Lactoperoxidase-catalyzed iodination of tyrosine peptides

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Abstract

  • 1.

    1.|The rate of lactoperoxidase (donor:H2O2 oxidoreductase, EC 1.11.1.7)-catalyzed iodination of tyrosine derivatives and tyrosine containing peptides was studied at neutral pH. Lactoperoxidase catalyzes the iodination of tyrosine, its ester and amide at comparable rates while derivatives of the amino group, such as N-acetyltyrosine, are iodinated at significantly lower rates. Neutral amino acid containing dipeptides in which tyrosine is N-terminal are more readily iodinated than those in which tyrosine is C-terminal. Tyr > Tyr-Gly = Tyr-Ala ≥ Tyr-Val = Tyr-Leu = Tyr-Phe = Phe-Tyr > Leu-Tyr = Val-Tyr = Ala-Tyr ≥ Gly-Tyr. Lysine containing dipeptides also are iodinated in the same manner. Tyr > Tyr-Lys > Lys-Tyr. The converse appears to be true in the case of glutamic acid containing dipeptides. Tyr > Glu-Tyr ≥ Tyr-Glu. At pH 3.6 the rate of iodination is the same for tyrosine and all the peptides.

  • 2.

    2.|At pH 7.4 horseradish peroxidase does not catalyze the direct iodination of tyrosine or any of the tyrosine derivatives studied. It is pointed out that spectrophotometric assays for iodination of tyrosine may not distinguish between oxidative coupling and iodination.

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