Sweet tyrosinase

A crnde extract of sweet potato Ipomoea-batatas (L.) Lam.) was nsed as a source of phenol oxidases (polyphenoloxidase, tyrosinase, catecholoxidase, EC 1.14.18.1). The extract was directly placed in the carrier of a FIA system with UVD, to promote oxidation of phenolic compounds to o-quinones that condense to form melanin-like pigments with a strong absorption at 410 nm. The determination of phenols in industrial wastewaters showed good agreement with conventional methods (correlation coefficient 0.9954) LOD was 10 p,M, with RSD <2.7% (w = 6). Under optimal storage conditions the enzymatic activity did not vary for at least five months . 

The crystal structures of tyrosinase from Streptomyces castaneoghbisporus HUT 6202 and catechol oxidase from the sweet potato Ipomoea batata have been determined. They confirm that the coordination of the type-3 copper site in tyrosinase and catechol oxidase is very similar to that found in hemocyanin. This had been deduced before from the similarity of spectroscopic properties and a comparison of many tyrosinase and hemocyanin primary structures. On the basis of the biological source of the proteins seven different domain organizations could be identified. Plant catechol oxidases of different organisms have a sequence identity of about 40-60%. The sequence identity between catechol oxidases and mulluscan hemocyanins is about 35% over almost the whole length of the sequences. In contrast, the sequence identity between plant catechol oxidases and other type-3 copper proteins from any nonplant source is limited to the two copperbinding regions. 

Tyrosinase is an enzyme complex (phenolase, polyphenol oxidase are other names which have been used for this enzyme), which catalyses of the ortho hydroxylation of monohydric phenols. The enzyme, which should not be confused with L-tyrosine hydroxylase mentioned above, contains Cu (I) and catalyses two distinct reactions—the hydroxylation of monohydric phenols to o-diphenols (cresolase activity) and the oxidation of o-diphenols to o-quinones (catecholase or catechol oxidase activity) . Most enzymes of this type, which are widely distributed in both the plant and animal kingdoms, exhibit both cataljrtic functions. Thus typically, the conversion of L-tyrosine (5) to L-dopa (15) and dopaquinone (36) which occurs in melanin biosynthesis is catalysed by an enzyme of the tyrosinase category. The two activities appear, in the majority of cases, to be functions of the same enzyme. However, certain o-diphenol oxidases such as those from tea , sweet potato and tobacco have been reported to show no capacity to catalyse the hydroxylation reaction but this is most probably due to destruction of the cresolase activity during purification. 

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