Mannosylation of endogenous and exogenous phosphatidic acid by liver microsomal membranes. Formation of phosphatidylmannose

High-performance liquid chromatography of phosphatidic acid

This paper reviews existing high-performance liquid chromatographic (HPLC) methods for the analysis of phosphatidic acid (PA) in various sample matrices. In addition to the introductory background discussion on important aspects of PA in lipid biochemistry, the review provides comprehensive coverage in the areas of derivatization techniques, detection methods, and HPLC separation techniques. Conversions of PA to suitable derivatives enhance the detection sensitivity and improve the chromatographic behavior of the analytes. Detection methods include the use of state-of-the-art detectors and are discussed in terms of sensitivity, specificity, and compatibility with analytical systems. Pertinent normal-phase and reversed-phase HPLC data for PA are compiled from published methods.

N-glycosylation of membrane glycoproteins in retinol-deficient rat liver

The effect of vitamin A deficiency on N-linked oligosaccharides of membrane glycoproteins was studied in rat liver in order to evaluate the suggested role of retinol in protein N-glycosylation. First, oligosaccharides of newly synthesized glycoproteins from rough endoplasmic reticulum of vitamin A deficient liver were compared with that of pair-fed controls. Oligosaccharides were metabolically labelled with D-[2-3H]mannose, released from the glycoproteins with endoglycosidase H, purified by reversed phase HPLC and ion exchange chromatography, and were reduced with sodium borohydride. HPLC fractionation of the oligosaccharide alditols showed that the glycoproteins carried mainly four oligosaccharide species, Glc1Man9GlcNAc2, Man9GlcNAc2, Man8GlcNAc2 and Man7GlcNAc2, in identical relative amounts in the vitamin A deficient and the control tissue. In particular, no increase in the proportion of short chain oligosaccharides was noted in vitamin A deficient liver. Second, the number of N-linked oligosaccharides was estimated in dipeptidylpeptidase IV (DPP IV), a major glycoprotein constituent of the hepatic plasma membrane, comparing the newly synthesized glycoprotein from rough endoplasmic reticulum and the mature form of DPP IV from the plasma membrane. No evidence was obtained that retinol deficiency caused incomplete glycosylation of this membrane glycoprotein. From these data, the suggested role of retinol as a cofactor involved in the synthesis of N-linked oligosaccharides of glycoproteins must be questioned.

Intracellular actions of vitamin A

Because the effects of vitamin A vary with tissue type and often with the form of vitamin A itself, a complete understanding of the mechanism(s) of action still has not been attained. The action of vitamin A may be at the level of genomic expression, at the membrane level, or both. Intercellular and intracellular transport of vitamin A are facilitated by specific binding proteins but probably not in the cellular uptake of vitamin A. Subcellularly, vitamin A may exert a direct effect on transit through the Golgi apparatus, as observed from both biochemical and morphological studies. In my laboratory, recent work using cell-free systems has shown that retinol stimulates transition vesicle formation from endoplasmic reticulum in a GTP-requiring step.

Phosphatidyl-Tris rather than N-acylphosphatidylserine is synthesized by Rhodopseudomonas sphaeroides grown in Tris-containing media

We have synthesized 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho(N-oleoyl)serine (N-acyl-PS) and 1,2-dioleoyl-sn-glycero-3-phospho-Tris (phosphatidyl-Tris) and have characterized both phospholipids by their chemical and chromatographic properties, as well as by their IR, 13C NMR, and 1H NMR spectra. Comparison of these data with those reported for a phospholipid isolated from Rhodopseudomonas sphaeroides grown in Tris-supplemented media [Donohue et al. (1982) Biochemistry 21, 2765-2773] indicates that R. sphaeroides synthesizes phosphatidyl-Tris rather than N-acyl-PS.

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.

Hydrolysis of N-acylated glycerophospholipids by phospholipases A2 and D: a method of identification and analysis

We have previously identified N-acylethanolamine phospholipids in infarcted dog heart and in normal fish brain by chemical and enzymatic degradation. We now report that hydrolysis with phospholipase D from Streptomyces chromofuscus removes N-acylethanolamine from N-acylethanolamine phospholipids and lyso N-acylethanolamine phospholipids, or N-acylserine from lyso N-acylserine phospholipids. At acidic pH, a phosphatase present in the phospholipase D preparation further hydrolyzes the resulting phosphatidic acid (PA) or lyso-PA to diacyl- or monoacylglycerol. Because N-acylserine phospholipids are a poor substrate for the phospholipase D, pretreatment with phospholipase A2 (Trimeresurus flavoviridis venom) is used to remove the 2-O-acyl group. Thus, both types of N-acylated phospholipids can be analyzed by consecutive phospholipase A2 and phospholipase D treatment. Reaction products, i.e., free fatty acids, monoacylglycerols and N-acylethanolamine or N-acylserine, are separable by thin-layer chromatography. Both N-acyl components can be further characterized by conversion to the t-butyldimethylsilyl derivatives. The method was used to identify and analyze the N-acylserine phospholipids of bovine brain.