Enzymatic oxidation and reduction of retinal by mouse epidermis

Retinoids in cosmeceuticals

Retinoids are natural and synthetic vitamin A derivatives. They are lipophilic molecules and easily penetrate the epidermis. Their biologically active forms can modulate the expression of genes involved in cellular differentiation and proliferation. Retinoic acid (tretinoin), its 13-cis isomer isotretinoin, as well as various synthetic retinoids are used for therapeutic purposes, whereas retinaldehyde, retinol, and retinyl esters, because of their controlled conversion to retinoic acid or their direct receptor-independent biologic action, can be used as cosmeceuticals. These natural retinoic acid precursors are thus expected to be helpful in (i) renewing epidermal cells, (ii) acting as UV filters, (iii) preventing oxidative stress, (iv) controlling cutaneous bacterial flora, and (v) improving skin aging and photoaging. Retinol and retinyl esters are not irritant, whereas demonstrating only a modest clinical efficiency. On the other hand, retinaldehyde, which is fairly well tolerated, seems to be the most efficient cosmeceutical retinoid; it has significant efficiency toward oxidative stress, cutaneous bacterial flora, epidermis renewing, and photoaging.

Drug-Metabolizing Ability of Molybdenum Hydroxylases

Molybdenum hydroxylases, which include aldehyde oxidase and xanthine oxidoreductase, are involved in the metabolism of some medicines in humans. They exhibit oxidase activity towards various heterocyclic compounds and aldehydes. The liver cytosol of various mammals also exhibits a significant reductase activity toward nitro, sulfoxide, N-oxide and other moieties, catalyzed by aldehyde oxidase. There is considerable variability of aldehyde oxidase activity in liver cytosol of mammals: humans show the highest activity, rats and mice show low activity, and dogs have no detectable activity. On the other hand, xanthine oxidoreductase activity is present widely among species. Interindividual variation of aldehyde oxidase activity is present in humans. Drug-drug interactions associated with aldehyde oxidase and xanthine oxidoreductase are of potential clinical significance. Drug metabolizing ability of molybdenum hydroxylases and the variation of the activity are described in this review.

INVOLVEMENT OF MOLYBDENUM HYDROXYLASES IN REDUCTIVE METABOLISM OF NITRO POLYCYCLIC AROMATIC HYDROCARBONS IN MAMMALIAN SKIN

Molybdenum hydroxylases, aldehyde oxidase and xanthine oxidoreductase, were shown to be involved in the nitroreduction of 2-nitrofluorene (NF), 1-nitropyrene, and 4-nitrobiphenyl, environmental pollutants, in the skin of various mammalian species. NF was reduced to 2-aminofluorene by hamster skin cytosol in the presence of 2-hydroxypyrimidine, 4-hydroxypyrimidine, N(1)-methylnicotinamide, or benzaldehyde, but not hypoxanthine or xanthine. Inhibitors of aldehyde oxidase markedly inhibited these nitroreductase activities, but oxypurinol, an inhibitor of xanthine oxidoreductase, had little effect. In DEAE column chromatography of hamster skin cytosol, the major fraction exhibiting nitroreductase activity also showed aldehyde oxidase activity. 2-Hydroxypyrimidine-linked nitroreductase activities of skin cytosol from rabbits and guinea pigs were also inhibited by an inhibitor of aldehyde oxidase. In contrast, nitroreductase activities of skin cytosols of rats and mice were markedly inhibited by oxypurinol. When aldehyde oxidase activity was estimated in skin cytosol of various mammals using benzaldehyde oxidase activity as a marker, considerable variability of the activity was found. The highest activity was observed with hamsters, and the lowest activity with rats. On the other hand, the highest xanthine oxidoreductase activity was observed with rats, and the lowest activity with rabbits. These skin cytosols of various mammals also exhibited significant 2-hydroxypyrimidine-linked nitroreductase activities toward 1-nitropyrene and 4-nitrobiphenyl catalyzed by aldehyde oxidase and xanthine oxidoreductase. Thus, NF was mainly reduced by aldehyde oxidase and xanthine oxidoreductase in skins of animals. However, the contributions of these two molybdenum hydroxylases were considerably different among animal species.

