Subunit level crosslinking of rabbit muscle pyruvate kinase by o-phthaldialdehyde

Evidence for proximal cysteine and lysine residues at or near the ative site of arginine kinase of Stichopus japonicus

Inactivation of arginine kinase (AK) of Stichopus japonicus by o-phthalaldehyde (OPTA) was investigated. The modified enzyme showed an absorption peak at 337 nm and a fluorescent emission peak at 410 nm, which are characteristic of an isoindole derivative formed by OPTA binding to a thiol and an amine group in proximity within the enzyme. Loss of enzymatic activity was concomitant with an increase in fluorescence intensity at 410 nm. Stoichiometry studies by Tsou's method showed that among the cysteine residues available for OPTA modification in the enzyme, only one was essential for the enzyme activity. This cysteine residue is located in a highly hydrophobic environment, presumably near ATP and ADP binding region. This conclusion was verified by 5,5 -dithiobis(2-nitrobenzoic acid) modification. In addition, these results were supported by means of electrophoresis and ultraviolet, fluorescence, circular dichroism spectroscopy and fast performance liquid chromatography. Sequence comparison suggested that this essential cysteine residue maybe the conservative Cys274.

Multiple effects of chemical reagent on enzyme: o-phthalaldehyde-induced inactivation, dissociation and partial unfolding of lactate dehydrogenase from pig heart

The effects of o-phthalaldehyde (OPTA) on lactate dehydrogenase (LDH) have been studied by following changes in enzymatic activity, aggregation state and conformation. Treatment with OPTA resulted in pseudo first-order inactivation of LDH over a wide concentration range of the inhibitor, and the second-order rate constant was estimated to be 1.52M(-1)s(-1). The loss of enzyme activity was concomitant with the increases in absorbance at 337nm and fluorescence intensity at 405nm. Complete loss of enzyme activity was accompanied by the formation of approximately 4mol isoindole derivatives per mole LDH subunit. Cross-linking experiments verified enzyme dissociation during OPTA modification, which could be attributed to the modification of both thiol groups and lysine residues. Circular dichroism (CD) spectra showed that the secondary structure of the OPTA-modified enzyme decreased correspondingly. Comparison of the inactivation with the conformational changes of the enzyme suggests that the active site of the enzyme exhibits greater conformational flexibility than the enzyme molecule as a whole. It is concluded that OPTA modification has multiple effects on LDH, including its inactivation, dissociation and partial unfolding.

Kinetic analysis of enzyme inactivation under second-order conditions by use of substrate-to-product progress curves: application to the inhibition of trypsin by alpha-1 proteinase inhibitor

The inhibition of bovine pancreatic trypsin by human alpha-1 proteinase inhibitor (alpha-1 PI) was studied under second-order conditions by continuously monitoring the fluorescence change due to the enzymatic hydrolysis of N alpha-benzoyl-L-arginine 7-amido-4-methyl-coumarin as substrate. Employing equimolar starting concentrations of enzyme and inhibitor (110-220 nM), the fluorescence progress curve was analyzed according to the equation Pt = (kcat[S]/kiKm)In[ki[E]0t + 1], where ki is the second-order rate constant for the reaction, E + alpha-1 PI-->E* alpha-1 PI (inactive). ki was found to be 1.8 +/- 0.16 x 10(7) M-1 min-1 (at pH 7.0 and 25 degrees C), in close agreement with results obtained by alternative kinetic methods. The method reported appears to be valid and should be useful in the study of fast reactions where one of the reaction partners is an enzyme. An extension of the second-order progress curve approach to cover unequimolar mixtures of E and I is also offered.

Involvement of sulfhydryl groups in the stable fluorescent derivatization of proteins by o-phthalaldehyde

Treatment of human alpha-1-proteinase inhibitor (alpha-1-PI) with o-phthalaldehyde (OPA) at pH 8.0 and 25 degrees C, in the absence of added thiol resulted in the formation of a mixed population of fluorescent and non-fluorescent isoindoles. The stoichiometry of isoindole formation was tentatively calculated to be 6:1 for unreduced alpha-1-PI and 10:1 for inhibitor treated with dithioerythritol, implicating not only cysteine but also non-sulfur nucleophilic centres as reaction partners. Despite the apparent involvement of the single cysteine residue in alpha-1-PI in the over-all derivatization process, the extent of fluorescent derivatization was independent of the redox state of the inhibitor. Hence the fluorescing moiety was not a 1-alkylthio-2-alkyl-substituted isoindole, as generally observed. The finding that isoindole formation in proteins is not limited by sulfhydryl content and that fluorescent products may originate from amino acid(s) other than cysteine cautions against interpreting fluorescent derivatization by OPA as evidence for cross-linking of lysine to cysteine residues.