Preparation, properties and metabolism of 5,6-monoepoxyretinoic acid

Cytochrome P450s in the regulation of cellular retinoic acid metabolism

The active metabolite of vitamin A, retinoic acid (RA), is a powerful regulator of gene transcription. RA is also a therapeutic drug. The oxidative metabolism of RA by certain members of the cytochrome P450 (CYP) superfamily helps to maintain tissue RA concentrations within appropriate bounds. The CYP26 family--CYP26A1, CYP26B1, and CYP26C1--is distinguished by being both regulated by and active toward all-trans-RA (at-RA) while being expressed in different tissue-specific patterns. The CYP26A1 gene is regulated by multiple RA response elements. CYP26A1 is essential for embryonic development, whereas CYP26B1 is essential for postnatal survival as well as germ cell development. Enzyme kinetic studies have demonstrated that several CYP proteins are capable of metabolizing at-RA; however, it is likely that CYP26A1 plays a major role in RA clearance. Thus, pharmacological approaches to limiting the activity of CYP26 enzymes may extend the half-life of RA and could be useful clinically in the future.

The role of CYP26 enzymes in retinoic acid clearance

Retinoic acid (RA) is a critical signaling molecule that regulates gene transcription and the cell cycle. Understanding of RA signaling has increased dramatically over the past decades, but the connection between whole body RA homeostasis and gene regulation in individual cells is still unclear. It has been proposed that cytochrome P450 family 26 (CYP26) enzymes have a role in determining the cellular exposure to RA by inactivating RA in cells that do not need RA. The CYP26 enzymes have been shown to metabolize RA efficiently and they are also inducible by RA in selected systems. However, their expression patterns in different cell types and a mechanistic understanding of their function is still lacking. Based on preliminary kinetic data and protein expression levels, one may predict that if CYP26A1 is expressed in the liver at even very low levels, it will be the major RA hydroxylase in this tissue. As such, it is an attractive pharmacological target for drug development when one aims to increase circulating or cellular RA concentrations. To further the understanding of how CYP26 enzymes contribute to the regulation of RA homeostasis, structural information of the CYP26s, commercially available recombinant enzymes and good specific and sensitive antibodies are needed.

4-hydroxyretinoic acid, a novel substrate for human liver microsomal UDP-glucuronosyltransferase(s) and recombinant UGT2B7

It is suggested that formation of more polar metabolites of all-trans-retinoic acid (atRA) via oxidative pathways limits its biological activity. In this report, we investigated the biotransformation of oxidized products of atRA via glucuronidation. For this purpose, we synthesized 4-hydroxy-RA (4-OH-RA) in radioactive and nonradioactive form, 4-hydroxy-retinyl acetate (4-OH-RAc), and 5,6-epoxy-RA, all of which are major products of atRA oxidation. Glucuronidation of these retinoids by human liver microsomes and human recombinant UDP-glucuronosyltransferases (UGTs) was characterized and compared with the glucuronidation of atRA. The human liver microsomes glucuronidated 4-OH-RA and 4-OH-RAc with 6- and 3-fold higher activity than atRA, respectively. Analysis of the glucuronidation products showed that the hydroxyl-linked glucuronides of 4-OH-RA and 4-OH-RAc were the major products, as opposed to the formation of the carboxyl-linked glucuronide with atRA, 4-oxo-RA, and 5,6-epoxy-RA. We have also determined that human recombinant UGT2B7 can glucuronidate atRA, 4-OH-RA, and 4-OH-RAc with activities similar to those found in human liver microsomes. We therefore postulate that this human isoenzyme, which is expressed in human liver, kidney, and intestine, plays a key role in the biological fate of atRA. We also propose that atRA induces its own oxidative metabolism via a cytochrome P450 (CYP26) and is further biotransformed into glucuronides via UGT-mediated pathways.

Properties of retinoids. Structure, handling, and preparation

Retinoids are unstable compounds being readily oxidized and/or isomerized to altered compounds, especially in the presence of oxidants including air, light, and excessive heat. They are labile toward strong acids and solvents that have dissolved oxygen or peroxides. In this review, procedures for handling and storage of retinoids and biological samples containing them have been described. The physical and chemical properties of retinoids have been reported. Simplified procedures for derivatizations and purification, and methods for quantitation of retinoids have been presented.

Synthesis and metabolism of all-trans-[11-3H]retinyl beta-glucuronide in rats in vivo

All-trans-[11-3H]retinyl beta-glucuronide (all-trans-[11-3H]ROG) was synthesized from [3H]retinol by an improved synthetic procedure. After its intraperitoneal injection into rats, ROG is initially found as the predominant labelled component in the serum, but then is distributed to the liver, intestine, kidney and other organs of the body. Esters of vitamin A, which constituted the major metabolite of ROG, were detected in the liver as well as in other tissues. Of the labelled vitamin A esters derived from tritiated ROG in the liver and intestine, about 50% contained 5,6-epoxyretinol, which was characterized by its chromatographic behaviour, formation of an acetyl ester and lack of reactivity with diazomethane. Thus ROG, although converted to retinol in vivo, might also act physiologically in an intact form.

