Metabolism of [11-3H]retinyl acetate in liver tissues of vitamin A-sufficient, -deficient and retinoic acid-supplemented rats

Retinal dehydrogenase gene expression in stomach and small intestine of rats during postnatal development and in vitamin A deficiency

Retinal dehydrogenase (RALDH) catalyzes the oxidation of retinal to all-trans and 9-cis retinoic acid, which function as ligands controlling RAR and RXR nuclear receptor-signaling pathways. We have recently shown the expression of RALDH transcript in the stomach and small intestine by reverse transcription polymerase chain reaction [Bhat, P.V., Labrecque J., Dumas, F., Lacroix, A. and Yoshida, A. (1995) Gene 166, 303-306]. We have examined RALDH expression in the stomach and small intestine before and during postnatal development and in vitamin A deficiency by assaying for mRNA levels and protein as well as for enzyme activity. In -2 day fetuses, RALDH expression was high in the small intestine, whereas RALDH protein was not detectable in the stomach. However, expression of RALDH was seen in the stomach after birth, and gradually increased with age and reached the highest level at postnatal day 42. In the intestine, RALDH expression decreased postnatally. Vitamin A deficiency up-regulated RALDH expression in the stomach and small intestine, and administration of retinoids down-regulated the RALDH expression in these tissues. These results show the differential expression of RALDH in the stomach and small intestine during postnatal development, and that vitamin A status regulates the expression of RALDH gene in these tissues.

Secretion pattern of retinol-binding protein in blood of goats: effect of vitamin A, provitamin A and their dosing schedule

Studies were conducted on the secretion of retinol-binding protein (RBP) in blood of goats given different treatments of preformed vitamin A, beta-carotene and plant carotenoids. Administration of these sources either in a single massive dose or massive dose split into four equal doses, markedly increased the secretion of RBP in blood. The secretion of RBP in blood occurred at least in two phases, one at early periods and the other at later periods.

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.

Fatty acid composition and kinetic behaviour of liver retinyl esters in vitamin A sufficient and deficient rats

Weanling rats were fed vitamin A deficient diets (-A) or diets supplemented with vitamin A (+A) (4.4 mg retinol equivalents/kg diet) for a period of 7 or 6 wk, respectively. In liver tissues of these two groups of animals both the subcellular localization as well as the fatty acid composition of the retinyl esters was studied. During vitamin A supplementation or deprivation, the kinetics of the different ester forms were investigated. Results indicate that the subcellular localization of all retinyl esters is similar and dependent on age. Two pools exist, ie one consisting of the nuclear/cell debris and mitochondrial-lysosomal fractions and the other containing the microsomal and cytosol fractions. HPLC analysis showed retinyl palmitate as the predominating (80%) form of the various retinyl esters. By supplementation clearly two kinetic behaviours can be demonstrated: one being a relatively stable storage of the palmitate and stearate, increasing with time and the second one being a more labile pattern for the ester forms with other saturated and unsaturated fatty acids. By vitamin A depletion all retinyl esters are affected indicating that the ester forms other than palmitate and stearate are also storage forms of vitamin A.

Vitamin A metabolism in rats chronically treated with 3,3',4,4',5,5'-hexabromobiphenyl

Chronic dietary administration of 3,3',4,4',5,5'-hexabromobiphenyl (HBB), 1 mg/kg diet, caused a decrease in retinol (20-fold) and retinyl esters (23-fold) in the livers of female rats, but resulted in a 6.4-fold increase in retinol and 7.4-fold increase in retinyl esters in the kidneys. Liver acyl-CoA:retinol acyltransferase and retinyl palmitate hydrolase activities were reduced while serum concentration of retinol was unaffected by HBB feeding. Metabolism of a physiological dose of [11-3H]retinyl acetate (10 micrograms), was examined in rats fed either vitamin A-adequate diet, or marginal amounts of vitamin A, or vitamin A-adequate diet containing HBB. A 13-fold greater amount of the administered vitamin A was found in kidneys of HBB-treated rats. In rats fed adequate or low amounts of vitamin A, kidney radioactivity was primarily in the retinol fraction, while in HBB-fed rats the radioactivity was associated mostly with retinyl esters. Fecal and urinary excretion of radioactivity was greatly increased in HBB-treated rats. Chronic HBB feeding results in a loss of ability of liver to store vitamin A, and severely alters the uptake and metabolism of vitamin A in the kidneys. We conclude that HBB causes major disturbances in the regulation of vitamin A metabolism.

Hydrolysis of retinyl esters by non-specific carboxylesterases from rat liver endoplasmic reticulum

The four most important non-specific carboxylesterases from rat liver were assayed for their ability to hydrolyse retinyl esters. Only the esterases with pI 6.2 and 6.4 (= esterase ES-4) are able to hydrolyse retinyl palmitate. Their specific activities strongly depend on the emulsifier used (maximum rate: 440 nmol of retinol liberated/h per mg of esterase). Beside retinyl palmitate, these esterases cleave palmitoyl-CoA and monoacylglycerols with much higher rates, as well as certain drugs (e.g. aspirin and propanidid). However, no transacylation between palmitoyl-CoA and retinol occurs. Retinyl acetate also is a substrate for the above esterases and for another one with pI 5.6 (= esterase ES-3). Again the emulsifier influences the hydrolysis by these esterases (maximum rates: 475 nmol/h per mg for ES-4 and 200 nmol/h per mg for ES-3). Differential centrifugation of rat liver homogenate reveals that retinyl palmitate hydrolase activity is highly enriched in the plasma membranes, but only moderately so in the endoplasmic reticulum, where the investigated esterases are located. Since the latter activity can be largely inhibited with the selective esterase inhibitor bis-(4-nitrophenyl) phosphate, it is concluded that the esterases with pI 6.2 and 6.4 (ES-4) represent the main retinyl palmitate hydrolase of rat liver endoplasmic reticulum. In view of this cellular localization, the enzyme could possibly be involved in the mobilization of retinol from the vitamin A esters stored in the liver. However, preliminary experiments in vivo have failed to demonstrate such a biological function.