[Inhibition of alkaline phosphatase by thioureido derivatives of methylenebisphosphonic acid].

A series of thioureido derivatives of methylenebisphosphonic acid were synthesized by the reaction of aminomethylenebisphosphonic acid with the corresponding isothiocyanates, and their effect on the activity of alkaline phosphatases from bovine small intestine mucosa (BSIM) and human placenta was studied. It was found that (3-phenylthioureido)methylenebisphosphonate is approximately one order of magnitude more effective in inhibiting the activity of alkaline phosphatase from BSIM than the alkyl derivatives of thioureidomethylenebisphosphonic acid with methyl, ethyl, tert-butyl, or cyclohexyl substituents. The introduction of substituents into the benzene ring of (3-phenylthioureido)methylenebisphosphonate decreased the effect of the inhibitor on the activity of the enzyme. The affinity of (3-phenylureido)methylenebisphosphonate to the alkaline phosphatase of BSIM was also weaker as compared with the corresponding thioureidomethylenebisphosphonate. The insertion of thioureidobisphosphonates into the active site of alkaline phosphatase of human placenta by the method of molecular docking indicated that the methylenebisphosphonate residue and the substituted amino groups of the inhibitor are involved in the mechanisms of complex formation with the enzyme. It is supposed that the improvement of the inhibitory activity of (3-phenylthioureido)methylenebisphosphonate toward alkaline phosphatase of BSIM is due to the additional fixation of the phenyl substituent in the active site of the enzyme.

[Relative reactivity of thiamine monophosphate and thiamine diphosphate upon interaction with alkaline phosphatase]. / Otnositel'naia reaktsionnaia sposobnost' tiaminmonofosfata i tiamindifosfata pri vzaimodeistvii so shchelochnoi fosfatazoi.

Reactivity of thiamin monophosphate (TMP) as calf intestinal alkaline phosphatase substrate in model transformations is lower comparing with thiamin diphosphate (TDP) reactivity. Under these conditions alkaline phosphatase catalyzes TDP, ADP and AMP hydrolysis approximately at same rate. It was shown that TDP competes with p-nitrophenyl phosphate more effectively than TMP for the binding in the active site. At pH 8.5 and 30 degrees C Km values are as follows: (5.2 +/- 1.6) x 10(-3) M for TMP and (3.0 +/- 0.8) x 10(-4) M for TDP. Under the same conditions the Vmax/Km value for TDP hydrolysis is 53 times higher than the one for corresponding reaction of TMP. It was suggested that positively charged thiazolium ion of TMP interacts with the nearest environment at the active center and by this way reduces enzyme activity.

[Thiamine: a mechanistic model of interaction with protein]. / Tiamin: mekahnisticheskaia model' vzaimodeistviia s belkom.

Mechanistic model of thiamine-binding protein functioning which is based on the potential role of prototropic groups and hydrophobic environment around 5-beta-hydroxyethyl substituent of ligand has been proposed. As a model the chemical transformations of thiamine and its structural O-acyl substituted analogues in the presence of ferricyanide and phosphatic buffer in pH range 7,2-7,8 were investigated. The oxidation to the thiochrome and thiochrome derivatives is first order in substrate and ferricyanide concentrations. It is found that the reciprocal of the pseudo-first-order rate constant increases in ferrocyanide concentration at the constant oxidant concentration. Rate constants and partition ratios for reaction of thiamine, O-benzoylthiamine, O-(4-nitrobenzoyl)thiamine, O-(2-norbornoyl) thiamine, O-(1-norbornoyl)thiamine, O-(1-adamantoyl) thiamine, O-(2-adamantoyl) thiamine, O-(5-methyl-1-adamantyl)acetylthiamine, O-(2-adamantyl)acetylthiamine, O-(1-adamantyl)acetylthiamine were determined. The acceleration effect of hydrophobic fragment of O-acyl substituent is attributed to the formation of neutral tricyclic form in the step followed by electron transfer to ferricyanide. Mechanistic implications for possible transformation of thiamine in neutral tricyclic form at interaction with thiamine-binding protein are discussed.

[Inactivation of yeast pyruvate decarboxylase in the presence of substrate and quinone electron acceptors]. / Inaktivatsiia drozhzhevoi piruvatdekarboksilazy v prisutstvii substrata i khinonovykh aktseptorov élektronov.

The rate constants of paracatalytic inactivation of pyruvate decarboxylase in the presence of 1,4-naphthoquinones and 1,4-benzoquinones are determined by redox potentials of the oxidant. The logarithm of k2 depends hyperbolically on the redox potential of quinone E0(Q/Q-.) with the coefficient of proportionality which approximates 8.4, The absence of considerable deviations in this correlation for the oxidants of different structures with closed values Eo(Q/Q-.) indicates that the enzyme produces no additional steric barrier.

[The interaction of pyruvate decarboxylase with the quinone-ferricyanide oxidative system]. / Vzaimodeistvie piruvatdekarboksilazy s okislitel'noi sistemoi khinon--ferritsianid.

Inactivation of yeast pyruvate decarboxylase in the presence of substrate and oxidative system containing substituted quinone and ferricyanide has been investigated. It was established that ferricyanide at pH 5.2-6.4 can prevent irreversible inactivation of the pyruvate decarboxylase caused by the concerted action of pyruvate and substituted quinone. The influence of ferricyanide which depends on the redox potential of the substituted quinone is decreasing in a series tetramethyl-p-benzoquinone, trimethyl-p-benzoquinone, 2-methyl-5-isopropyl-p-benzoquinone. It is supposed that the effect of the oxidative system partially converting the nonoxidative to oxidative function of pyruvate decarboxylase is attributed to the oxidation of active acetaldehyde by substituted quinone and reaction of resultant semiquinone radical with ferricyanide.

