The preparation and biological activity of methyl 5,6-epoxy-retinoate

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.

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.

Infrared spectroscopy of retinoids

The Fourier transform infrared spectra of 15 purified retinoids were compared. Retinoids with conjugated C = O groups revealed the presence of a band between 1730 and 1630 cm-1 X A characteristic of retinoids is the presence of a band between 1650 and 1620 cm-1 due to C = C stretching where three to four conjugated C = O and C = C bonds were present or between 1610 and 1555 cm-1 where more conjugated unsaturations were present. The presence of cis double bonds was confirmed by a band at 1380 cm-1 while unsubstituted trans double bonds gave absorbances at 990 to 955 cm-1. Epoxy rings, which are present in some retinoids, resulted in bands at 1250, 880, and 790 cm-1 while a furan structure was confirmed by bands at 1175, 1083, and 1065 cm-1.

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.

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.

The biological activity of retinoic acid in the domestic fowl and the effects of vitamin A deficiency on the chick embryo

1. Male and female chickens were reared from hatching on vitamin A-free diets, either unsupplemented or containing retinoic acid (vitamin A acid), methyl retinoate or retinyl acetate (vitamin A acetate). The birds given retinyl acetate grew well and had a normal appearance, but those given the unsupplemented diet died before 4 weeks of age after developing typical signs of avitaminosis A. The birds given retinoic acid or methyl retinoate did not show overt signs of vitamin A deficiency or other abnormalities except for a progressive failure of vision. Minimal histological changes were found in their retinas, and their vision was rapidly restored after feeding with retinyl acetate.

2. The cocks maintained with retinoic acid or methyl retinoate had normal testes and the hens laid eggs at a normal rate, but although their eggs could be obtained fertile the development of the embryo became abnormal after 2 days incubation and it always died. The development of the embryos could be stimulated and sometimes restored to normal by injection of various forms of vitamin A into the eggs before incubation, or by previous administration of retinyl acetate to the hens.

3. It is concluded that feeding retinoic acid as the sole source of vitamin A enables the hen to produce eggs that lack vitamin A but are otherwise normal, thus permitting the demonstration of a hitherto undescribed requirement of the early chick embryo for vitamin A.

4. The toxicity of vitamin A derivatives to chick embryos was investigated; injected retinoic acid was found to be extremely toxic.

The retinol requirements of rats for spermatogenesis and vision

1. Hooded rats were fed from weaning on a basal retinol-deficient diet containing retinoic acid. Such a diet maintains growth and general health but does not prevent the appearance of lesions associated with vitamin A deficiency in the retina and testis. Some animals were also given supplements of retinol averaging 0.1, 0.25, 1 or 5 μg retinyl acetate per day. Rats were killed at intervals up to 28 weeks after weaning. The weights of the testes and the histological appearance of the testes and epididymides indicated that 5 μg retinyl acetate per day had maintained spermatogenesis throughout the experimental period. Doses averaging 1 μg retinyl acetate per day were only partially effective and the two smaller doses had little beneficial effect.

2. In a second similar experiment rats were given doses of retinyl acetate averaging 0.25, 0.5, 1, 2 or 100 μg per day. Measurements of the electroretinogram thresholds of the rats indicated that a dose of 1 μg retinyl acetate per day maintained mainly normal vision until the end of the experiment 29 weeks after weaning. Additional histological observations made 21 weeks after weaning showed that this dose level had not maintained spermatogenesis but that doses of 2 μg retinyl acetate per day had been effective.

3. The experiments show that the differing functions of vitamin A in spermatogenesis and vision are reflected in the hooded rat in differences in the dietary retinol levels needed to maintain these processes.

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.