Xanthine oxidase catalyzes the synthesis of retinoic acid

Milk xanthine oxidase (xanthine: oxygen oxidoreductase; XO; EC 1.1.3.22) was found to catalyze the conversion of retinaldehyde to retinoic acid. The ability of XO to synthesize all trans-retinoic acid efficiently was assessed by its turnover number of 31.56 min-1, determined at pH 7.0 with 1 nM XO and all trans-retinaldehyde varying between 0.05 to 2 microM. The determination of both retinoid and purine content in milk was also considered in order to correlate their concentrations with kinetic parameters of retinaldehyde oxidase activity. The velocity of the reaction was dependent on the isomeric form of the substrate, the all trans- and 9-cis-forms being the preferred substrates rather than 13-cis-retinaldehyde. The enzyme was able to oxidize retinaldehyde in the presence of oxygen with NAD or without NAD addition. In this latter condition the catalytic efficiency of the enzyme was higher. The synthesis of retinoic acid was inhibited 87% and 54% by 4 microM and 2 microM allopurinol respectively and inhibited 48% by 10 microM xanthine in enzyme assays performed at 2 microM all trans-retinaldehyde. The Ki value determined for xanthine as an inhibitor of retinaldehyde oxidase activity was 4 microM.

Topical delivery of retinoids counteracts the UVB-induced epidermal vitamin A depletion in hairless mouse

UVB irradiation depletes all-trans-retinol (ROL) and all-trans-retinyl esters (RE) from the hairless mouse epidermis. Prevention of this may be of relevance in counter-acting the long-term side effects of UVB exposure. We studied the effects of a topical treatment with natural retinoids before and after UVB exposure on three parameters involved in vitamin A metabolism: the amount of epidermal ROL and RE, the level of functional cellular retinol-binding protein I (CRBP-I), which is likely to protect ROL from UVB, as well as the cytosolic and microsomal enzyme activities which generate ROL and RE, i.e. all-trans-retinaldehyde (RAL) reductase, acylCoA:retinol acyltransferase (ARAT) and retinyl-ester hydrolase (REH). Topical pretreatment with retinoids promoted a dramatic increase of epidermal ROL, RE and CRBP-I levels, a transient increase of RAL reductase and ARAT activities as well as a decreased activity of REH, indicating a direction of epidermal vitamin A metabolism toward storage. In untreated mice UVB irradiation induced a depletion of epidermal ROL and RE in 10 min and a 50% decrease of CRBP-I after 24 h. In mice treated with topical retinoids, and then exposed to UVB, epidermal RE levels were higher than in vehicle-treated, nonirradiated mice. In contrast, ROL was as much depleted after UVB in pretreated as in untreated animals in spite of an induction of CRBP-I, indicating that CRBP-I does not actually protect ROL from UVB-induced depletion in this model. However, the reconstitution of both epidermal ROL and RE, after their depletion induced by UVB, was accelerated by previous topical treatment with RAL. Our results indicate that topical delivery of retinoids partly counteracts UVB-induced vitamin A depletion and promotes recovery.

Purification and characterization of a novel cytosolic NADP(H)-dependent retinol oxidoreductase from rabbit liver

