Retinol Glycolipids

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.

Vitamin A-deficiency impairs the normal mannosylation, conformation and iodination of thyroglobulin: a new etiological approach to endemic goitre

This study was undertaken in order to validate the hypothesis that vitamin A-deficiency alters the structure of thyroglobulin (Tg). For that purpose, four groups of 20 Sprague-Dawley rats were submitted during two months to varying dietary conditions, namely a control diet (C+), a vitamin A-deficient diet (A-), an iodine-deficient diet (I-) and a diet characterized by the association of both deficiencies (A-I-). Both the conventional parameters of thyroid function, the intracellular steps of Tg glycosylation and iodination were analyzed. In the A- and A-I- groups, blood levels of retinol fell to one tenth of the control mean and circulating concentrations of total and free T4 and T3 increased significantly. This biochemical hyperthyroidism contrasted with the maintenance of normal TSH plasma values, suggesting a generalized peripheral refractoriness to thyroid hormones. In both A- and A-I- groups, thyroid cytosol 3H-RPM (retinyl-phosphate-mannose) and 3H-mannose incorporation into the core of the 12S-Tg and 19S-Tg species were reduced by 40-50%. In contrast, cytosolic concentrations of 3H-DPM (dolichyl-phosphate-mannose) rose, suggesting that the N-glycosylation pathways are affected in opposite direction. The sedimentation coefficient in sucrose gradient of the purified dimeric 125I-19S-Tg after guanidine 6M and dithiothreitol denaturation showed that most of the A- Tg molecules were transformed into monomeric 12S species, implying alterations of both noncovalent and covalent bonds. Finally, the radiochromatogram of 125I-iodothyronines recovered after Tg pronase digestion revealed a significant increase in the mono- (MIT) and diiodothyronine (DIT) fractions in contrast with a significant decrease in the T3 and T4 hormonal compounds. These findings are consistent with the view that vitamin A-depletion impairs the endogenous RPM synthesis and, therefore, the normal Tg 0-mannosylation. The growing peptide is characterized by steric hindrance, leading to abnormal closure of disulphide bonds, reduced MIT-DIT coupling reactions and depressed generation of physiologically active thyroid hormones. pure iodine deficit (I-) induces no effects on the above-mentioned glycosylation reactions, but iodine shortage superimposed on preexisting vitamin A-deficit (A-I-) aggravates the Tg dysmaturation.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

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.

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 biosynthesis of mannolipids and mannose-containing complex glycans by the retina

Large-scale incubations were carried out with homogenates of the retinas of the 15--16-day-old chick embryo in the presence of GDP[U-14C] mannose, from which there were isolated mannolipid (Lipid I), oligosaccharide-lipids (Lipid II), and glycoprotein (residue). These incubations were performed in the presence of endogenous acceptors as well as dolichyl phosphate. [14C] Mannolipid I was subjected to chromatography on DEAE cellulose and silicic acid. The response to these, as well as TLC, enzymatic, and chemical treatments, were consistent with the product being dolichyl phosphomannose. [14C] Lipid II was purified by DEAE cellulose chromatography and gel filtration on LH-20. Responses to these treatments, as well as TLC and paper chromatography, were consistent with this product being of the class of the oligosaccharide-pyrophosphate-lipids. The residue remaining after removal of the lipids was shown to contain glycoproteins by conversion of high-molecular-weight radioactive material to low-molecular-weight [14C] mannose-containing glycopeptides by the action of pronase. These reactions and their products are consistent with there being in the retina, the pathway for glycoprotein synthesis involving the participation of the lipid-activated carbohydrates. When the incubations were performed in the presence of ATP or ADP there was a decrease in the labeling of Lipid I, accompanied by an increase in the labeling of Lipid II and glycoprotein. When incubated in the presence of dolichyl phosphate and detergent, however, the stimulatory effect of ATP did not occur. The effect on these activities of a variety of other nucleotide phosphates was also examined.

Cell surface mannosyl transferase activity in the liver of embryonic chick. A mannose containing glycolipid as intermediate in glycoprotein synthesis

Trypsin dissociated intact cells of embryonic chick liver catalyze the transfer of mannose from exogenous GDP-mannose to glycoprotein in vitro. In cells of 8 day old embryos a surface bound mannosyltransferase-system forms several mannose containing isoprenoid-lipids in a primary step. One of these compounds serves as a substrate in the highly specific second step of the overall reaction, the transfer of mannose to glycoprotein. The isoprenoic intermediate has been isolated and its effectivity as substrate for the incorporation of mannose into glycoprotein has been examined.