[Paracatalytic interaction of pyruvate decarboxylase with quinones in the presence of an organic solvent. Formation of an active site which hinders proton transfer]. / Parakataliticheskoe vzaimodeistvie piruvatdekarboksilazy s khinonami v prisutstvii organicheskogo rastvoritelia. Formirovanie aktivnogo tsentra, zatrudniaiushchego perenos protona.

Inactivation kinetics of pyruvate decarboxylase under joint action of substrate and substituted quinones in aqueous solutions which contain 1.0-13.5 vol.% of methyl alcohol has been investigated. The observed inactivation rate constant of pyruvate decarboxylase sharply decreases with the increase of methanol concentration from 4 up to 7 vol.% at pH 5.8-6.4. The decrease of the rate constant is independent of quinone order in the kinetic inactivation equation. The result is that the decrease of microscopic dielectric permeability by interaction of methanol with hydrophobic cities of enzyme active surface hinders the transfer of proton at the stage which is limiting in the inactivation process. It is assumed that the organization of active centre of pyruvate decarboxylase may depend on hydrophobic contact.

[Paracatalytic inactivation of pyruvate decarboxylase in the presence of quinones]. / Parakataliticheskaia inaktivatsiia piruvatdekarboksilazy v prisutstvii khinonov.

Pyruvate promotes the yeast pyruvate decarboxylase inactivation under the influence of substituted p-benzoquinones. Pyruvate decarboxylase activity is not renewed after the removal of low-molecular impurities by gel filtration and subsequent addition of dithiothreitol, thiamine diphosphate, magnesium chloride. The inactivation rate under joint action of 2-methyl-5-isopropyl-p-benzoquinone and pyruvate is regulated by the pseudo-first-order equation. The relationship between pseudo-first-order rate constant and pyruvate concentration takes the shape of hyperbola. The inactivation order with respect to quinone is determined by oxidant concentration and pH value. Maximum pseudo-first-order rate constant values in the presence of the excess substrate and 2-methyl-5-isopropyl-p-benzoquinone are observed at pH 5.9-6.0. The data obtained evidence for the fact that during inactivation quinone interacts with "active acetaldehyde" being the intermediate in the process of catalysis with pyruvate decarboxylase.

[Mechanism of interaction between quinones and 2alpha-carbanion in the active center of pyruvate decarboxylase]. / Mekhanizm vzaimodeistviia khinonov s 2alpha-karbanionom v aktivnom tsentre piruvatdekarboksilazy.

The kinetics of paracatalytic inactivation of pyruvate decarboxylase by joint action of the substrate and exogenous oxidant p-benzoquinone, methyl-p-benzoquinone, 2-methyl-5-isopropyl-p-benzoquinone, trimethyl-p-benzoquinone, tetramethyl-p-benzoquinone has been investigated. Nonlinear correlation between the observed second-order rate constants and redox potentials of quinones has been found. It is supposed that negative deviations from linear dependence was caused by changing the rate determining step by the interaction of quinone with 2 alpha-carbanion in the active centre of pyruvate decarboxylase. According to structure of the oxidant the inactivation rate is limited by one electron transfer or the following protonating of the formed anion radical of quinone.

[Pyruvate decarboxylase inactivation by interaction with substrate and molecular oxygen]. / Inaktivatsiia piruvatdekarboksilazy pri vzaimodeistvii s substratom i molekuliarnym kislorodom.

Pyruvate may promote the yeast pyruvate decarboxylase inactivation when affected by molecular oxygen. In the presence of pyruvate and O2 inactivation of enzyme increases with the initial substrate concentration increasing. pH-dependence of pyruvate decarboxylase inactivation under joint action of substrate and O2 has maximum in the region 6.9-7.5. It is suggested that the influence of pyruvate and molecular oxygen is connected with the coenzyme-substrate complex oxidation in an active site of yeast pyruvate decarboxylase on the steps preceding the release of free acetaldehyde.

[Inhibition of yeast pyruvate decarboxylase by alkyl phosphates]. / Ingibirovanie drozhzhevoi piruvatdekarboksilazy alkilfosfatami.

It is found that yeast pyruvate decarboxylase is inhibited by alkyl phosphates. Inhibition is competitive with respect to a substrate. The inhibition constants with n-butyl and n-heptyl esters of phosphoric acid are the values of the same order of magnitude. With an increase in the length of the alkyl phosphates hydrocarbon chain from 7 to 10 carbon atoms inhibition constants change drastically. For n-heptyl phosphate and n-decyl phosphate values KI are equal to 1.6 x 10(-4) M and 1.7 x 10(-6) M, respectively. A further increase in the number of carbon atoms in the alkyl substituent of phosphoric acid ester induces no reduction of the inhibition constant. Multiple-inhibitor experiments of pyruvate decarboxylase show that inorganic phosphate and n-decyl ester of phosphoric acid are mutually exclusive. It is suggested that the inhibition mechanism with alkyl phosphates includes the competition of the phosphoric acid residue with alpha-ketocarboxyl group of pyruvate as well as the interaction between a hydrocarbon radical and hydrophobic parts on the enzyme surface, one of them being outside the substrate binding site.