Binding of aflatoxin B1 to melanin

Implications of melanin binding in ocular drug delivery

Pigmented ocular tissues contain melanin within the intracellular melanosomes. Drugs bind to melanin at varying extent that ranges from no binding to extensive binding. Binding may lead to drug accumulation to the pigmented tissues and prolonged drug retention in the melanin containing cells. Therefore, melanin binding is an important feature that affects ocular drug delivery and biodistribution, but this topic has not been reviewed since 1998. In this review, we present current knowledge on ocular melanin, melanosomes and binding of drugs to pigmented cells and tissues. In vitro, in vivo and in silico methods in the field were critically evaluated, because the literature in this field can be confusing if the reader does not properly understand the methodological aspects. Literature analysis includes a comprehensive table of literature data on melanin binding of drugs. Furthermore, we aimed to give some insights beyond the current literature by making a chemical structure based classification model for melanin binding of drugs and kinetic simulations that revealed significant interplay between melanin binding and drug permeability across the melanosomal and plasma membranes. Overall, more mechanistic and systematic research is needed before the impact of melanin binding on ocular drug delivery can be properly understood and predicted.

Magnetically assisted solid phase extraction using Fe3O4 nanoparticles combined with enhanced spectrofluorimetric detection for aflatoxin M1 determination in milk samples

A novel, facile and inexpensive solid phase extraction (SPE) method using ethylene glycol bis-mercaptoacetate modified 3-(trimethoxysilyl)-1-propanethiol grafted Fe(3)O(4) nanoparticles coupled with spectrofluorimetric detection was proposed for determination of aflatoxin M1 (AFM1) in liquid milk samples. The method uses the advantage fluorescence enhancement by β-cyclodexterin complexation of AFM1 in 12% (v/v) acetonitrile-water and the remarkable properties of Fe(3)O(4) nanoparticles namely high surface area and strong magnetization were utilized to achieve high enrichment factor (57) and satisfactory extraction recoveries (91-102%) using only 100 mg of magnetic adsorbent. Furthermore, fast separation time of about 15 min avoids many time-consuming column-passing procedures of conventional SPE. The main factors affecting extraction efficiency including pH value, desorption conditions, extraction/desorption time, sample volume, and adsorbent amount were evaluated and optimized. Under the optimal conditions, a wide linear range of 0.04-8 ng mL(-1) with a low detection limit of 0.015 ng mL(-1) was obtained. The developed method was applied for extraction and preconcentration of AFM1 in three commercially available milk samples and the results were compared with the official AOAC method.

Distribution of BDE-99 and effects on metamorphosis of BDE-99 and -47 after oral exposure in Xenopus tropicalis

The high concentrations of polybrominated diphenylethers (PBDEs) in the environment have raised the need for generating more information about the impact of these substances on animals. To study the distribution of (14)C-labelled 2,2',4,4',5-pentabromodiphenyl ether ((14)C-BDE-99) in Xenopus tropicalis (West African clawed frog) (14)C-BDE-99 was administered by dietary exposure to tadpoles at stage 54 or to juvenile frogs at stage 66. Whole-body autoradiography and liquid scintillation counting were used to examine the distribution of the substance at different survival times. Further, X. tropicalis tadpoles were dietarily exposed to the PBDE congeners BDE-47 and BDE-99 to study the effects on metamorphosis process. Measurements like body weight, body length, hind limb length and developmental stage as well as histological measurements on thyroid glands were performed after 14 days of exposure. Autoradiograms revealed high concentrations and long term retention of (14)C-BDE-99 in adipose tissue and melanin in frogs exposed both as tadpoles and juveniles. Further, a difference in uptake was recorded between the exposures at stages 54 and 66, implying that the juvenile frogs have higher uptake and more prolonged retention of the chemical than the tadpoles. Hind limb length was reduced in tadpoles dietarily exposed to 1mg/g feed of both BDE congeners. This was associated with reduced body weight and body length for BDE-47, suggesting general toxicity. Tadpoles exposed to BDE-99 also showed lower developmental stage but no effects on body weight or body length, suggesting possible thyroid hormone disruption. Higher concentrations of both congeners caused increased mortality. Thus, it can be concluded that in the present study, BDE-99 was retained for a longer period in the juvenile frogs than in metamorphosing tadpoles and that BDE-99 had an impact on X. tropicalis metamorphosis that might be of thyroid disrupting origin.

