Enzymatic synthesis of mannosyl retinyl phosphate from retinyl phosphate and guanosine diphosphate mannose

Influence of vitamin A status and DDT on vitamin A-dependent protein mannosylation in rat liver

Male Wistar rats of different vitamin A status (total depletion to moderate deficiency) were administered DDT (5 mg/kg/day) or vehicule (corn oil) i.p. daily for 14 days. Vitamin A-dependent protein mannosylation was measured either by in vivo incorporation of [3H]mannose into liver glycoprotein or by in vitro assay of incorporation of [14C]mannose into mannosylretinyl phosphate. Vitamin A deficiency resulted in a significantly impaired in vivo incorporation of mannose in liver glycoprotein but had no effect on the in vitro transport of mannose via retinyl phosphate. Although DDT induced an increase synthesis of liver proteins in smooth endoplasmic reticulum and caused a diminution of the hepatic vitamin A content, it did not affect vitamin A-dependent protein mannosylation.

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.

In vivo formation of tritium-labeled lactic acid from [2-3H]mannose or [15-3H]retinol by hamster intestinal epithelial cells

In studies designed to reexamine the in vivo occurrence of retinyl phosphate mannose we injected hamsters intraperitoneally with either [2-3H]mannose or [15-3H]retinol and sacrificed the animals 15 min later. The small intestine was removed, the epithelial cells were scraped, and a methanolic extract of the labeled cells was prepared and chromatographed on a Mono Q anion-exchange column. Intraperitoneal administration of either [2-3H]mannose or [15-3H]retinol lead to the formation of a tritium-labeled anionic compound with a retention time on the Mono Q column similar to that of standard retinyl phosphate mannose. However, the biochemical properties of this labeled anionic compound were those expected of an organic acid and not retinyl phosphate mannose. The compound was resistant to both strong acid hydrolysis and mild base hydrolysis, as well as digestion with alpha- or beta-mannosidase, phosphodiesterase I, nucleotide pyrophosphatase, or beta-glucuronidase. When chromatographed on an Aminex HPX-87H organic acid analysis column or a silicic acid column the labeled anionic compound derived from either [2-3H]mannose or [15-3H]retinol comigrated with standard lactic acid. Treatment of the anionic compound derived from [2-3H]mannose with lactate oxidase or L-lactate 2-monooxygenase resulted in the formation of a tritium-labeled product that cochromatographed, respectively, with pyruvate or acetate on the Aminex HPX-87H column. However, treatment of the anionic compound derived from [15-3H]retinol with these same two enzymes resulted in a labeled product that migrated on the Aminex column at the same position as tritiated water. This result demonstrated that the labeled hydrogen was removed during enzymatic digestion and suggested that it was present on the second carbon of lactic acid. During the course of these studies no evidence for the in vivo labeling of a compound with the properties of retinyl phosphate mannose was found. Since [2-3H]mannose leads to labeled lactic acid in vivo the tritium label must not always be lost, as expected, during the entry step into glycolysis in which mannose 6-phosphate is converted to fructose 6-phosphate. The results suggest that an intramolecular hydrogen transfer from the C-2 position of mannose 6-phosphate to the C-1 position of fructose 6-phosphate can occur during the phosphomannose isomerase reaction. The finding that the position of the tritium label on lactic acid derived from [15-3H]retinol is on the second carbon is consistent with it coming from NADH labeled with tritium in the transferable hydrogen which was formed intracellularly during the NAD+-linked oxidation of retinol to retinaldehyde.

Effect of teprenone on the content of phospholipids in gastric secretion in man

The phospholipid content of basal and pentagastrin-stimulated gastric secretion was measured in 9 healthy volunteers. One week oral administration of teprenone did not alter the gastric acid and pepsin secretion. The phospholipid content was increased by teprenone. Changes in subclasses of phospholipids in gastric mucus by teprenone treatment, namely, increased phosphatidylcholine and decreased lysophosphatidylcholine, were observed.

