Preparation, characterization, biological activity and metabolism of all-trans retinoyl fluoride

Chemical synthesis of all-trans-[11-3H]retinoyl beta-glucuronide and its metabolism in rats in vivo

All-trans-[11-3H]retinoyl beta-glucuronide (RAG) was synthesized in a single step from all-trans-[11-3H]retinoyl fluoride, with a 24% yield. After its intraperitoneal injection into rats, RAG was detected in the blood, liver, intestine and kidney during the following 24 h period. Although the concentration of radiolabelled metabolites decreased with time, RAG predominated at nearly all times in nearly all tissues. Small amounts of retinoic acid (RA) were also universally present, together with unidentified polar metabolites and small amounts of non-polar esters of RA. The major excretion products of RAG in faeces and urine were RA and polar metabolites. Thus RAG, although converted in part to RA in vivo, persists as a major component in blood and tissues for at least 24 h. These observations support the concept that the retinoid beta-glucuronides might serve a physiologically significant role in the function of vitamin A.

A novel photopigment candidate in membranes of cultured chick pineal cells

The photopigment of avian pineal which mediates light sensitivity was sought via its chromophore. Chick pineal cells in primary cultures were incubated overnight in the dark with [3H]retinol. Reduction of Schiff's bases with cyanoborohydride prior to SDS-PAGE revealed radioactivity bound to a 30 kDa component in pinealocyte membranes. All-trans-retinal, but not retinol or retinoic acid, incubated with pinealocyte homogenates prior to reduction, resulted in a loss of radioactivity from the 30 kDa region of the gel. The radioactivity was also displaced by NH2OH in the dark. Incubation of cultured cells or homogenates with retinoyl fluoride, an acylating agent specific for the retinal binding site of opsins, also displaced radioactivity from the protein. Furthermore, retinoyl fluoride, added to chick pineal cells in culture, blocked the suppressive effect of light on melatonin output by these cells. Taken together these results raise the possibility that the 30 kDa protein mediates photosensitivity in the chick pineal.

Chemical synthesis and growth-promoting activity of all-trans-retinyl beta-D-glucuronide

All-trans-retinol reacts with methyl (2,3,4-tri-O-acetyl-1-bromo-1-deoxy-beta-D-glucopyran)uronate in the presence of Ag2CO3 to give the triacetate methyl ester of retinyl beta-glucuronide. Hydrolysis of this ester with sodium methylate in methanol gives retinyl beta-D-glucuronide in about 15% yield. The water-soluble retinyl beta-D-glucuronide was characterized by u.v.-visible, n.m.r. and mass spectra, by elemental analysis and by its susceptibility to hydrolysis by bacterial beta-glucuronidase. Retinyl beta-glucuronide, when administered intraperitoneally in saline (0.9% NaCl), supports well the growth of vitamin A-deficient rats.

Enhancement of acyl coenzyme A:retinol acyltransferase in rat liver and mammary tumor tissue by retinyl acetate and its competitive inhibition by N-(4-hydroxyphenyl) retinamide

Retinol esterification by microsomal acyl coenzyme A:retinol acyltransferase was quantified in rat mammary tumor and liver tissue. Acyltransferase activity in the livers of mammary tumor-bearing rats was 40% of that in normal animals. In response to daily oral doses of 2 mg retinyl acetate for 18-19 days, activity increased 2.8-fold in transplanted rat mammary tumors, 4.1-fold in the livers of tumor-bearing rats, and 1.5-fold in the livers of normal rats. The in vitro esterification of retinol was competitively inhibited by all-trans-N-(4-hydroxyphenyl) retinamide (Ki = 154 microM).

Metabolism and biological activity of all-trans 4,4-difluororetinyl acetate

All-trans [11-3H]4,4- difluororetinyl acetate was synthesized by treating methyl all-trans [11-3H]4- oxoretinoate with diethylaminosulfurtrifluoride , followed by reduction and acetylation of the product. After oral administration of the radioactive difluoro analog in oil to rats, difluororetinol , difluororetinyl palmitate and related esters, 4- oxoretinol , 4- oxoretinoic acid and polar conjugated derivatives were identified in the intestine, liver, kidney and/or blood. The major metabolic products were difluororetinyl palmitate and related esters, which were stored in the liver. The presence of the difluoro analog in liver oil from treated rats was confirmed by 19F-NMR spectroscopy. Neither retinol nor retinyl esters were detected as products of the metabolism of the difluoro analog. Nonetheless, all-trans difluororetinyl acetate showed 26 +/- 12% of the biological activity of all-trans retinyl acetate in the rat growth assay. Presumably, the difluoro analog is active per se in growth rather than by conversion to retinol or to one of its known growth-promoting metabolites. In general, however, the difluoro analog was metabolized in a manner very similar to vitamin A. The vitamin A moiety of administered difluororetinyl acetate and retinyl acetate was poorly stored (1.8-3.3%) in the liver of vitamin A-depleted rats, confirming and extending past reports that the liver storage mechanism is severely impaired when initial liver stores are very low.