Isolation, characterization, and biological activity of retinyl phosphate from hamster intestinal epithelium

Presence of a cellular retinylphosphate binding protein in rat liver

A retinylphosphate binding activity, resolved during purification, has been discovered in rat liver cytosol. The partial purification includes ammonium sulfate precipitation and DEAE-cellulose chromatography. The macromolecular component responsible for the binding has a sedimentation coefficient of about 2 S and is sensitive to pronase. This binding is reversible and specific for retinylphosphate, since retinol, retinoic acid and retinoylphosphate do not compete with [3H]retinylphosphate.

Retinoic acid alters the metabolic 3H-labelling of glycosphingolipids

Retinoic acid increases the incorporation of radioactivity from a mixture of [3H]-galactose and [3H]-glucosamine into glycosphingolipids of serum-starved quiescent human foreskin fibroblasts with a preferential labelling of ceramide mono- and dihexoside as compared to ceramide tri- and tetrahexoside. Under the conditions used, no similar change in the specific labelling of glycoprotein is observed. Alteration in [3H]-precursor uptake into glycolipids comparable to that seen under the influence of retinoic acid does not occur in the presence of phorbolester, colchicine, butyrate or after infection with cytomegalovirus.

Friend erythroleukemia cell differentiation: induction by retinoids

Growth in the presence of retinoids was found to induce erythroid differentiation in Friend murine erythroleukemia (MEL) cells in culture. The program of differentiated functions expressed by retinoid-treated cells was quite similar to that promoted by other inducers of MEL cell differentiation. For example, 70% or more of induced cells synthesized hemoglobin which accumulated to a level of 8 micrograms-10 micrograms per 10(6) cells. The level of acetylcholinesterase activity increased two to five-fold in induced cells, and induction by retinoids, like induction by dimethylsulfoxide (DMSO), promoted the appearance of cell surface lumps or 'blebs'. All-trans retinaldehyde, which promoted maximum hemoglobin and acetylcholinesterase synthesis at a concentration of 5 X 10(-7) M, was found to be a more potent inducer than all-trans retinoic acid or retinol, which both showed maximum induction at 1 X 10(-5) M. Like differentiation promoted by DMSO, retinoid-induced differentiation was inhibited by 10(-7) M dexamethasone.

Vitamin A in human skin: I. detection and identification of retinoids in normal epidermis

In an attempt to identify vitamin A and derivatives (retinoids) specimens of breast skin epidermis (0.5 g) were homogenized, freeze-dried and extracted with chloroform/methanol. The evaporated extract was partitioned repeatedly between petroleum ether and a mixture of ethanol and pH-adjusted water. This yielded 3 fractions of partially purified retinoids. High-pressure liquid chromatography (HPLC) of these fractions revealed the presence of the following retinoids given in order to their abundance in the epidermis: retinyl acyl esters, retinol, 3-dehydroretinyl acyl esters and retinoic acid. Small amounts of other retinoids may also be present. In order to obtain quantitative data it was essential to add internal retinoid standards and to completely hydrolyze the skin in KOH-ethanol before extraction. The retinoids were deconjugated by this procedure but, with the exception of retinaldehyde, were otherwise unchanged. The recoveries of the endogenous retinoids at HPLC were identical to those of the internal standards. The technique was reproducible and could be applied to the analysis of nanograms of retinol and dehydroretinol in small (10-30 mg) skin specimens. The amounts of acidic retinoids were usually below the detection limit of the method (less than 10 ng/g) but the approach may be useful at the higher levels attained during retinoid therapy.

Recent studies on the involvement of retinyl phosphate as a carrier of mannose in biological membranes

Rat liver microsomes synthesized [14C]mannosylretinylphosphate and dolichyl [14C]mannosylphosphate from guanosinedisphosphate [14C]mannose, retinylphosphate and dolichylphosphate. Two distinct enzyme activities were shown to be responsible for the biosynthesis of the two mannolipids. A higher affinity mannosyl transferase (EA I), responsible for dolichylmannosylphosphate synthesis, displayed a Km for GDP-mannose of 1.7 microM; while a lower affinity enzyme (EA II), responsible for mannosylretinylphosphate synthesis, displayed a Km for GDP-mannose of 12.5 microM. These Km values were unaffected by the addition of either dolichylphosphate for EA II, or retinylphosphate for EA I. The same Km values were found before and after solubilization of the enzyme activity with 1% Triton X-100. Differential solubilization of EA I and EA II was demonstrated, utilizing different concentrations of Triton X-100. Triple-labeled mannosylretinylphosphate was prepared from [3H]retinylphosphate, retinyl[32P]phosphate and GDP-[14C]mannose from incubations containing rat liver microsomes. This compound was shown to donate [14C]mannose to endogenous acceptors of rat liver microsomes.

