Activation of tyrosinase in microsomes and melanosomes from B16 and Harding-Passey melanomas

Glucose and sulfate conjugation of phenolic compounds by the spiny lobster (Panulirus argus)

Previous work has shown that the hepatopancreas of the spiny lobster (Panulirus argus) contains a mixed-function oxidase system capable of catalyzing the monooxygenation of polycyclic aromatic hydrocarbons to highly toxic products similar to those formed by mammalian tissues. Studies were designed to determine the ability of the spiny lobster to conjugate the phenolic compounds 4-methylumbelliferone, p-nitrophenol, beta-naphthol, and 3-hydroxybenzo[a]pyrene with endogenous molecules. The hepatopancreas contained UDP-glucose (UDPG) dependent glucosyltransferase, while no activity was detected when UDP-glucuronic acid was used as the cosubstrate. Atypical Michaelis-Menten kinetics result with varying concentrations of UDPG, indicating that multiple forms of glucosyltransferase may exist in this organ. The activity was localized in the microsomal fraction, exhibited a pH optimum at 8.0-8.5, and a temperature optimum of 30 degrees C. Sulfate conjugation was found only in the cytosolic fraction of the antennal gland and used adenosine 3'-phosphate 5'-phosphosulfate (PAPS) as the sulfate donor (Km(apparent) = 9.0 +/- 4.9 microM). Hepatopancreas cytosol inhibited sulfotransferase activity. The pH optimum of antennal gland sulfotransferase was a function of the substrate and ranged from 5.5 to 7.4. Analysis of spiny lobster urine 24 hr following exposure to 3-hydroxybenzo[a]pyrene demonstrated the ability of the lobster to form both the sulfate and glucoside conjugate in vivo.

Bovine retinal pigment epithelial cells cultured in vitro: growth characteristics, morphology, chromosomes, phagocytosis ability, tyrosinase activity and effect of freezing

We compared the bovine melanotic and amelanotic retinal pigment epithelial (RPE) cells cultivated in vitro using a number of parameters. Both cell types could be readily harvested from the eye by dissection or by enzyme treatment of the RPE. Both types of cells grew equally well in the primary culture, and in serial subcultures for over 600 days, and also after being stored frozen for over two months. The two types of cells were similar in their growth patterns, morphology, chromosome characteristics and phagocytic activity but they differed in their tyrosinase activity. Although in long-term cultures cells showed chromosome loss, a high proportion of the cells retained their normal diploid chromosome number (2n = 60) with apparently normal karyotype.

The effect of temperature on tyrosinase activity in Himalayan mouse skin

The tyrosinase activity of Himalayan mouse skin homogenates was measured over a range of temperatures using two sensitive radiometric assay--namely, (1) the measurement of 14C-tyrosine incorporation into melanin, and (2) the measurement of 3HOH released as a by-product of 3H-tyrosine hydroxylation. Results show that Himalayan tyrosinase is maximally active at temperatures well below normal body temperature (15 degree C to 25 degree C). These results are in support of Danneel's visual observations ('41) that "ferment" activity of Himalayan rabbit skin is absent at temperatures above 25 degree C. Further results suggest the presence of a tyrosinase inhibitor in Himalayan mouse skin. First, removal of a low molecular weight fraction from Himalayan skin homogenates resulted in an increase in tyrosinase activity. Second, recombination of the low molecular weight fraction to the homogenate from which it was originally separated resulted in a decrease in tyrosinase activity when assayed at 37 degree C, but no decrease when assayed at 25 degree C. It is proposed that at the normal body temperature of 37 degree C, tyrosinase from Himalayan skin is strongly bound to an inhibitor. At lower body temperatures, the affinity of the enzyme for the inhibitor decreases, thus allowing the synthesis of melanin to increase. This change in affinity of the enzyme for the inhibitor could be regulated by temperature-induced conformational changes in either the enzyme or the inhibitor or both.

