The oxidation of catechol and homocatechol by tyrosinase in the presence of amino-acids

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Abstract
The component of crude mushroom extracts responsible for the appearance of a purple color when such extracts oxidize catechol was identified by isolation of L-proline. The pigment-forming reaction was studied in respect of color intensity developed, oxygen absorbed and hydrogen ion liberated, in systems containing purified tyrosinase, catechol or homocatechol, and one of the following: proline,hydroxyproline,hy-droxyproline ethyl ester, pyrrolidlne, glycine, dimethylamine and methylamine. A few expts. were also carried out with alanine, glutamic acid and arginine. The same intense purple color was obtained in each case in which the nitrogenous substance present had a secondary amino group; the compounds with primary amino N gave a much less intense orange-red color. From homocatechol and hydroxyproline ethyl ester, by oxidation with AgO in ethanol, 4- (4[image]-hydroxy-2[image]-carbethoxy-1[image] -pyrrolidyl)-5-methyl-o-benzo-quinone was prepared in a pure state. It appeared to be identical with the pigment formed from the same precursors in the enzymic reaction mixture, and it was concluded that all the pigments had an analogous structure, and that the pigment-forming reaction was essentially a condensation between 1 mol. of o-quinone and 1 mol. of nitrogenous base to give a leuco pigment which was spontaneously oxidized to the colored form by oxygen. In the case of glycine, when present in excess, the formation of pigment was followed by oxidation of the excess amino acid, with liberation of ammonia. Further oxidation of this type did not take place in the case of the amino-acid pigments.
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