Polyisoprenoid glycolipids involved in glycoprotein biosynthesis

Until five years ago, it was believed that the oligosaccharide chains of most, if not all, glycoproteins were assembled by the stepwise transfer of single sugar residues from their nucleotide derivatives to growing oligosaccharide chains attached to a polypeptide core. It is now becoming widely accepted that polyisoprenol-linked mono- and oligosaccharides function as activated glycosyl carriers in the biosynthesis of some glycoproteins in animal tissues. The lipophilic glycosyl carrier of monosaccharides is the phosphomonoester of dolichol, the C(80-100)-polyisoprenol, containing a saturated terminal isoprene unit. In this biosynthetic process, sugars are initially transferred to dolichol monophosphate from their nucleotide derivatives by membrane-associated glycosyltransferases. These dolichol-linked monosaccharides serve as glycosyl donors in the glycosylation of oligosaccharide phospholipids. It appears likely that dolichol is also the lipid moity of the oligosaccharide intermediates. Detailed enzymatic studies with oligosaccharide phospholipids formed by rat liver, a mouse myeloma tumor and hen oviduct have revealed that these intermediates function as oligosaccharide donors in the assembly of at least one class of glycoproteins. The exact nature of the glycoproteins glycosylated by lipid intermediates and the sub-cellular site(s) of this assembly process remain to be established. The possibility, that the mannose and GlcNAc-containing core found in many glycoproteins, is assembled at the lipid-level is now being investigated. At the current rate of progress in this area of research, the identity of the glycoproteins glycosylated via lipid intermediated and the subcellular site of this assmebly process will soon be known.

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.

Biosynthesis of retinol in bovine corpus luteum tissue

The bovine corpus luteum was found to contain retinol, as analyzed by fluorometry and the formation of anhydroretinol. After the incubation of beta-[15,15 foot-3H] carotene with slices of corpus luteum tissue the biosynthesis of radioactive retinol was demonstrated. Alumina column chromatography, thin layer chromatography, and high speed liquid chromatography were used to isolate and identify radioactive vitamin A. Retinol was also found to form a polar complex, from which could be released by acid hydrolysis.

Synthesis P1-dolichyl P2-alpha-D-mannopyranosyl pyrophosphate. The acid and alkaline hydrolysis of polyisoprenyl alpha-D-mannopyranosyl mono- and pyrophosphate diesters

P1-Dolichyl P2-ALPHA-D-mannopyranosyl pyrophosphate (9) has been chemically synthesized by a method developed for the corresponding citronellyl derivative, which also contains a saturated alpha isoprene residue. In each case, the P1-polyisoprenyl P2-diphenyl pyrophosphate was treated with 2,3,4,6-tetra-O-acetyl-alpha-D-mannopyranosyl phosphate to give a fully acetylated pyrophosphate diester, which was purified chromatographically and subsequently deacetylated. The citronellyl and dolichyl pyrophosphate diesters were compared with the previously synthesized citronellyl and dolichyl alpha-D-mannopyranosyl phosphate, respectively, by chromatography and by hydrolysis experiments. Good separations of the monophosphate from the corresponding pyrophosphate were achieved by silica gel tlc in a variety of solvent systems. Brief dilute acid hydrolysis of both the mono- and pyrophosphate diesters gave D-mannose and no alpha-D-mannosyl phosphate, the other products being polyprenyl phosphate and pyrophosphate, respectively. When the polyprenyl alpha-D-mannopyranosyl mono- and pyrophosphate diesters were treated with hot dilute alkali, the major products were polyprenyl phosphate and substances arising from the breakdown of D-mannose, indicating that the alpha-D-mannosyl phosphate bond was the most labile linkage in both compounds. However, the formation of a small proportion of free dolichol indicated that alpha-D-mannosyl phosphate was also formed to a minor extent. The interpretation of the results of the alkaline hydrolysis was complicated by the instability of D-mannose under basic conditions, it being almost completely degraded by even a brief treatment.