Enzymatic glucosylation of dolichol monophosphate and transfer of glucose from isolated dolichyl-D-glucosyl phosphate to ceramides by BHK-21 cell microsomes

Enzymatic glucosylation of dolichol monophosphate (dolichol-P) from UDP-D-[3H]glucose was studied using the microsomal fraction of BHK-21 cells. The reaction product was separated by preparative thin-layer chromatography, further purified by DEAE-cellulose acetate column chromatography, and characterized as dolichyl-beta-D-glucosyl phosphate (Dol-P-Glc). The microsomal fraction of BHK cells catalyzed the incorporation of glucose from UDP-[3H]glucose into ceramides (endogenous and exogenous) and Dol-P; both reactions required Mn2+. Maximal glucosylation of Dol-P was achieved at pH 5.6-5.8 in the presence of a non-ionic detergent, Zonyl A. Glucosylation of exogenous Dol-P, from UDP-Glc, was non-competitively inhibited by exogenous ceramides. Incubation of Dol-P-[3H]Glc or Dol-P-[14C]Glc with liposomes (containing ceramides) and the microsomal fraction of BHK-21 cells resulted in the formation of a radioactive glucolipid which comigrated with the same RF value as glucosylceramide (Glc-Cer) on silica gel thin-layer chromatography. Transfer of [14C]glucose from Dol-P-[14C]Glc to exogenous ceramides was confirmed by double-labeling techniques. The pH dependence for transfer of radio-labeled glucose from Dol-P-[3H]Glc to ceramides was multi-phasic (optima at pH 4.0 and 7.0); glycosylation occurred within 5 min and Zonyl A was absolutely essential for the transfer reaction. These results indicate that Dol-P-Glc may also participate in the synthesis of ceramide hexosides.

Studies on mannosyl carrier function of retinol and retinoic acid in epithelial and mesenchymal tissues

The mannosyl derivative of phosphorylated vitamin A, mannosylretinylphosphate, is synthesized in vivo and in vitro by most epithelial tissues, including mouse epidermal cells. This compound is but one component of the complex biosynthetic machinery responsible for the assembly of glycoproteins in mammalian membranes; its specific role remains to be understood, even though it has been established that it functions as a carrier of mannose mostly in a direct transfer to protein. The system of five conjugated double bonds appears necessary for this function, since upon saturation the resulting perhydroretinylphosphate is incapable of acting in mannosyl transfer to endogenous microsomal proteins. Using vitamin A - deficient hamsters and rats, retinoic acid was shown to be as active as retinol in restoring the in vivo in corporation of mannose into glycoproteins to normal levels within a short time, in liver and tracheal tissues. Retinoic acid was also active in increasing adhesive properties of spontaneously transformed mouse dermal fibroblasts (3T12 cells) in a reversible manner. The free carboxyl group appeared to be a requirement for this activity inasmuch as the lactonized form of 11-hydroxyretinoic acid was inactive. Even though retinoic acid, as well as retinol, was able to enhance adhesion of 3T12 cells, only the cellular retinoic acid-binding protein was detected, suggesting that the cellular retinol-binding protein is not a requirement for this activity. A metabolite of retinoic acid, compound X, different from retinol, was found incorporated into mannosylretinoidphosphate (MXP) by 3T12 cells. This derivative constitutes up to 40% of the total radioactive pool of derivatives of retinoic acid after 48 hours of labeling. These data suggest that retinol, as mannosylretinylphosphate, and retinoic acid, as mannosylretinoidphosphate, function as mannosyl carriers in biologic membranes.

Formation of alpha-1,2- and alpha-1,3-linked mannose disaccharides from mannosyl retinyl phosphate by rat liver membrane enzymes

Mannosyl retinyl phosphate (MRP) was an active substrate for the transfer of mannose to methyl-alpha-D-mannose (CH3-alpha-man), p-nitrophenyl-alpha-D-mannose, and free mannose. The products formed during MRP incubation with CH3-alpha-man or with mannose were alpha-linked. The disaccharides formed by incubation of MRP with mannose were identified by paper chromatography and electrophoresis as mannose-alpha-1,2-mannose and mannose-alpha-1,3-mannose. Triton X-100 greatly inhibited mannose-alpha-1,3-mannose synthesis. In the absence of detergent, MnCl2, NiCl2, and ZnCl2 inhibited synthesis of both products. Formation of mannose-alpha-1,3-mannose was more sensitive to preincubation of the enzyme at 40 degrees C then was synthesis of mannose-alpha-1,2-mannose. No differences in membrane mannosyltransferase activity with MRP, compared to DMP, could be demonstrated.

Vitamin A and galactosyl transferase of tracheal epithelium

We developed an assay for galactosyl transferase (uridine diphosphategalactose:glycoprotein galactosyltransferase, EC 2.4.1.22) in microsomes of rat tracheal epithelium and characterized the properties of this enzyme system. We found that vitamin A deficiency greatly depressed galactosyl transferase activity. Activity could be restored within 48 h by dosing the animal with retinyl acetate. Adding retinol to the microsomes from tracheal epithelium of vitamin A-deficient rats also restored galactosyl transferase activity to some extent. Full restoration was achieved by pre-incubating retinol with the microsomal preparation for 30 min. Optimal concentration of pre-incubated retinol was 10(-8) M. Pre-incubation with retinyl phosphate and retinoic acid stimulated galactosyl transferase activity in microsomes from tracheas of deficient rats; pre-incubation with dolichyl phosphate, anhydroretinol and the dimethylacetylcyclopentenyl analog of retinoic acid did not. We concluded that vitamin A is involved in the galactosyl transferase system of rat tracheal epithelium, possibly by being linked to galactose.