Melanin counter act puromycin-induced inhibition of collagen and DNA biosynthesis in human skin fibroblasts

Puromycin is an experimental anti-tumor antibiotic acting through inhibition of protein synthesis. Because of its untoward side effects (as inner ear and renal lesions) the antibiotic was not approved for clinical trials. The mechanism underlying the organ specificity of the side effect is not understood. In view of the fact that a number of drugs form with melanin complexes that affect their pharmacological activity, we determined whether puromycin interacts with melanin and how this process affects biosynthesis of collagen in cultured human skin fibroblasts. Our results indicate that puromycin forms complexes with melanin. The amount of puromycin bound to melanin increases with increase of initial drug concentration. The Scatchard plot analysis of the drug binding to melanin has shown that at least two classes of independent binding sites are implicated in the puromycin-melanin complex formation: one class of strong binding sites with the association constant K1 = 1.84 x 10(6) M(-1), and the second class of weak binding sites with the association constant K2 = 5.26 x 10(3) M(-1). The number of total binding sites were n1 = 0.1260 and n2 = 0.2861 mumol puromycin per 1 mg melanin. We found that puromycin induced inhibition of collagen and DNA biosynthesis (IC50 approximately 2 microM). Melanin at 100 microg/ml produced about 20% inhibition of DNA synthesis, but it had no effect on collagen biosynthesis in cultured fibroblasts. However, the addition of melanin (100 microg/ml) to puromycin - treated cells (2 microM) abolished the inhibitory action of puromycin on collagen and DNA biosynthesis. We have suggested that IGF-I receptor expression, involved in collagen metabolism, may be one of the targets for puromycin - induced inhibition of collagen biosynthesis. It was found that melanin abolished puromycin induced decrease in the expression of IGF-I receptor as well MAP kinases expression: ERK1 and ERK2 as shown by Western immunoblot analysis. These data suggest that tissue specific pharmacological activity of puromycin may depend on the melanin abundance in tissues.

Melanin potentiates gentamicin-induced inhibition of collagen biosynthesis in human skin fibroblasts

One of the recognized side effects of gentamicin is ototoxicity. The mechanism underlying the organ specificity of this side effect of gentamicin has not been fully established. In view of the fact that a number of pharmacologic agents are known to form complexes with melanin and melanin is an abundant constituent of the inner ear tissues, we determined whether gentamicin interacts with melanin and how this process affects the biosynthesis of collagen in cultured human skin fibroblasts. Our results indicate that gentamicin forms complexes with melanin. The amount of gentamicin bound to melanin increases with increasing of initial drug concentration. The Scatchard plot analysis of drug binding to melanin showed that at least two classes of independent binding sites are implicated in gentamicin-melanin complex formation: one class with an association constant K(1) approximately 4 x 10(3) M(-1), and the second class with an association constant K(2) approximately 3 x 10(2) M(-1). The number of total binding sites (n(1)+n(2)) was calculated as about 1.36 micromol gentamicin per 1 mg melanin. We have suggested that prolidase, an enzyme involved in collagen metabolism, may be one of the targets for gentamicin-induced inhibition of collagen biosynthesis. We found that gentamicin-induced inhibition of prolidase activity (IC(50) approximately 100 microM) and collagen biosynthesis (IC(50) approximately 100 microM). At this concentration of gentamicin, DNA biosynthesis in human skin fibroblasts was inhibited only by about 30%. Melanin at 100 microg/ml produced about 25% inhibition of DNA synthesis and about 30% inhibition of prolidase activity, but it had no effect on collagen biosynthesis in cultured fibroblasts. However, the addition of melanin (100 microg/ml) to gentamicin-treated cells (100 microM) augmented the inhibitory action of gentamicin on collagen and DNA biosynthesis and partially reversed its inhibitory effect on prolidase activity. A melanin-induced augmentation of the inhibitory effects of gentamicin on collagen and DNA biosynthesis may explain the mechanism for the organ specificity of gentamicin-induced hearing loss in patients administered this drug.