Melanocytotoxicity and the mechanism of activation of gamma-L-glutaminyl-4-hydroxybenzene

gamma-L-Glutaminyl-4-hydroxybenzene is converted by the tyrosinase of the common mushroom, Agaricus bisporus, to the toxic, dormancy-inducing metabolite 2-hydroxy-4-imino-2,5-cyclohexadiene-1-one. Hydroxylation of gamma-L-glutaminyl-4-hydroxybenzene by mammalian tyrosinase was monitored by determining tritium water release from gamma-L-glutaminyl-[3,5-(3)H[4-hydroxybenzene and occurred at only 25% of the rate found with tyrosine. The dihydroxy product of the hydroxylation reaction, gamma-L-glutaminyl-3,4-dihydroxybenzene, was not oxidized by the mammalian enzyme. Therefore, oxidation of gamma-L-glutaminyl-4-hydroxybenzene to sulfhydryl-reactive quinones by mammalian tyrosinase is an unlikely explanation for the hair depigmentation and inhibition of melanocarcinoma growth observed following administration of this compound. Cleavage of gamma-L-glutaminyl-4-hydroxybenzene by gamma-glutamyl transpeptidase releasing p-aminophenol was demonstrated. p-Aminophenol was an active depigmenting and melanocytotoxic compound. N2-Methyl-gamma-L-glutaminyl-4-hydroxybenzene was synthesized, differing from gamma-L-glutaminyl-4-hydroxybenzene only by the presence of a methylated amide linkage. This chemical modification resulted in a compound resistant to cleavage by gamma-glutamyl transpeptidase and lacking in melanocytotoxic activity. gamma-Glutamyl transpeptidase cleavage is proposed as the route for transformation of gamma-L-glutaminyl-4-hydroxybenzene into an active inhibitor of melanocytes.

Chymotrypsin activation of melanosome tyrosinase in hamster melanotic melanoma

The tyrosinase activity in two sucrose gradient isolated melanosome fractions from a melanotic hamster melanoma was found to increase after alpha-chymotrypsin treatment. The enhancement in tyrosinase activity had its maximum at a concentration of 1 mg/ml alpha-chymotrypsin after 120 min incubation at 37 degrees C. No direct activating effect of alpha-chymotrypsin was found either on the soluble tyrosinase fraction from freshly prepared untreaed whole-tumor homogenate or on purified mushroom tyrosinase. The activating effect of alpha-chymotrypsin upon the melanosome tyrosinase is believed to be due to the endopeptidic hydrolysis of the--CO--NH--bound existing between tyrosinase and tyrosine and phenylalanine residues in the melanin molecule. Although alternative interpretations are not excluded, the observed enhancement in tyrosinase activity after alpha-chymotrypsin treatment of melanosomes might indicate the existence of an "enzyme liberating" mechanism in the melanosomes.

Characterization of human hairbulb tyrosinase: properties of normal and albino enzyme

Human hairbulb tyrosinase from normally pigmented and tyrosinase-positive oculocutaneous albino (TPA) hairbulbs was studied by single hairbulb and by pooled hairbulb assay procedures. The response to temperature and pH was the same for TPA and normal enzyme. The Km for tyrosine as substrate and the Km or dopa as cofactor was the same for TPA and normal enzyme. These studies show that TPA tyrosinase is kinetically normal and that the defect with this form of albinism must be elsewhere in the melanin pathway.

Mammalian tyrosinase. Structural and functional interraltionship of isozymes

The isozymes of tyrosinase from normal and malignant melanocytes were studied; the data indicates that each consists of a basic tyrosinase polypeptide, and differs by post-translational modifications. T3 represents the de novo form of the enzyme; it is converted to T1 in vivo by the addition of sialic acids and neutral sugars, and in turn, to T4 by complexing with mealanosomal membrane constituents. The T2 isomer is suggested to be an artefact of the electrophoretic procedure, and due to deamidation of T3. It is shown that the apparent kinetics of enzyme activity are unafffected by any of these modifications.

The heterogeneity of uridine diphosphate glucuronyltransferase from rat liver

1. The glucuronide conjugation of p-nitrophenol, phenolphthalein, o-aminophenol and 4-methylumbelliferone by rat liver microsomes has been studied. The detergent Triton X-100 activated UDP-glucuronyltransferase activity towards all these substrates, therefore the optimum activating concentration was added in all experiments. 2. Mg(2+) enhanced the conjugation of the substrates. 3. With phenolphthalein substrate inhibition occurred but this could be relieved by adding albumin, which binds excess of phenolphthalein. 4. Kinetic constants of the substrates and UDP-glucuronate have been determined. Mutual inhibition was found with the substrates p-nitrophenol, 4-methylumbelliferone and phenolphthalein. p-Nitrophenol conjugation was inhibited competitively by phenolphthalein and 4-methylumbelliferone. 5. o-Aminophenol did not inhibit the conjugation of the other three substrates because these are conjugated preferentially to o-aminophenol. 6. It is concluded that the four substrates are conjugated by one enzyme at the same active site.