Inhibition by chlorogenic acid of haematin-catalysed retinoic acid 5,6-epoxidation

Chlorogenic acid (3-O-caffeoylquinic acid) inhibited haematin- and haemoglobin-catalysed retinoic acid 5,6-epoxidation. Some other phenol compounds (caffeic acid and 4-hydroxy-3-methoxybenzoic acid) also showed inhibitory effects on the haematin- and haemoglobin-catalysed epoxidation, but salicylic acid did not. Of the above compounds, caffeic acid and chlorogenic acid were potent inhibitors compared with the other two, suggesting that the o-hydroquinone moiety of chlorogenic acid and caffeic acid is essential to the inhibition of the epoxidation. Although caffeic acid inhibited retinoic acid 5,6-epoxidation requiring the consumption of O2, formation of retinoic acid radicals was not inhibited on the addition of caffeic acid to the incubation mixture. The above results suggest that caffeic acid does not inhibit the formation of retinoic acid radicals but does inhibit the step of conversion of retinoic acid radical into the 5,6-epoxide.

Detection of radical species in haematin-catalysed retinoic acid 5,6-epoxidation by using h.p.l.c.-e.p.r. spectrometry

E.p.r. signals were detected in an all-trans-retinoic acid/haematin incubation mixture by using an e.p.r. spin-trapping technique. The spin adducts are presumably attributable to some intermediates in haematin-catalysed retinoic acid 5,6-epoxidation, since addition of nitrosobenzene to the reaction mixture dose-dependently inhibited the epoxidation. Analysing the reaction mixture by h.p.l.c.-e.p.r. spectrometry resulted in the detection of three peaks (III-1, III-2, IV) ascribable to the radical species. Two (peaks III-1 and -2) of the three peaks, which appeared 10 min after the reaction had started, seem to be attributable to the radical species directly participating in the epoxidation. The radicals trapped by nitrosobenzene do not appear to be derived from active oxygen, since none of these peaks were detected in a similar h.p.l.c. analysis of O2- and OH.-generating systems. They are presumably derived from retinoic acid. This view is also supported by the following results: none of these peaks were detected in the h.p.l.c. elution profile of the reaction mixture when retinoic acid was absent; peaks III-1 and 2 were detected even under anaerobic conditions, and their peak heights were unchanged under aerobic conditions.

Quantitation of biological retinoids by high-pressure liquid chromatography: primary internal standardization using tritiated retinoids

A single method is described for quantitation of 14 retinoids found in biological material. The method consists of reversed-phase HPLC, internal standardization, and carrier extraction procedures with three synthetic retinoids. Primary standardization of HPLC uv detector is achieved using tritiated all-trans-retinoic acid, all-trans-retinol, all-trans-retinyl palmitate, and all-trans-retinyl acetate. Extraction methods are standardized by correlating the uv absorbance of retinoids at 340 nm with radioactivity of tritiated retinoids of known specific activity. Quantitation of 10 pg of tritiated or 5 ng of nonradioactive retinoid per 0.1 g sample in a polarity range from 4-oxo-retinoic acid to retinyl stearate can be achieved in a single, 50-min chromatographic run. A single HPLC pump, a C18 reversed-phase analytical column, a multistep three-solvent gradient, and inexpensive solvents based on methanol, water, and chloroform comprise this cost-effective chromatographic system. Our primary standardization method allows investigators employing different procedures to compare results between laboratories by standardizing the HPLC uv detector with commercially available tritiated retinoids. With this method we were able to quantitate nanomolar amounts of endogenous retinoic acids and retinyl esters, that "HPLC uv only" conditions usually would not detect in the circulation and liver of rats under physiological conditions.

Haemoglobin-catalysed retinoic acid 5,6-epoxidation

Examination of the subcellular distribution of retinoic acid 5,6-epoxidase activity in rat liver and human liver homogenates showed that there is a prominent peak of activity in a high-density fraction. A corresponding peak was also detected in rat blood and human blood. Retinoic acid 5,6-epoxidation was catalysed by human blood cells but not by human plasma, and purified human haemoglobin also catalysed the epoxidation of retinoic acid to 5,6-epoxyretinoic acid. These results suggest that retinoic acid 5,6-epoxidase activity in human liver and rat liver homogenates is partially due to the presence of residual blood cells, and particularly haemoglobin, in the homogenates. In the retinoic acid 5,6-epoxidation catalysed by human haemoglobin, molecular O2 was required and its reaction was stimulated by Triton X-100. Boiling of haemoglobin solution resulted in an 94% decrease in the activity. NADPH (1 mM) and NADH (1 mM) completely [2-mercaptoethanol (5 mM) almost completely] inhibited the 5,6-epoxidation catalysed by haemoglobin, but catalase, superoxide dismutase and mannitol showed no inhibitory effect. CN- ion (100 mM) inhibited the reaction, but N3- ion (100 mM) did not.