Melanin potentiates daunorubicin-induced inhibition of collagen biosynthesis in human skin fibroblasts

One of the recognized side effects of antineoplastic anthracyclines is poor wound healing, resulting from an impairment of collagen biosynthesis. The most affected tissue is skin. The mechanism underlying the tissue specificity of the side effects of anthracyclines has not been established. In view of the fact that a number of pharmacologic agents are known to form complexes with melanin and melanins are abundant constituents of the skin, we determined whether daunorubicin interacts with melanin and how this process affects collagen biosynthesis in cultured human skin fibroblasts. Results indicated that daunorubicin forms complexes with melanin. Scatchard analysis showed that the binding of daunorubicin to melanin was heterogeneous, suggesting the presence of two classes of independent binding sites with K1 = 1.83 x 10(5) M(-1) and K2 = 5.52 x 10(3) M(-1). The number of strong binding sites was calculated as n1 = 0.158 micromol/mg of melanin and the number of weak binding sites as n2 = 0.255 micromol/mg of melanin. We have suggested that prolidase, an enzyme involved in collagen metabolism, may be one of the targets for anthracycline-induced inhibition of collagen synthesis. We found that daunorubicin induced inhibition of prolidase activity (IC50 = 10 microM), collagen biosynthesis (IC50 = 70 microM) and DNA biosynthesis (IC50= 10 microM) in human skin fibroblasts. Melanin (100 microg/ml) by itself produced about 25% inhibition of DNA synthesis and prolidase activity but it had no effect on collagen biosynthesis in cultured fibroblasts. However, the addition of melanin (100 microg/ml) to daunorubicin-treated cells (at IC50 concentration) augmented the inhibitory action of daunorubicin on collagen and DNA biosynthesis without having any effect on prolidase activity. The same effect was achieved when the cells were treated with daunorubicin at one-fourth of the IC50 given at 0, 6, 12 and 18 h during a 24-h incubation. The data suggest that the melanin-induced augmentation of the inhibitory effects of daunorubicin on collagen and DNA biosynthesis may result from: (i) accumulation of the drug in the extracellular matrix, (ii) gradual dissociation of the complex, and (iii) constant action of the released drug on cell metabolism. The phenomenon may explain the potential mechanism for the organ specificity of daunorubicin-induced poor wound healing in patients administered this drug.

Pheomelanin as a binding site for drugs and chemicals

Certain drugs and chemicals, such as chloroquine, chlorpromazine, and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), are bound to melanin and retained in pigment cells for long periods. This specific retention in pigmented tissues can cause adverse effects in the skin, eye, inner ear, and pigmented nerve cells of the substantia nigra of the brain. To date, all studies have been focused on eu- and neuromelanin. In the present study, we show that chloroquine, chlorpromazine, chlomipramine, paraquat, acridine orange, and nickel, which are bound to eumelanin, also bind to synthetic pheomelanin, but the binding to pheomelanin is lower. The binding varied with the cysteine content and pH, and the results indicate that the binding is complex and includes ionic interactions. In addition, we have shown that these substances also bind to synthetic thiourea-containing melanin, but to quite a low extent. We also present a microautoradiographic study on the binding of 14C-chloroquine to natural pheomelanin in vivo in yellow mice C57BL (Ay/a). Black (C57/BL) and albino (NMRI) mice were used as controls. The autoradiography demonstrated a pronounced uptake of chloroquine in the hair follicles and the dermal melanocytes in the ear of yellow mice, which was comparable to the corresponding accumulation of label in black mice. In the albino mouse, the uptake was lower and more homogeneously distributed in the skin. These results suggest that the toxicological risks of melanin-related adverse effects are applicable to persons with a high content of pheomelanin in the skin and hair.

Evaluation of the metabolism and hepatotoxicity of styrene in F344 rats, B6C3F1 mice, and CD-1 mice following single and repeated inhalation exposures

Styrene is used for the manufacture of plastics and polymers. The metabolism and hepatotoxicity (mice only) of styrene was compared in male B6C3F1 mice, CD-1 mice, and F344 rats to evaluate biochemical mechanisms of toxicity. Rats and mice were exposed to 250 ppm styrene for 6 h/day for 1 to 5 days, and liver (mice only) and blood were collected following each day of exposure. Mortality and increased serum alanine aminotransferase (ALT) activity were observed in mice but not in rats. Hepatotoxicity in B6C3F1 mice was characterized by severe centrilobular congestion after one exposure followed by acute centrilobular necrosis. Hepatotoxicity was delayed by 1 day in CD-1 mice, and the increase in ALT and degree of necrosis was less than observed for B6C3F1 mice. Following exposure to unlabeled styrene for 0, 2, or 4 days, rats and mice were exposed to [7-14C]-styrene (60 microCi/mmol) for 6 h. Urine, feces, and expired air were collected for up to 48 h. Most styrene-derived radioactivity was excreted in urine. The time-course of urinary excretion indicates that rats and CD-1 mice eliminated radioactivity at a faster rate than B6C3F1 mice following a single 250 ppm exposure, consistent with a greater extent of liver injury for B6C3F1 mice. The elimination rate following 3 or 5 days of exposure was similar for rats and both mouse strains. Following three exposures, the total radioactivity eliminated in excreta was elevated over that measured for one exposure for both mouse strains. An increased excretion of metabolites on multiple exposure is consistent with the absence of ongoing acute necrosis following 4 to 5 daily exposures. These data indicate that an induction in styrene metabolism occurs after multiple exposures, resulting in an increased uptake and/or clearance for styrene.

