Defective calcium uptake in keratinocyte cell cultures from vitiliginous skin

45Ca2+ has been used to measure kinetics for the uptake, efflux, and "steady state" of this regulatory cation in keratinocytes grown from the involved and uninvolved skin of one donor (JM) with vitiligo. Cells grown from uninvolved skin yielded a very rapid uptake and efflux of this isotope before reaching "steady state". A similar profile has been found for keratinocytes from normal healthy adult controls. However, cells established from vitiliginous skin showed a slow uptake of 45Ca2+ before reaching the same "steady state" as the controls. 45Ca2+ efflux has not been observed in vitiliginous keratinocytes. Furthermore, vitiliginous keratinocytes yielded a higher concentration of extracellular bound 45Ca2+ compared with keratinocytes from uninvolved skin. Since Ca2+ has been found to be an allosteric inhibitor of membrane-associated thioredoxin reductase, this defect in Ca2+ transport may explain the proposed breakdown in free radical defense in vitiligo. These findings may also shed more light on the etiology of this disorder.

Thioredoxin reductase activity in Hermansky-Pudlak syndrome: a method for identification of putative heterozygotes

Recent studies indicate that membrane-associated thioredoxin reductase (TR) is a possible regulator of melanin biosynthesis via the inhibition of tyrosinase by reduced thioredoxin. In normal individuals, the levels of TR activity in skin correlate linearly to the Fitzpatrick classification of skin type, being lowest in type I skin and highest in skin type VI. In this study, TR was measured in 3-mm skin biopsies in Hermansky-Pudlak syndrome (HPS) patients and their relatives. Forty-five individuals from seven Puerto Rican kindreds were tested, including 12 homozygotes, nine obligate heterozygotes, and 24 unclassified individuals. In addition, seven separate nonkindred HPS patients were tested. With one exception, TR activity was markedly decreased in 18 homozygotes. TR activity was decreased in eight obligate heterozygotes and in 12 unclassified kindred members, whereas 10 subjects had normal TR activity when compared to the expected activity of their skin type. Four individuals were excluded from the analysis because of inadequate controls for their age group or immunosuppressive treatment for kidney transplant. The results indicate that decreased TR activity assayed in 3-mm skin punch biopsies is a useful method for detecting carriers of the HPS gene.

Anthralin inhibits elevated levels of thioredoxin reductase in psoriasis. A new mode of action for this drug

Membrane-associated thioredoxin reductase (TR) activity has been measured in 3-mm punch biopsy specimens from psoriatic and uninvolved skin in eight patients with chronic plaque-type psoriasis vulgaris. The mean specific activity for this free radical reducing enzyme in psoriatic vs uninvolved skin was 28.5 +/- 8.0 U vs 16.8 +/- 4.25 U. Because TR contains two reactive thiolate groups at its active site, this enzyme reacts with anthralin to form a covalent anthralin-TR complex causing irreversible enzyme deactivation. This mode of action for anthralin was confirmed by using pure TR from Escherichia coli. Keratinocyte cell cultures, grown from normal and psoriatic skin of one donor, revealed 24% and 42% inhibition of cell surface TR activity, respectively, in the presence of 2 X 10(-5)M anthralin. Time-dependent topical application of anthralin on guinea pig skin gave 70% inhibition of TR with concentrations of 0.25% to 1.0% after 24 hours in open and occlusive applications of the drug. Short contact with 1% anthralin showed 70% inhibition after 120 minutes.

Azelaic acid as a competitive inhibitor of thioredoxin reductase in human melanoma cells

Azelaic acid has been shown to inhibit thioredoxin reductase (TR) at the surface of guinea pig and human skin, on cultures of human keratinocytes, melanocytes, melanoma cells, murine melanoma cells (Cloudman S91), and on purified enzymes from Escherichia coli, rat liver, and human melanoma. Human melanoma cells are more resistant to inhibition by azelaic acid than murine melanoma or human melanocytes. Kinetic studies with pure TRs indicate that azelaic acid is a reversible competitive inhibitor. Fluorescence spectroscopy has been used to show that azelaic acid does not interfere with electron transfer from NADPH to FAD on TR. However, azelaic acid does inhibit electron transfer from the dithiolate active site of this enzyme. Inhibition by azelaic acid is pH-dependent, requiring the dissociation of both carboxylate groups, and also the dissociation of the active site dithiol groups. Binding studies with [14C]azelaic acid at different pHs, indicate that inhibition is first due to the formation of a thioester on the active thiolate groups followed by transacylation of a basic amino acid residue in the active site. A comparative study of TR inhibition by C6, C9, C10 and C12 saturated dicarboxylic acids was also determined on guinea pig skin in vivo. These homologous dicarboxylic acids gave greater inhibition with increasing size (i.e. mol wt.).

Thioredoxin reductase. Role in free radical reduction in different hypopigmentation disorders

Membrane-associated thioredoxin reductase (TR) has been discovered to reduce free radicals at the surface of the skin. An accurate bioassay for this enzyme has been developed by using a spin-labeled quaternary ammonium salt as a free radical substrate. Enzyme activity has been correlated with the surface area, and units of specific activity have been determined as the sequential decrease in nitroxide radical reduction per 3-mm punch biopsy per ten minutes. The TR activity in a random population of 30 healthy volunteers with different skin types (Fitzpatrick classification I through VI) could be correlated to the skin type. Ten patients with untreated vitiligo, two with piebaldism, three with albinism, and two with postinflammatory leukoderma were examined and the findings were compared with the expected rates for the individuals' skin types. The results from this survey on the human population support our previous molecular experiments on the control of melanin biosynthesis by TR in the epidermis.