Rabbit liver cytosol exhibits very high retinol dehydrogenase activity. At least two retinol dehydrogenases were demonstrated to exist in rabbit liver cytosol, and the major one, a cytosolic NADP(H)-dependent retinol dehydrogenase (systematic name: retinol oxidoreductase) was purified about 1795-fold to electrophoretic and column chromatographic homogeneity by a procedure involving column chromatography on AF-Red Toyopearl twice and then hydroxyapatite. Its molecular mass was estimated to be 34 kDa by SDS-PAGE, and 144 kDa by HPLC gel filtration, suggesting that it is a homo-tetramer. The enzyme uses free retinol and retinal, and their complexes with CRBP as substrates in vitro. The optimum pH values for retinol oxidation of free retinol and CRBP-retinol were 8.8-9.2 and 8.0-9.0, respectively, and those for retinal reduction of free retinal and retinal-CRBP were the same, 7.0-7.6. Km for free retinol and Vmax for retinal formation were 2.8 microM and 2893 nmol/min per mg protein at 37 degrees C (pH 9.0) and the corresponding values with retinol-CRBP as a substrate were 2.5 microM and 2428 nmol/min per mg protein at 37 degrees C (pH 8.6); Km for free retinal and Vmax for retinol formation were 6.5 microM and 4108 nmol/min per mg protein, and the corresponding values with retinal-CRBP as a substrate were 5.1 microM and 3067 nmol/min per mg protein at 37 degrees C, pH 7.4. NAD(H) was not effective as a cofactor. 4-Methylpyrazole was a weak inhibitor (IC50 = 28 mM) of the enzyme, and ethanol was neither a substrate nor an inhibitor of the enzyme. This enzyme exhibits relatively broad aldehyde reductase activity and some ketone reductase activity, the activity for aromatic substitutive aldehydes being especially high and effective. Whereas, except in the case of retinol, oxidative activity toward the corresponding alcohols was not detected. This novel cytosolic enzyme may play an important role in vivo in maintaining the homeostasis of retinal, the substrate of retinoic acid synthesis, at least in rabbit liver, since a high concentration of retinol in liver and the lower Km of the enzyme for retinol force the oxidative reaction, while higher activity of retinal reductase at physiological pH forces the reductive reaction.

Characterization of esterase and alcohol dehydrogenase activity in skin. Metabolism of retinyl palmitate to retinol (vitamin A) during percutaneous absorption

Retinyl palmitate, a widely used ingredient in cosmetic products, is promoted for its beneficial effects on the appearance of skin. Previous studies suggest that enzymes are available in skin to metabolize this ingredient during skin absorption. Esterase activity hydrolyzes retinyl palmitate to retinol (vitamin A), which is oxidized in many tissues to retinoic acid primarily by alcohol dehydrogenase. The activities of esterase and alcohol dehydrogenase were characterized in hairless guinea pig skin by using flow-through diffusion cells and radiolabeled model compounds (methyl salicylate and benzyl alcohol) previously shown to be metabolized by these enzymes. Methyl salicylate was hydrolyzed by esterase to a greater extent in viable skin than in nonviable skin. Glycine conjugation of salicylic acid and benzoic acid occurred only in viable skin. The metabolism of methyl salicylate and benzyl alcohol occurred to a greater extent in male guinea pig skin than in female guinea pig skin. The percutaneous absorption of both radiolabeled compounds was similar in viable and nonviable skin. About 30 and 18% of topically applied retinyl palmitate were absorbed from an acetone vehicle by hairless guinea pig skin and human skin, respectively. Less than 1% of the applied dose of this lipophilic compound diffused from skin into the receptor fluid. Retinol was the only detectable metabolite of retinyl palmitate in both hairless guinea pig and human skin. In human skin, 44% of the absorbed retinyl palmitate was hydrolyzed to retinol. The use of retinyl palmitate in cosmetic formulations may result in significant delivery of retinol into the skin.

Identification of mouse liver aldehyde dehydrogenases that catalyze the oxidation of retinaldehyde to retinoic acid