Uptake of 7,12-dimethylbenz(a)anthracene and benzo(a)pyrene in melanin-containing tissues

It is widely accepted that UV exposure is the main etiological factor for malignant melanoma. Epidemiologic studies, however, have indicated that also chemical carcinogens may be a risk factor for the disease. Polycyclic aromatic hydrocarbons such as 7,12-dimethylbenz(a)anthracene and benzo(a)pyrene represent an important class of carcinogenic chemicals. It is known that 7,12-dimethylbenz(a)anthracene can induce melanotic tumours in various animal species, and human melanocytes in culture have been found to be capable of metabolizing benzo(a)pyrene to its proximate carcinogen benzo(a)pyrene-7,8-diol. In the present study the disposition of 14C- and 3H-7,12-dimethylbenz(a)anthracene and 14C-benzo(a)pyrene was studied in pigmented and albino mice and Syrian golden hamsters by whole-body autoradiography. The results showed pronounced retention of label in the melanin-containing structures of the eyes and the hair follicles in the pigmented animals. The labelling of the corresponding structures in the albino animals was low. Additional experiments showed that 7,12-dimethylbenz(a)anthracene and benzo(a)pyrene as well as some of their metabolites are bound to melanin in vitro. The specific localization of the polycyclic aromatic hydrocarbons in pigmented tissues due to melanin affinity, combined with bioactivating capacity of melanocytes, suggest that these substances may play a role in the induction of malignant melanoma.

Human oral mucosal melanocytes: a review

Despite the complex role of melanocytes in skin physiology, the function of oral mucosal melanocytes has attracted little research interest and remains largely unclear. This article reviews what is known about oral mucosal melanocytes and identifies areas of research that may shed further light on their role in oral biology.

Interaction between chemicals and melanin

Various drugs and other chemicals, such as organic amines, metals, polycyclic aromatic hydrocarbons, etc., are bound to melanin and retained in pigmented tissues for long periods. The physiological significance of the binding is not evident, but it has been suggested that the melanin protects the pigmented cells and adjacent tissues by adsorbing potentially harmful substances, which then are slowly released in nontoxic concentrations. Long-term exposure, on the other hand, may build up high levels of noxious chemicals, stored on the melanin, which ultimately may cause degeneration in the melanin-containing cells, and secondary lesions in surrounding tissues. In the eye, e.g., and in the inner ear, the pigmented cells are located close to the receptor cells, and melanin binding may be an important factor in the development of some ocular and inner ear lesions. In the brain, neuromelanin is present in nerve cells in the extrapyramidal system, and the melanin affinity of certain neurotoxic agents may be involved in the development of parkinsonism, and possibly tardive dyskinesia. In recent years, various carcinogenic compounds have been found to accumulate selectively in the pigment cells of experimental animals, and there are many indications of a connection between the melanin affinity of these agents and the induction of malignant melanoma.

Extrahepatic disposition of 3H-aflatoxin B1 in the rainbow trout (Oncorhynchus mykiss)

Whole-body autoradiography in rainbow trout (Oncorhynchus mykiss) after oral and intravenous administration of 3H-labelled aflatoxin B1 showed labelling of several extrahepatic tissues, such as the uveal melanin and the vitreous humour of the eyes, the trunk and head kidney, the olfactory rosettes and the pyloric caecae. Liquid chromatography of extracts of the vitreous humour showed that unmetabolized 3H-AFB1 was the main labelled material present at this site. Liquid chromatography of extracts of the uveal melanin showed presence of aflatoxicol and aflatoxin B1 in proportions of about 3:1. The binding to the pigment is probably due to a hydrophobic type of interaction with the melanin. Microautoradiography showed that melanin-containing cells in the trunk and head kidney and in the olfactory rosettes also accumulated high amounts of radioactivity. In the trunk kidney there was, in addition, a labelling of the second segment of the proximal tubules and of the distal tubules and the collecting ducts. Studies in vitro with microsomal and 12,000 x g supernatant preparations of the trunk kidney showed formation of DNA- and protein-bound metabolites from the aflatoxin B1. It is probable that the bioactivation of the aflatoxin B1 is confined to the cytoplasm of the cells, may be related to excretion and/or absorption processes. Microautoradiography of the olfactory rosettes, showed labelling of the sensory epithelium, but not the indifferent epithelium. A low formation of protein-bound aflatoxin B1-metabolites was found in incubations with microsomal preparations of this tissue. The same observation was made in incubations with microsomal preparations of the head kidney. In the pyloric caeca bound metabolites were observed in vivo at a level comparable to that found in the trunk kidney. Our results suggest that retention and metabolism in some extrahepatic tissues might be of importance as concerns the toxicologic potential of aflatoxin B1 in the rainbow trout.

