As determined by in vitro tests, anhydroretinol, a metabolic product of retinol, was bound specifically by serum retinol-binding protein and by cellular retinol-binding protein but not by cellular retinoic acid-binding protein or the nuclear receptors, RARs and RXRs. For rats dosed with the mammary carcinogen, N-methyl-N-nitrosourea (45 mg/kg body weight) and given diets containing either the retinoid vehicle, anhydroretinol (67, 134, 268, or 536 mg/kg of diet), or retinyl acetate (328 mg/kg of diet), there were, over a 90-day observation period, no significant differences in body weights. The compound did not accumulate in liver tissue or cause an increase in hepatic levels of retinyl palmitate (potential problems observed with other retinoids). The numbers of mammary cancers were as follows: no retinoid, 4.5/rat; retinyl acetate, 2.1/rat; and increasing doses of anhydroretinol, 2.9, 3.3, 3.0, and 1.7/rat, respectively. Thus, anhydroretinol, at non-toxic levels, was effective as a preventive agent in this experimental model of breast cancer.

Animals↗

PubMedDate not supplied

Conversion of retinoid ethers to alcohols by enzymatic activity present in rat liver microsomes.

An enzyme present in rat liver microsomes catalyzes the conversion of retinyl methyl ether (RME) to retinol; NADPH is required for activity. The optimum pH for the reaction is 7.4; the KM and Vmax values are 120 microM RME and 14.3 nmol of retinol/mg protein/hr, respectively. As a substrate, the 2,3,6-trimethyl-4-methoxyphenyl analog of RME is as effective as RME. There is, however, no measurable activity for dealkylation of retinyl ethyl ether or retinyl butyl ether. Hepatic enzyme activity for the metabolism of RME is induced by 3-methylcholanthrene but not by phenobarbital or RME itself. The induced activity also requires NADPH as a cofactor. The optimum pH for the induced enzyme is 8.4; the KM and Vmax values are 50 microM RME and 111 nmol of retinol/mg protein/hr, respectively. For this enzyme, RME is a better substrate than the 2,3,6-trimethyl-4-methoxyphenyl analog of RME; retinyl ethyl ether is less effective; and again, there is no measurable activity with retinyl butyl ether as a substrate. Neither constitutive nor induced activity is detectable in microsomes from lung, spleen, stomach, kidney, small intestine, or large intestine. The enzyme activity that cleaves retinoid ethers appears to be similar to other microsomal NADPH-requiring O-dealkylases and different from a reported tetrahydropteridine-requiring dealkylase.

Alcohols↗

PubMedDate not supplied

Nonenzymatic isomerization of all-trans- and 13-cis-retinoids catalyzed by sulfhydryl groups.

Certain thiol-containing compounds catalyze, in a chemical reaction, the isomerization of all-trans-retinoic acid (RA) to 13-cis-RA and of 13-cis-RA to RA. Reactions approaching equilibrium contain more RA than 13-cis-RA. Small molecules effective as catalysts are glutathione, mercaptoethanol, and L-cysteine methyl ester. L-Cysteine is not a catalyst and inhibits the reaction catalyzed by glutathione or mercaptoethanol. Apoferritin (a thiol-containing protein), native microsomes, and, to a lesser extent, boiled microsomes catalyze the reaction, but their activity is reduced or eliminated by prior incubation with iodoacetate. Other cis and trans isomeric retinoids are also substrates for this reaction; the reactions proceed more readily for the cis isomers. The thiol-catalyzed isomerization of RA and 13-cis-RA may account for the observations of both cis and trans forms of retinoids in tissues of animals after administration of either.

Animals↗

PubMedDate not supplied

Enzymatic hydrolysis of retinamides.

Enzymatic activity present in liver microsomes from rats slowly hydrolyzed N-(4-hydroxyphenyl)retinamide (4HPR). A product of the reaction was all-trans-retinoic acid. The reaction, which had a pH optimum greater than 8.6, was stimulated by divalent cations, particularly Mn2+. Enzyme activity was highest in liver microsomes but was also present in kidney microsomes, liver cytoplasm, and spleen cytoplasm. Of 10 possible substrates tested, the 13-cis- and all-trans-forms of N-ethylretinamide were most active. The all-trans-form of 4HPR was much more active than the 13-cis-form. Neither 13-cis- nor all-trans-retinoyl leucine was a substrate. Because no detectable [14C]all-trans-retinoic acid could be found in the livers of rats after doses of [14C]4HPR, we conclude that this enzyme is not extensively active in intact animals.

Amidohydrolases↗

PubMedDate not supplied

Biochemical characteristics and differentiating activity of 4-oxo analogs of retinoic acid.

3-Methyl-4-oxoretinoic acid and 3-cinnamyl-4-oxoretinoic acid bind to a cellular retinoic acid-binding protein (CRABP-II) and to a retinoic acid-receptor protein (RARa). These analogs of 4-oxoretinoic acid, as well as the parent compound, have less binding affinity than retinoic acid. Cotransfection assays in CV-1 cells with plasmids containing cDNAs for RAR alpha, RAR beta and RAR gamma (homodimers) and RAR alpha-RXR alpha and RAR beta-RXR alpha (heterodimers), indicate that 3-cinnamyl-4-oxoretinoic acid induces relatively less transcriptional activity than 4-oxoretinoic acid and its 3-methyl analog, both of which are less effective than retinoic acid. In differentiating mouse F9 embryocarcarcinoma cells, the order of effectiveness is retinoic acid > 4-oxoretinoic acid = 3-methyl-4-oxoretinoic acid > 3-cinnamyl-4-oxoretinoic acid. This order of potency is similar to that for inhibition of induction of ornithine decarboxylase (ODC) activity and for prevention of papillomas on the skin of mice. Binding to CRABP-II and activation of RARs appear to be important factors for expression of differentiating activity, inhibition of induction of ODC activity and prevention of papillomas on the skin of mice.

Amino Acid Sequence↗