasma particulate 5'-nucleotidase brought about by Con A is not completely reversed by addition of alpha-methyl-D-mannoside.

Calorimetric study of the binding reaction of concanavalin A with immunoglublins

The thermal effects associated with the binding reaction of concanavalin A and immunoglobulins have been measured by calorimetry. IgG solutions do not generate heat on mixing with concanavalin A, confirming the low reactivity of IgG for the lectin molecule. IgM solutions, on the other hand, show a substantial enthalpic contribution of delta H = -24 +/- 1kJ/site for the binding reaction between the polysaccharide chains of human IgM macroglobulin and concanavalin. A stoichiometry is 10 monovalent concanavalin A molecules per intact macroglobulin and two bivalent concanavalin A molecules per two sites on the heavy chain of the reduced macroglobulin subunit.

Calorimetric study of concanavalin A binding to saccharides

The binding of concanavalin A in the dimer form to various saccharides has been studied by calorimetry, and estimates of the binding enthalpy and binding constants have been calculated. Methyl alpha-D-mannoside and methyl alpha-D-glucoside have a -- delta H0 of 21.5 and 11.5 kJ/mol, respectively, at both pH 4 and 4.5. The p-nitrophenyl derivatives react with enthalpic values of 15.6 and 14.6 kJ/mol. The galactosepyranosides show no heat effects during mixing with the protein solutions. The apparent binding enthalpies calculated from the variations of the equilibrium constants with temperature are in good agreement with the values measured experimentally. The two binding sites of the dimer form of concanavalin A are equal and independent, and the low enthalpies obtained do not justify a large conformational change during the reaction. The binding reaction has also been estimated for other sugars normally contained in glycoproteins.

Effect of surface modifiers on an ectoenzyme: granulocyte 5'-nucleotidase

Several agents that react with plasma membranes, namely the native lectins concanavalin A, Ricinus communis agglutinin, and wheat germ agglutinin, the modified lectin succinyl concanavalin A, and sodium meta-periodate, inhibited the ecto-5'-nucleotidase of intact guinea pig granulocytes. Stimulation of the enzyme was not observed at any lectin concentration. Inhibition by native lectins could be blocked or reversed by appropriate competing hapten sugars. In the case of concanavalin A, reversal could be achieved at 37 degrees C, but not at 5 degrees C. When lectins were used in combination with each other, the effects were found to be largely independent. However, when concanavalin A and R. communis agglutinin were applied together, complications arose because the former lectin binds to the latter as well as to the cell surface. To avoid some of the complexities inherent in studying intact cell 5'-nucleotidase and to gain additional information about the system, two broken cell enzyme preparations were also examined. The enzyme of plasma membrane-enriched fractions was inhibited by all five agents mentioned above. 5'-Nucleotidase solubilized in sodium deoxycholate was inhibited by the four lectins but stimulated by periodate. The effects of the surface modifiers on kinetic data for all three enzyme preparations are consistent with the hypothesis that direct interactions with the enzyme molecule give rise to changes in Vmax; interactions at membrane sites other than 5'-nucleotidase itself could cause increases in apparent Km values. Effects of interactions of ectoenzymes with plant lectins may serve as models for phenomena that result from cell-cell interactions or from interactions of animal cells with lectin-like components of the cellular environment.

Properties of galactosyltransferase-enriched vesicles of Golgi membranes from lactating-rat mammary gland

Lactose was synthesised within the lumen of purified Golgi membrane vesicles, prepared from lactating rat mammary gland, from externally added glucose and UDP-galactose. An apparent Km of 1.5 mM was shown towards glucose at anomeric equilibrium, but only beta-glucose was utilised. Two apparent Km values, about 17 micro M and 112 micro M, were shown towards UDP-galactose. 5-D-Thioglucose, 6-deoxy-D-glucose and 6-deoxy-6-chloro-D-glucose were alternative substrates, acquiring alpha-lactalbumin dependence when the vesicles were lysed with detergent. Substrates independent of alpha-lactalbumin, or inhibited by it, included a wide range of N-acylated glucosamines as well as phenyl-beta-glucoside. The galactosylation of these by vesicle preparations could be ascribed to a proportion of leaky vesicles. Suitably low concentrations of Triton X-100 activated lactose synthesis by intact vesicles, indicating the membrane as a rate-limiting feature of the system.

Restoration of detergent-inactivated adenosine triphosphatase activity of human prostatic fluid with concanavalin A

A stimulation by concanavalian A OF Mg2+ -and Ca2+-dependent ATPase (ATP phosphohydrolase, EC 3.6.1.3) of human prostatic fluid has been observed after the enzyme system had been inactivated by a detergent such as 0.05% deoxycholate. The concanavalin A effect was specific since the positive effect was abolished in the presence o alpha-methyl-D-mannoside. Furthermore, the positive effect of concanavalin A was obtained with a low lectin concentration, equal to the concentration reported for optimal stimulation of other membrane enzymes.