The metabolism of retinoic acid to 5,6-epoxyretinoic acid, retinoyl-beta-glucuronide, and other polar metabolites

A description of the enzyme that produces 5,6-epoxyretinoic acid from all-trans-retinoic acid has been presented. This enzyme system is found in highest concentrations in the kidney followed by intestine, liver and spleen. The enzyme requires molecular oxygen, magnesium ions, ATP, and NADPH. In the kidney, it is found in the mitochondrial and microsomal fractions and has a Michaelis constant of 3.2 X 10(-6) M and 3.7 X 10(-6) M for 13-cis and all-trans-retinoic acid, respectively. The resultant product, 5,6-epoxyretinoic acid, has minimal activity in supporting growth of vitamin A-deficient rats, its activity estimated to be 0.5% that of retinoic acid. An investigation of the biliary excretion products of tritiated retinoic acid has revealed several unknown metabolites. A glucuronidase sensitive metabolite from these products has been isolated and identified as retinoyl-beta-glucuronide by ultraviolet absorption spectrometry and mass spectrometry. The retinoyl-beta-glucuronide originally discovered by Olson and collaborators accounts for only 12% of the total excreted biliary products of retinoic acid. At least four to six major unknown retinoic acid metabolites, in addition to retinoyl-beta-glucuronide, have been detected and will shortly be identified.

5,6-epoxyretinoic acid is a physiological metabolite of retinoic acid in the rat

5,6-Epoxyretinoic acid was detected in small intestine, kidney, liver, testes and serum of vitamin A-deficient rats 3 h after a single physiological dose of [3H]retinoic acid. The maximum concentration of 5,6-epoxide in intestinal mucosa was observed 3 h after intrajugular administration of retinoic acid. However, at 7 h post administration, no 5,6-epoxyretinoic acid was detected in mucosa, demonstrating the rapid intestinal metabolism or excretion of this metabolite. No 5,6-epoxy[3H]retinoic acid was detected in mucosa, liver or serum of retinoic acid-repleted rats 3 h after administration of 2 micrograms of [3H]retinoic acid.

The oxidation and decarboxylation of retinoic acid by horseradish peroxidase

The decarboxylation of retinoic acid by horseradish peroxidase was investigated. A marked increase in the yield of products was obtained. However, the data indicated the reaction was a nonenzymatic, heme catalyzed peroxidation. Previously reported requirements for phosphate, oxygen and ferrous ion were eliminated when hydrogen peroxide was provided. Peroxide also eliminated the EDTA and cyanide induced inhibition of the phosphate dependent system. In the presence of hydrogen peroxide, horseradish peroxidase was not essential to the reaction; heme equivalent amounts of hemoglobin decarboxylated retinoic acid with equal facility. However, hemoglobin was ineffective in the absence of hydrogen peroxide. Attainment of 50--60% decarboxylation represented complete utilization of the available retinoic acid. Thus the products of the reaction can be divided into two groups, products of retinoic acid oxidation and products of an oxidative decarboxylation of retinoic acid.

Identification of 5,8-oxyretinoic acid isolated from small intestine of vitamin A-deficient rats dosed with retinoic acid

A retinoid was isolated by a multistep procedure from the small intestines of vitamin A-deficient rats given a single dose of retinoic acid. The compound, designated 8II, was pure, as demonstrated by four high-pressure liquid chromatographic procedures. It was positively identified as 5,8-oxyretinoic acid by ultraviolet spectrophotometry, mass spectrometry, and spectral and chromatographic comparison to known compounds. It is probable that 5,8-oxyretinoic acid was produced from 5,6-epoxyretinoic acid under the acidic conditions used in the isolation. It is highly probable, therefore, that the natural product is 5,6-epoxyretinoic acid.

Preparation, properties and metabolism of 5,6-monoepoxy-3-dehydroretinal

1. 5,6-Monoepoxy-3-dehydroretinal was synthesized from 3-dehydroretinyl acetate and characterized. 2. When fed to vitamin A-deficient rats, 5,6-monoepoxy-3-dehydroretinal was converted into 5,6-monoepoxy-3-dehydrovitamin A and stored in the liver. 3. It was demonstrated that the rat possesses the necessary enzymes for the reduction and oxidation of 5,6-monoepoxy-3-dehydroretinal to the corresponding alcohol and acid respectively. 4. The biological potency of the epoxy-3-dehydroretinal by the rat-growth assay (determined by USP XIV procedure) was 1.07% of that of vitamin A.