Mannosylation of endogenous proteins of rough and smooth endoplasmic reticulum and of Golgi membranes

Mannosylation of the proteins of microsomal and Golgi membranes was investigated both after incubation in vitro of the isolated subfractions with GDP-[14C]mannose and after injection of [3H]mannose into rats followed by separation of these subfractions. Mannosylation of endogenous and added exogenous dolichol phosphate and also of dolichol pyrophosphate-oligosaccharide occurs in all three fractions. It was essential to inhibit antagonistic enzymes during incubation and to centrifuge after incubation. The presence of detergent in the incubation mixture influences the incorporation pattern of the different fractions in very different ways. In a system in vitro predominantly membrane proteins and not secretory proteins are mannosylated. Trypsin treatment of intact vesicles removes components from the outer surface only; such treatment liberates about one third of the radioactive mannose associated with lipid, releases radioactivity from the protein acceptor to the same extent and causes some inactivation of the transferase activities. It appears that a part of the mannosyl transferase system in rough and smooth endoplasmic reticulum and in Golgi membranes is localized at the cytoplasmic side of these membranes. This activity is probably involved in the glycosylation of proteins localized at the cytoplasmic surface of the endoplasmic reticulum.

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.

Effect of retinyl acetate on sulfated glycosaminoglycan biosynthesis in dermal and epidermal cells in vitro

The effects of retinyl acetate on the biosynthesis of sulfated glycosaminoglycans in dermal and epidermal cells isolated from newborn mice was investigated. Three compartments were analysed for [35S]-glycosaminoglycans; the culture medium, the cellular matrix, and the cells. The individual levels of chondroitin 4-sulfate, dermatan sulfate and heparin and/or heparan sulfate in these compartments as a function of retinyl acetate was also analysed. The addition of retinyl acetate resulted in a dose dependent increase 35SO4 incorporation in the cellular and matrix compartments of the dermis in vitro. At the optimum concentration of 1.8 X 10(-6) M, this increase was 50%. The levels of35SO4 incorporated into medium glycosaminoglycans were relatively constant. There were also changes in the levels of the individual sulfated glycosaminoglycans. The glycosaminoglycan profile was modified differently in each of the three compartments analysed. In the epidermal cells, retinyl acetate at an optimum concentration of 1.8 X 10(-6) M resulted in a dose dependent increase in 35SO4 incorporation in the cellular, matrix and medium compartments. There was no apparent change in the glycosaminoglycan profile, with heparin and/or heparan sulfate being the major sulfated glycosaminoglycan.

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.

Biosynthetic studies on mannolipids and mannoproteins of normal and vitamin A-depleted hamster livers

The incorporation of [1-14C]mannose into hamster liver glycolipids and glycoproteins was studied in normal and vitamin A-depleted hamsters. Severly (25% weight loss) and mildly (no weight loss) deficient animals were compared to vitamin A-fed controls. The incorporation of [14C]mannose into glycolipids and glycoproteins decreased in mild and severe vitamin A deficiency by 63-90% compared to vitamin A-fed animals. These results were essentially the same whether expressed per g of wet liver, per DNA or per protein. The size of the pools of mannose, glucose and galactose and their specific radioactivity in liver were determined by gas-liquid chromatography of the boronates of the hexitols (Eisenberg, Jr, F. (1972) Methods Enzymol. XXVIIIB, 168-178) in normal and vitamin A-deficient conditions. It was found that the amount of free hexoses per g of liver was very similar in normal and vitamin A-deficient conditions. The specific radioactivities for mannose and glucose were greater in vitamin A deficiency, thus excluding the possibility that the observed severe decrease in glycopeptide and glycolipid synthesis is a reflection of a similar decrease in the specific radioactivity of the precursor pools. Quantitation of mannose in glycoprotein showed a 79% decrease in vitamin A deficiency. Specific radioactivity of mannose in glycoproteins, 20 min after injection of the label, was 187 dpm/mug of mannose in the normal and 48 kpm/mug of mannose in the vitamin A-deficient livers. It is concluded that vitamin A is necessary for the biosynthesis of liver mannose-containing glycoproteins and glycolipids.