NAD(P)-linked aldehyde dehydrogenases catalyze the oxidation of a wide variety of aldehydes. Thirteen of these enzymes have been identified in mouse tissues; eleven are found in the liver. Some are substrate-nonspecific; others are relatively substrate-specific. The present investigation sought to determine which of these enzymes are operative in catalyzing the oxidation of retinaldehyde to retinoic acid, a metabolite of vitamin A that promotes the differentiation of epithelial and other cells. Spectrophotometric and HPLC assays were used for this purpose. Enzyme-catalyzed oxidation of retinaldehyde (25 microM) was restricted to the cytosol (105,000 g supernatant fraction) and occurred at a rate of 211 nmol/min/g liver; oxidation of acetaldehyde (4 mM) by this fraction proceeds about ten times faster. At least 90% of this activity was NAD dependent. Of the approximately 10% that was apparently NAD independent, two-thirds was inhibited by 1 mM pyridoxal, a known inhibitor of aldehyde oxidase. Of the six cytosolic aldehyde dehydrogenases, only two, viz. AHD-2 and AHD-7, catalyzed the oxidation of retinaldehyde to retinoic acid. An additional NAD-dependent enzyme, viz. xanthine oxidase (dehydrogenase form), also catalyzed the reaction. Catalysis by AHD-2 accounted for more than 90% of the total NAD-dependent activity. Km values were 0.7, 0.6 and 0.9 microM, respectively, for the AHD-2-, AHD-7- and xanthine oxidase (dehydrogenase form)-catalyzed reaction. AHD-4, an aldehyde dehydrogenase found in the cytosol of mouse stomach epithelium and cornea, did not catalyze the reaction.

Increased hepatic retinal dehydrogenase activity after phenobarbital and ethanol administration

Cytosolic retinal dehydrogenase (EC 1.2.1.36, retinal: NAD+ oxidoreductase) activity was measured by assessing the conversion of retinal to retinoic acid by HPLC. In vitro, acetaldehyde, chloral hydrate and disulfiram were found to be inhibitors, whereas 95% of the activity remained in the presence of cyanide or in the absence of oxygen. In rats, retinal dehydrogenase activity prevailed over that of retinal oxidase. By contrast, in deermice, 80% of retinal oxidation was due to the oxidase rather than the retinal dehydrogenase activity in a normal strain (ADH+) as well as in one lacking alcohol dehydrogenase (ADH-). In ADH- deermice, retinal oxidase activity was greater than in ADH+ animals. In vivo, in the rat, chronic ethanol administration resulted in a significant increase of the dehydrogenase activity in the liver, but not in other tissues. After phenobarbital administration, hepatic retinal dehydrogenase activity was increased 8-fold, but no extrahepatic induction was observed. Conversely, feeding rats with a diet devoid of the precursor for the substrate (retinal) by replacing retinyl acetate with an equivalent amount of retinoic acid resulted in decreased retinal dehydrogenase activity. Under conditions in which retinal dehydrogenase activity is rate-limiting for the metabolism of retinal to retinoic acid, its induction after phenobarbital or ethanol administration may contribute to hepatic vitamin A depletion.

Properties of retinal-oxidizing enzyme activity in rat kidney

An enzyme activity which converts retinal to retinoic acid was found in the cytosol of rat kidney. The oxidation of retinal was pH-, temperature-, time- and protein-dependent. Under the assay conditions employed, the oxidase activity had an apparent Km of 125 microM toward all-trans retinal. n-Propylgallate, butylated hydroxytoluene and quinacrine inhibited the reaction. The inhibition caused by quinacrine can be partly reversed by FAD. p-Hydroxymercuribenzoate, a sulfhydryl cross-linking agent, was a potent inhibitor. 4'-(9-Acridinylamino)methanesulfon-anisidide, an inhibitor of aldehyde oxidase, inhibited the reaction by 77% at a concentration of 3 mM. All-trans retinal reversed the inhibition caused by acetaldehyde and 2-aminobenzaldehyde. Retinol inhibited the reaction, but retinoic acid did not. The specific activity of the enzyme was increased by vitamin A deficiency. These data indicate that retinal-oxidizing enzyme activity found in the kidney is a sulfhydryl flavoprotein and its activity is dependent on the vitamin A levels of the tissues.