Glycoprotein biosynthesis in intestinal epithelial cells during differentiation. Incorporation of [14C]mannose from GDP-[14C]mannose into dolichol derivatives

Epithelial cells of the rat small intestine were collected as a gradient of villus to crypt cells. Homogenates of these cells incubated with GDP-D-[14C]mannose in the presence of MnCl2 incorporated radioactivity into dolichyl mannosyl phosphate and a mixutre of dolichyl pyrophosphate oligosaccharides varying in the size of their oligosaccharide moiety. The labeled oligosaccharides formed in villus cell homogenates appeared shorter than those formed in crypt cell homogenates. The addition of dolichyl phosphate greatly stimulated the synthesis of dolichyl mannosyl phosphate. The initial rate of synthesis of dolichyl mannosyl phosphate from GDP-D-[14C]mannose and exogenous dolichyl phosphate was highest in an intermediate cell fraction having a low specific activity of sucrase and alkaline phosphatase and an intermediate specific activity of thymidine kinase. To compare the rates of dolichyl mannosyl phosphate synthesis in the different cell fractions, it was essential to control degradation of GDP-D-[14]mannose by the addition of AMP to the incubation, since villus cells degraded GDP-D-[14C]mannose much faster than crypt cells.

The synthesis, biological activity and metabolism of 15-[6,7-14C2]- and 15-[21-3H]methyl retinone, 15-methyl retinol and 15-dimethyl retinol in rats

15-Methyl retinone, 15-methyl retinol and 15-dimethyl retinol were synthesized from all-trans retinoic acid. The absorption maxima and molar absorption coefficients (epsilon) in ethanol are: 15-methyl retinone (372 nm, 47 400), 15-methyl retinol (325 nm, 72 000) and 15-dimethyl retinol (325 nm, 72 200). The latter two compounds show fluorescence at 470 nm when excited around 320 nm, but with intensities 40 and 70%, respectively, that of retinol. All react with trifluoroacetic acid in chloroform, and the alcohols are dehydrated in ethanolic HCl. Relative to all-trans retinyl acetate, the biological activities in rat growth assay (+/-S.E.) are: 15-methyl retinone (4.7 +/- 1.5%), 15-methyl retinol (16.2 +/- 2.3%) and 15-dimethyl retinol (0.34 +/- 0.07%). The monomethyl derivatives actively support testicular development and spermatogenesis, but none of the three analogues prevents degeneration of the retina in retinoate-treated vitamin A-deficient rats. By using [6,7-14C2]-, [11,12-3H2]- and [21-3H]-labeled analogues, the 15-methyl derivatives were shown to be well absorbed in the gut but poorly stored (1-3% of the dose) in the liver. Metabolites are excreted extensively (25-65%) in the bile, however, largely as glucuronides, and to some extent (15%) in the urine. 15-Methyl retinone is reduced to 15-methyl retinol in the intestinal mucosa but not in the liver, and fatty acyl esters of the alcohols are present in liver. Insofar as we could judge, none of these 15-methyl analogues is converted into retinol. Both monomethyl and dimethyl retinols are transported in plasma in vivo in the retinol-binding protein fraction. Interestingly, the dosages required for half saturation and saturation of the plasma transport system are inversely related, in a rough way, to the biological activities in growth.

Incorporation of mannose and glucose into prenylphosphate sugars in isolated human platelet membranes

Isolated platelet membranes synthesize mannosylretinyl phosphate and dolichylmannosyl phosphate from GDP-[14C]mannose, but only dolichylglucosyl-phosphate is synthesized from UDP-[14C]glucose. Addition of exogenous retinylphosphate specifically stimulates the biosynthesis of mannosylretinylphosphate.