Induction of contact dermatitis in guinea pigs by quaternary ammonium compounds: the mechanism of antigen formation

Eight quaternary ammonium compounds were tested for their ability to induce contact dermatitis in guinea pigs by using a modified Freund's complete adjuvant test together with the guinea pig maximization test. Only two quaternary ammonium salts of the eight tested could be designated as strong allergens. These two active substances were shown to be capable of stable association with membrane lipids in forming immunogenic complexes. This surface complexation phenomenon was confirmed by using a spin-labeled quaternary ammonium salt which competed for binding sites at the surface of epidermal cells in vivo. Electron spin resonance was used to demonstrate that stable "ion-pairs" are formed between binding sites and the two allergenic preservatives. Furthermore, information was obtained on the kinetics of immunogenic complex formation as well as on the position and orientation of the quaternary ammonium ion at the cell surface.

Free radical reduction by thioredoxin reductase at the surface of normal and vitiliginous human keratinocytes

Cell cultures of human keratinocytes contain membrane-associated thioredoxin reductase that is extremely active in reducing radicals on the outer plasma membrane. This enzyme activity was confirmed by its purification from cultures of stratified human keratinocytes by affinity column chromatography. The enzyme was assayed both in vivo and in vitro using a spin-labeled quaternary ammonium compound as the substrate, under saturating conditions in free radical substrate. Specific activities were determined by monitoring the sequential decrease in the amplitude of the electron spin resonance signal per unit of cell protein. The following properties were found: Cultures of adult stratified cells have approximately twice the thioredoxin reductase activity of neonatal cells. The enzyme is inhibited by thioprotein inhibitors (i.e., parachloromecuribenzoate and dinitrochlorobenzene). The activity is regulated by calcium concentrations of the cell culture medium. Stratified keratinocytes are half as active in medium containing 2 mM Ca++ compared with 0.1 mM Ca++ concentration. Product inhibition of the enzyme occurs with oxidized coenzyme NADP+ (i.e., 87% inhibition of enzyme activity over 30 min). The enzyme is heat stable at temperatures of 70 degrees C for 10 min. It is inactivated at 75 degrees C. A comparative study of thioredoxin reductase activity on stratified differentiated and undifferentiated rapidly growing cells was performed. Also, enzyme activity was quantitated for cultured keratinocytes isolated from vitiliginous and normal skin of the same donor. The results of this study, and the connection between this enzyme activity and UV-generated free radicals are reconciled in terms of the mechanism of action and metabolic activity of thioredoxin reductase.

The role of calcium in the regulation of free radical reduction by thioredoxin reductase at the surface of the skin

Membrane associated thioredoxin reductase has been previously shown to reduce free radicals on the outer plasma membranes of human keratinocytes and melanocytes to provide a possible first line of defense against free radical damage at the surface of the skin. Preliminary experiments with cell cultures of human keratinocytes and melanocytes grown in serum-free medium showed that the enzyme activity depends on extracellular calcium concentration in the medium. Thioredoxin reductase activity at the surface of the skin, at the surface of human keratinocytes and melanocytes, and purified thioredoxin reductase from E. coli and adult human keratinocytes all exhibited calcium-dependent allosteric control. Since thioredoxin reductase contains two extremely reactive thiolate groups at the active site with pK values close to neutrality, both of these anions can form covalent complexes with N-ethylmaleimide by nucleophilic attack on the double bond. In our experiments we used spin-labeled maleimide [4-maleimido-tempo] to examine the local environment in the active site of thioredoxin reductase in the presence and absence of calcium. Both spin-labeled thioethers are distinguishable by EPR spectroscopy, with one site being significantly more immobilized than the other. Hence, it has been possible to observe direct evidence for active site closure by calcium. These results suggest that extracellular calcium may play an important role in regulation of thioredoxin reductase activity for the defense mechanism against UV-mediated free radical damage at the surface of human skin.

The role of thioredoxin reductase in the reduction of free radicals at the surface of the epidermis

A study of guinea pig and human skin in vivo has revealed that keratinocytes contain a thioenzyme which reduces radicals. This enzyme has been purified by affinity column chromatography and identified as thioredoxin reductase. In vivo and in vitro bioassays were performed by using a spin-labelled surfactant as the radical substrate, because it can diffuse through the stratum corneum and react by surface complexation with the epidermis and also on the outer plasma membrane of keratinocytes from cell cultures. Thioredoxin, the native substrate for thioredoxin reductase effectively competes for electrons with radical substrates. Nicotinamide adenine dinucleotide phosphate (NADPH) is the electron donating coenzyme in both the reduction of radicals and thioredoxin. Reduced thioredoxin has been shown to be an inhibitor of tyrosinase, whereas oxidized thioredoxin has no effect on this enzyme activity. Taken together these results indicate that the thioredoxin/thioredoxin reductase system plays an important role in preventing cell damage from UV-generated free radicals on the skin.

The allergenicity of complex cations

A homologous series of eight quaternary ammonium salts (quats) were used as complex cations in a survey of contact hypersensitivity in guinea pigs. Two of the quats tested were found to be strong allergens which was due to stable association with membrane lipids at the surface of epidermal cells. This surface complexation reaction was studied in detail by using a spin-labelled quat of intermediate allergenicity. Electron spin resonance was used to show that stable "ion pairs" are formed between membrane receptor sites and the two strong allergens. Information was obtained on the specificity and kinetics of immunogenic complex formation as well as on the position and orientation of these haptens on epidermal receptor sites in vivo.