Lactoperoxidase-Catalyzed Oxidation of Thiocyanate: Equilibria between Oxidized Forms of Thiocyanate

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Abstract

Lactoperoxidase-catalyzed oxidation of thiocyanate ion (SCN-) was studied in the pH range 5-8 so as to obtain either hypothiocyanous acid (HOSCN) or hypothiocyanite ion (OSCN-) as the major observed product. A pKa value of 5.3 was calculated for HOSCN, from the effect of pH on two independent parameters: (1) the extent of extraction of HOSCN into organic solvents from aqueous mixtures of HOSCN and OSCN- and (2) the rate of decomposition of HOSCN-OSCN- mixtures. Partition coefficients of 5.0, 2.1, and 2.3 were calculated for extraction of HOSCN into ethyl acetate, 1-octanol, and 2-octanol. Identification of HOSCN was confirmed by the 2:1 stoichiometry for extraction of oxidizing equivalents and the SCN moiety. Decomposition of HOSCN-OSCN- was accelerated by SCN-, which also changed the apparent kinetic mechanism of decomposition. A second-order rate constant of 3 M-1 s-1 was calculated for decomposition of HOSCN in the presence of 0.1-0.2 mM SCN-, assuming the rate-limiting step to be the dismutation of HOSCN. A first-order constant of 2.6 X 10-3 s-1 was calculated at 10 mM SCN-, assuming the reaction of HOSCN with SCN- to be rate limiting. A number of pH buffering agents also accelerated the decomposition of HOSCN-OSCN-. These agents or SCN- had no effect on extraction of HOSCN into organic solvents. Certain nitrogenous compounds stabilized the oxidizing activity of HOSCN-OSCN- mixtures, apparently by lowering the concentration of free HOSCN. Stabilization by sulfonamides and aromatic imines was consistent with formation of derivatives containing the nitrogen-thiocyanate (N-SCN) linkage (thiocyanatosulfonamides and thiocyanatimines). These N-SCN derivatives retained the oxidizing equivalents of HOSCN or OSCN-, but differed from HOSCN in their solubility in organic solvents. The results indicate that HOSCN is formed in significant amounts and can be relatively stable in the pH range consistent with peroxidase-catalyzed oxidation of SCN- in exocrine secretions, leukocytes, and the thyroid gland. The neutral HOSCN molecule may be responsible for biological activities that have been attributed to the OSCN' anion. Also, the biological activity of HOSCN-OSCN- may be much greater at low pH, provided that the concentrations of SCN' and other components of the medium favor the stability of HOSCN.

Original languageEnglish (US)
Pages (from-to)3273-3280
Number of pages8
JournalBiochemistry
Volume20
Issue number11
DOIs
StatePublished - Jan 1 1981

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Lactoperoxidase
Oxidation
Decomposition
Organic solvents
Bioactivity
hypothiocyanite ion
thiocyanate
1-Octanol
Derivatives
Imines
Sulfonamides
Stoichiometry
Solubility
Peroxidase
Anions
Rate constants
Thyroid Gland
Leukocytes
Nitrogen
Stabilization

All Science Journal Classification (ASJC) codes

  • Biochemistry

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Lactoperoxidase-Catalyzed Oxidation of Thiocyanate : Equilibria between Oxidized Forms of Thiocyanate. / Thomas, Edwin.

In: Biochemistry, Vol. 20, No. 11, 01.01.1981, p. 3273-3280.

Research output: Contribution to journalArticle

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abstract = "Lactoperoxidase-catalyzed oxidation of thiocyanate ion (SCN-) was studied in the pH range 5-8 so as to obtain either hypothiocyanous acid (HOSCN) or hypothiocyanite ion (OSCN-) as the major observed product. A pKa value of 5.3 was calculated for HOSCN, from the effect of pH on two independent parameters: (1) the extent of extraction of HOSCN into organic solvents from aqueous mixtures of HOSCN and OSCN- and (2) the rate of decomposition of HOSCN-OSCN- mixtures. Partition coefficients of 5.0, 2.1, and 2.3 were calculated for extraction of HOSCN into ethyl acetate, 1-octanol, and 2-octanol. Identification of HOSCN was confirmed by the 2:1 stoichiometry for extraction of oxidizing equivalents and the SCN moiety. Decomposition of HOSCN-OSCN- was accelerated by SCN-, which also changed the apparent kinetic mechanism of decomposition. A second-order rate constant of 3 M-1 s-1 was calculated for decomposition of HOSCN in the presence of 0.1-0.2 mM SCN-, assuming the rate-limiting step to be the dismutation of HOSCN. A first-order constant of 2.6 X 10-3 s-1 was calculated at 10 mM SCN-, assuming the reaction of HOSCN with SCN- to be rate limiting. A number of pH buffering agents also accelerated the decomposition of HOSCN-OSCN-. These agents or SCN- had no effect on extraction of HOSCN into organic solvents. Certain nitrogenous compounds stabilized the oxidizing activity of HOSCN-OSCN- mixtures, apparently by lowering the concentration of free HOSCN. Stabilization by sulfonamides and aromatic imines was consistent with formation of derivatives containing the nitrogen-thiocyanate (N-SCN) linkage (thiocyanatosulfonamides and thiocyanatimines). These N-SCN derivatives retained the oxidizing equivalents of HOSCN or OSCN-, but differed from HOSCN in their solubility in organic solvents. The results indicate that HOSCN is formed in significant amounts and can be relatively stable in the pH range consistent with peroxidase-catalyzed oxidation of SCN- in exocrine secretions, leukocytes, and the thyroid gland. The neutral HOSCN molecule may be responsible for biological activities that have been attributed to the OSCN' anion. Also, the biological activity of HOSCN-OSCN- may be much greater at low pH, provided that the concentrations of SCN' and other components of the medium favor the stability of HOSCN.",
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AU - Thomas, Edwin