The function of melanin or six blind people examine an elephant

The pigment melanin is found in all living kingdoms and in many different structures and forms. When its various functions are examined separately, its behaviors seem disparate and conflicting. It has a clear role in camouflage and sexual display. Other major roles are examined critically. It can act as a sun screen but is not a very effective one. It can also scavenge active chemical species, but this, too, is not done very effectively. It produces active radicals that can damage DNA. It binds to drugs in ways that are either beneficial or deleterious. Aside from camouflage, its other roles can be brought together by a unifying hypothesis as first proposed by Proctor and McGinness nearly 20 years ago. Melanin is envisaged as an energy transducer with the properties of an amorphous semiconductor. It can absorb many different types of energy and dissipate them in the form of heat. However, if the energy input is too great, the output can be expressed in the form of activated chemical species that can damage cellular macromolecules resulting in cell death, mutations and cancer. The protective aspect of melanin in dark skin is seen as resulting from its high concentration and its confinement to ellipsoidal and densely packed organelles that can effectively shield the nucleus. In light skin, its radical nature is seen as potentially participating in the carcinogenic process, particularly when overwhelmed by intense episodes of sunburn.

Tissue disposition and excretion of 14C-labelled aflatoxin B1 after oral administration in channel catfish

The pharmacokinetics, tissue distribution and excretion of 14C-labelled aflatoxin B1 (AFB1) were examined after oral administration (250 micrograms/kg body weight) in channel catfish (Ictalurus punctatus). Plasma concentrations of parent AFB1 were best described by a one-compartment pharmacokinetic model, in which peak plasma concentration (503 ppb) occurred at 4.1 hr after dosing. The absorption and elimination half-lives were 1.5 and 3.7 hr, respectively. AFB1 was highly bound (95%) to plasma proteins. Concentrations of 14C (in AFB1 equivalents) measured in the tissues were highest at 4 hr, ranging from 596 ppb in the plasma to 40 ppb in the muscle. AFB1 residues were rapidly depleted; at 24 hr the concentrations in the plasma and muscle were 32 and less than 5 ppb, respectively. Concentrations in the bile exceeded 2000 ppb (at 24 hr), whereas the highest concentration in the urine was 51 ppb (4-6-hr collection interval). Renal and biliary excretion accounted for less than 5% of the administered dose, indicating incomplete absorption. Pharmacokinetic modelling and tissue data demonstrate a very low potential for the accumulation of AFB1 and its metabolites in the edible flesh of channel catfish through the consumption of AFB1-contaminated feed.

Disposition of 3H-aflatoxin B1 in mice: formation and retention of tissue bound metabolites in nasal glands

Whole-body autoradiography with 3H-labelled aflatoxin B1 (3H-AFB1) in C57B1-mice showed a pronounced accumulation and retention of radioactivity in some nasal glands. At long survival intervals the labelling of the nasal glands was much higher than that of the liver. Experiments in vitro showed a capacity of the nasal glands to form tissue-bound 3H-AFB1-metabolites. Incubations in the presence of glutathione decreased the levels of tissue-bound 3H-AFB1-metabolites both in the liver and in the nasal glands, but the decrease was more pronounced in the former than in the latter tissue. The 3H-AFB1-metabolite-binding to the nasal glands in vitro was inhibited by the cytochrome P-450-inhibitor metyrapone and by CO- and N2-atmospheres indicating a cytochrome P-450-dependent bioactivation of the AFB1 in these glands. Cytochrome P-450 was shown to be present in the glands although at a much lower level than in the liver. The glands in the nose, which were shown to have this AFB1-metabolizing capacity, were the lateral nasal gland (Steno's gland) situated ventrally and laterally to the maxillary sinus and the large group of glands in the lateral nasal wall ventrally to the ostium of the maxillary sinus. Our results also indicated an AFB1-metabolizing capacity of the serous glands which are present in the anterior part of the nasal septum.