Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: UMLS:C0027960 (mole)
 21,279 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Reaction of aldose reductase (ALR2) from pig muscle with pyridoxal 5'-phosphate (pyridoxal-P) and other lysine modifying reagents resulted in activation of the enzyme. The activation by pyridoxal-P showed pH and concentration dependence that was prevented by incubation with NADPH and various cofactor analogues but not by the aldehyde substrate. Spectral analysis of the reaction showed characteristic peaks associated with Schiff's base formation between a lysine amino group and the aldehyde of pyridoxal-P. Subsequent reduction produced spectra characteristic of a phosphopyridoxyllysine bond. Phosphopyridoxyllysine was isolated by amino acid analysis of modified ALR2. Determination of the stoichiometry of bound phosphopyridoxyllysine indicated one mole of pyridoxal-P per mole of enzyme under conditions that produced maximal activation. A single [3H]phosphopyridoxyllysine containing peptide was isolated by high performance liquid chromatography after enzymatic cleavage of the modified enzyme. This 34 residue peptide exhibited considerable sequence homology to the region comprising residues 242 to 275 of human liver ALR1 and a similar region in rat lens ALR2, human muscle ALR2 and human placental ALR2. The activation of ALR2 via formation of a Schiff's base suggests a possible mechanism of activation of the enzyme in vivo by glucose.
 ...
PMID:Enhancement of aldose reductase activity by modification of an active site lysine: a possible mechanism for in vivo activation. 211 50

Incubation of human placental aldose reductase (EC 1.1.1.21) with the sulfhydryl oxidizing reagents 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) and N-ethylmaleimide (NEM) results in a biexponential loss of catalytic activity. Inactivation by DTNB or NEM is prevented by saturating concentrations of NADPH. ATP-ribose offers partial protection against inactivation by DTNB, whereas NADP, nicotinamide mononucleotide (NMN), and the substrates glyceraldehyde and glucose offer little or no protection. The inactivation by DTNB was reversed by dithiothreitol and partially by 2-mercaptoethanol but not by KCN. When the release of 2-nitro-5-mercaptobenzoic acid was measured, 3 mol of sulfhydryl residues was found to be modified per mole of the enzyme by DTNB. Correlation of the fractional activity remaining with the extent of modification by the statistical method of C.-L. Tsou (1962, Sci. Sin. 11, 1535-1558) indicates that of the three reactive residues, one reacts at a faster rate than the other two, and that two residues are essential for the catalytic activity of the enzyme. Labeling of the total sulfhydryl by [14C]NEM and quantification of DTNB-reactive residues in the enzyme denatured by 6 M urea indicates that a total of seven sulfhydryl residues are present in the protein. The modification of the enzyme did not affect Km glyceraldehyde, but the modified enzyme had a lower Km NADPH. Kinetic analysis of the data suggests that a biexponential nature of inactivation could be due to the formation of a dissociable E:DTNB complex and the presence of a partially active enzyme species.
 ...
PMID:Functional cysteinyl residues in human placental aldose reductase. 251 May 98

Bovine lens aldose reductase (ALR2) is readily modified by glutathione disulphide (GSSG) to an enzyme form (GS-ALR2) exhibiting a reduced catalytic efficiency with all the substrates tested and a reduced susceptibility to inhibition. The modification, which is completely reversed by reduced glutathione (GSH) or dithiothreitol occurs by a pseudo-first-order process with respect to the enzyme and a second order rate constant of 30 +/- 0.1 mol-1 min-1 at 25 degrees C was determined. By measuring the residual activity of ALR2 incubated in different glutathione redox buffers at 25 degrees C, an apparent redox equilibrium constant of 1.4 +/- 0.1 was evaluated. Thus the rate and the maximal extent of ALR2 inactivation are proportional to the redox ratio of the thiol used as modifying agent (i.e. [GSH]/[GSSG]). The stoichiometric reversibility of the enzyme modification might be impaired by a reduced solubility of GS-ALR2 with respect to ALR2 and by an increased susceptibility of the modified enzyme to proteolysis. While the native enzyme form is rather insensitive to proteolytic breakdown. GS-ALR2 is easily degraded by chymotrypsin with the generation of a peptide of 26 kDa with an aminoacid sequence at the aminoterminal side compatible with proteolysis at level of Tyr 7 of aldose reductase. A reduced efficiency in the enzyme-cofactor binding following the GSSG dependent modification of ALR2, appears to be associated to the thiol accessibility of GS-ALR2 measured at different temperatures. GS-ALR2 is characterized by the presence of one glutathione residue, linked through a mixed disulphide bond. This is sustained by: (i) the isoelectric point for the modified enzyme of 4.75, which is 0.1 pH units lower than that observed for the native enzyme, which indicates the contribution of an acidic residue to the pI of GS-ALR2; (ii) the incorporation of radioactivity coming from [3H] labelled GSSG accounting for the presence of one equivalent of glutathione per mole of enzyme. Besides being a general feature of protein reactivity in oxidative conditions, the glutathione-mediated ALR2 modification might be part of a cell strategy to preserve reducing power in conditions of oxidative stress.
 ...
PMID:Glutathione dependent modification of bovine lens aldose reductase. 792 85

Bovine lens aldose reductase (alditol: NADP+ oxido-reductase, EC 1.1.1.21) undergoes a thiol-dependent oxidative modification catalyzed by the Fe(II)/Fe(III) redox system. The enzyme is inactivated by various oxygen radical generating systems. However, addition of 2-mercaptoethanol to the oxygen radical generating systems resulted in an initial increase followed by a decrease in the activity of aldose reductase. The net maximal increase in the enzyme activity was observed with 3 mM 2-mercaptoethanol, 0.3 mM FeSO4, and 0.9 mM EDTA, either with or without 1 mM hypoxanthine and 37 mU/ml of xanthine oxidase. The formation of the stable, activated intermediate, ARa, appears to proceed through the reaction between the enzyme and the oxidized form of 2-mercaptoethanol which in the presence of iron, forms a mixed disulfide with a cysteine residue. Reduction of ARa with dithiothreitol released 0.7 mol of 2-mercaptoethanol per mole of enzyme and converted it to a form that resembled the native aldose reductase.
 ...
PMID:Thiol-dependent metal-catalyzed oxidation of bovine lens aldose reductase. I. Studies on the modification process. 842 75