Tyrosine hydroxylase Tyrosinase

A. The correct response is dihydropteridine reductase. This enzyme reduces dihydrobiopterin to tetrahydrobiopterin the obligate electron donor for phenylalanine hydroxylase. Tyrosinase is the first enzyme on the pathway to melanin. Dopamine hydroxylase and tyrosine transaminase are enzymes on other tyrosine metabohc tracts. Homogentisic acid oxidase is an enzyme on the pathway of tyrosine to fumarate and acetoacetate. 

Mushroom tyrosinase was extracted as described by Ingebrigtsen and Flurkey (J. Food Sci., in press). Tyrosinase activity was monitored using either catechol, dopa or tyrosine as the substrates. All assays were carried out in the presence and absence of 0.1% SDS (w/v) to detect active and latent enzyme activities. The catechol oxidase activity of tyrosinase was assayed in 50 mM phosphate (pH 6.0) containing 10 mM catechol and the absorbance monitored at 410 nm (25-26). The dopa oxidase activity of tyrosinase was assayed in 50 mM phosphate (pH 6.0) containing 5 mM L-dopa and the absorbance monitored at 475 nm. The tyrosine hydroxylase activity of tyrosinase was assayed in 33 mM phosphate (pH 6.0) containing 0.33 mM L-tyrosine and the absorbance monitored at 280 nm. Protein content was determined by the method of Lowry et al. (26). 

The sequence His-Phe-Arg-Trp (of the green-shaded sequence in Fig. 30-2) is essential for binding. In kerat-inocytes a-MSH may form a 1 1 complex with tetrahy-drobiopterin, the coenzyme for tyrosine hydroxylase, and a regulator of tyrosinase, an essential enzyme for melanin formation (Fig. 25-6). 

It has also been observed that tryptophan, like dopa, inhibits tyrosine hydroxylase and dopa oxidase activity of melanosomal tyrosinase and that its inhibitory mechanism differs from inhibition caused by non-substrate type compounds like cysteine and ascorbic acid (36). In fact, tyrosinase is inhibited by its own substrate in vitro and this inhibition mechanism differs from that caused by cysteine and ascorbic acid (242, 268). 

The second step of dopamine production is hydroxylation of L-tyrosine to form l-DOPA. l-DOPA is a biologically important chemical employed for the treatment of Parkinson s disease, so l-DOPA has been produced employing chemical, enzymatic, or microbial processes [43-46]. In plants and animals, l-DOPA is mainly synthesized from L-tyrosine by tyrosine hydroxylase (TH). Since TH requires a tetrahydrobiopterin as a cofactor, TH cannot be utilized for the conversion of L-tyrosine to l-DOPA in E. coli. Therefore, tyrosinase or p-hydroxy phenylacetate 3-hydroxylase has been employed for the hydroxylation of L-tyrosine to form l-DOPA [43, 46, 47]. 

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. 

The phenol-oxidizing enzyme tyrosinase has two types of activity (/) phenol o-hydroxylase (cresolase) activity, whereby a monophenol is converted into an o-diphenol via the incorporation of oxygen, and (2) cathecholase activity, whereby the diphenol is oxidized. The two reactions are illustrated in Figure 2-6, in the conversion of tyrosine (2.40) to L-DOPA (3,4-dihydroxyphenylalanine (2.41), dopaquinone (2.42), and indole-5,6-quinone carboxylate (2.43), which is further converted to the brown pigment... 

Japanese publications report that vitamin E can be used elfectively in the treatment of facial pigmentation. It inhibits tyrosinase indirectly by inhibiting its hydroxylase activity. It also acts by limiting the oxidative phases required in the first stages of the transformation of tyrosine into indole derivatives. Vitamin E oxidizes easily, and more stable derivatives than pure a-tocopherol have to be used in cosmetic preparations. Tocopheryl acetate is one of the more stable derivatives of vitamin E. Tocopheryl ferulate is also frequently used. 

The hydroxylation of tyrosine into dopa is catalysed by tyrosinase, that of tryptophane into 5-hydroxytryptophane is probably catalysed by a specific tryptophane-5-hydroxylase but other workers were not able to isolate the latter from the mucosa of the small intestine in rats and guine2i-pigs/ " But recently it was possible by means of isotope techniques to demonstrate tryptophanehydroxylase in carcinoid tissue as well as in human platelets. (1864) 5-Hydroxylation of tryptophane seems to be the rate-limiting step in the synthesis of serotonin. Tryptophanehydroxylase is strongly inhibited in vivo by / -chlorophenylalanine. Initial therapeutic trials with the called drug in carcinoid patients resulted in a marked improvement of intestinal symptoms but did not prevent the attacks of flushing. ... 

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