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N2 - Lactoperoxidase-catalyzed oxidation of thiocyanate ion (SCN-) was studied in the pH range 5-8 so as to obtain either hypothiocyanous acid (HOSCN) or hypothiocyanite ion (OSCN-) as the major observed product. A pKa value of 5.3 was calculated for HOSCN, from the effect of pH on two independent parameters: (1) the extent of extraction of HOSCN into organic solvents from aqueous mixtures of HOSCN and OSCN- and (2) the rate of decomposition of HOSCN-OSCN- mixtures. Partition coefficients of 5.0, 2.1, and 2.3 were calculated for extraction of HOSCN into ethyl acetate, 1-octanol, and 2-octanol. Identification of HOSCN was confirmed by the 2:1 stoichiometry for extraction of oxidizing equivalents and the SCN moiety. Decomposition of HOSCN-OSCN- was accelerated by SCN-, which also changed the apparent kinetic mechanism of decomposition. A second-order rate constant of 3 M-1 s-1 was calculated for decomposition of HOSCN in the presence of 0.1-0.2 mM SCN-, assuming the rate-limiting step to be the dismutation of HOSCN. A first-order constant of 2.6 X 10-3 s-1 was calculated at 10 mM SCN-, assuming the reaction of HOSCN with SCN- to be rate limiting. A number of pH buffering agents also accelerated the decomposition of HOSCN-OSCN-. These agents or SCN- had no effect on extraction of HOSCN into organic solvents. Certain nitrogenous compounds stabilized the oxidizing activity of HOSCN-OSCN- mixtures, apparently by lowering the concentration of free HOSCN. Stabilization by sulfonamides and aromatic imines was consistent with formation of derivatives containing the nitrogen-thiocyanate (N-SCN) linkage (thiocyanatosulfonamides and thiocyanatimines). These N-SCN derivatives retained the oxidizing equivalents of HOSCN or OSCN-, but differed from HOSCN in their solubility in organic solvents. The results indicate that HOSCN is formed in significant amounts and can be relatively stable in the pH range consistent with peroxidase-catalyzed oxidation of SCN- in exocrine secretions, leukocytes, and the thyroid gland. The neutral HOSCN molecule may be responsible for biological activities that have been attributed to the OSCN' anion. Also, the biological activity of HOSCN-OSCN- may be much greater at low pH, provided that the concentrations of SCN' and other components of the medium favor the stability of HOSCN.

AB - Lactoperoxidase-catalyzed oxidation of thiocyanate ion (SCN-) was studied in the pH range 5-8 so as to obtain either hypothiocyanous acid (HOSCN) or hypothiocyanite ion (OSCN-) as the major observed product. A pKa value of 5.3 was calculated for HOSCN, from the effect of pH on two independent parameters: (1) the extent of extraction of HOSCN into organic solvents from aqueous mixtures of HOSCN and OSCN- and (2) the rate of decomposition of HOSCN-OSCN- mixtures. Partition coefficients of 5.0, 2.1, and 2.3 were calculated for extraction of HOSCN into ethyl acetate, 1-octanol, and 2-octanol. Identification of HOSCN was confirmed by the 2:1 stoichiometry for extraction of oxidizing equivalents and the SCN moiety. Decomposition of HOSCN-OSCN- was accelerated by SCN-, which also changed the apparent kinetic mechanism of decomposition. A second-order rate constant of 3 M-1 s-1 was calculated for decomposition of HOSCN in the presence of 0.1-0.2 mM SCN-, assuming the rate-limiting step to be the dismutation of HOSCN. A first-order constant of 2.6 X 10-3 s-1 was calculated at 10 mM SCN-, assuming the reaction of HOSCN with SCN- to be rate limiting. A number of pH buffering agents also accelerated the decomposition of HOSCN-OSCN-. These agents or SCN- had no effect on extraction of HOSCN into organic solvents. Certain nitrogenous compounds stabilized the oxidizing activity of HOSCN-OSCN- mixtures, apparently by lowering the concentration of free HOSCN. Stabilization by sulfonamides and aromatic imines was consistent with formation of derivatives containing the nitrogen-thiocyanate (N-SCN) linkage (thiocyanatosulfonamides and thiocyanatimines). These N-SCN derivatives retained the oxidizing equivalents of HOSCN or OSCN-, but differed from HOSCN in their solubility in organic solvents. The results indicate that HOSCN is formed in significant amounts and can be relatively stable in the pH range consistent with peroxidase-catalyzed oxidation of SCN- in exocrine secretions, leukocytes, and the thyroid gland. The neutral HOSCN molecule may be responsible for biological activities that have been attributed to the OSCN' anion. Also, the biological activity of HOSCN-OSCN- may be much greater at low pH, provided that the concentrations of SCN' and other components of the medium favor the stability of HOSCN.

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