The biosynthesis of retinoic acid from retinol by rat tissues in vitro

This report shows that a spectrum of vitamin A-dependent tissues can produce retinoic acid by synthesis in situ, indicates that cellular retinol and retinoic acid binding proteins are not obligatory to retinoic acid synthesis, and provides initial characterization of retinoic acid synthesis by rat tissues. Retinoic acid synthesis from retinol was detected in homogenates of rat testes, liver, lung, kidney, and small intestinal mucosa, but not spleen. Zinc did not stimulate the conversion of retinol into retinoic acid by liver homogenates. Retinoic acid synthesis was localized in cytosol of liver and kidney, where its rate of synthesis from retinol was fourfold (liver) and sevenfold (kidney) slower than from retinal. The synthesis of retinoic acid from retinol required NAD and was not supported by NADP. NADH (0.5 mM) reduced retinoic acid synthesis from retinol, supported by NAD (2 mM), by 50-70%, but was fivefold less potent in reducing retinoic acid synthesis from retinal. Dithiothreitol enhanced the conversion of retinol, but not retinal, into retinoic acid. EDTA inhibited the conversion of retinol into retinoic acid slightly (13%, liver; 29%, kidney). A high ethanol concentration (100 mM), relative to retinoid substrate (10 microM), inhibited retinoic acid synthesis from retinol (liver, 54%; kidney, 30%) and from retinal (30%, liver; 9%, kidney). 4'-(9-Acridinylamino)methansulfon-m-anisidine, an inhibitor of aldehyde oxidase, and disulfiram, a sulfhydryl-group crosslinking agent, were potent inhibitors of retinoic acid synthesis at 10 microM or less, and seemed equipotent in liver and kidney. 4-Methylpyrazole, an inhibitor of ethanol metabolism, also inhibited retinoic acid synthesis from retinol, but was less potent than the former two inhibitors, and affected liver to a greater extent than kidney, particularly with retinal as substrate.

The formation of all-trans-retinoic acid from all-trans-retinol in hairless mouse skin

All-trans-retinoic acid formation from topically applied retinol has been demonstrated in the skin of skh/hr1 (hairless) mice. The all-trans-retinoic acid was identified on the basis of its chromatographic properties on HPLC at various pH values, its photoisomerization to reaction products identical to those formed from authentic all-trans-retinoic acid, and its co-chromatography with methyl retinoate after methylation with diazomethane. Topically applied retinol is about 2-fold less potent at inducing epidermal hyperplasia and 7-fold less potent at inhibiting the induction of epidermal ornithine decarboxylase by phorbol esters than all-trans-retinoic acid in this strain of mice. To elucidate the possible role all-trans-retinoic acid formation from retinol may have in these pharmacological activities, the epidermal and dermal all-trans-retinoic acid levels were compared in mice treated topically with retinol or [11-3H]-all-trans-retinoic acid. The levels of all-trans-retinoic acid found after retinol treatment were several orders of magnitude lower than those found after [11-3H]-all-trans-retinoic acid treatment, and they were insufficient to account for the difference in potencies between all-trans-retinoic acid and retinol. Retinol was eliminated from the epidermis at a rate similar to that of all-trans-retinoic acid after topical administration, but the initial tissue levels achieved were lower. These results suggest that the lower potencies of retinol may simply reflect lower tissue uptake.

Vitamin A uptake by human skin in vitro

Results are presented establishing that epidermis accumulates vitamin A from serum retinol-binding protein (RBP). Strips of human breast skin (0.2-0.3-mm thick) were incubated in a serum-free medium. From the rate of glucose oxidation, the tissue was viable for at least 48 h at 32 degrees C in 5% CO2 air. [3H]-Retinol-RBP (10(-6) M) was added to the medium for 1-24 h, after which epidermis and dermis were split and separately extracted with hexane after saponification. [3H]-Retinol was isolated by high performance liquid chromatography (HPLC). Epidermis had 6-7 times higher affinity for [3H]-retinol than dermis. The uptake could be saturated by substrate and was inhibited with unlabelled retinol-RBP but not with serum albumin. Furthermore, although the uptake was temperature-dependent, it seemed independent of cellular energy production. The epidermal accumulation of [3H]-retinol was reduced by the filtering action of dermis. On the basis of these observations, an in vitro model for the delivery of vitamin A to human skin has been proposed.