Characterization of polyisoprenyl phosphate phosphatase activity in rat liver

Defects in the N-linked oligosaccharide biosynthetic pathway in a Trypanosoma brucei glycosylation mutant

Concanavalin A (ConA) kills the procyclic (insect) form of Trypanosoma brucei by binding to its major surface glycoprotein, procyclin. We previously isolated a mutant cell line, ConA 1-1, that is less agglutinated and more resistant to ConA killing than are wild-type (WT) cells. Subsequently we found that the ConA resistance phenotype in this mutant is due to the fact that the procyclin either has no N-glycan or has an N-glycan with an altered structure. Here we demonstrate that the alteration in procyclin N-glycosylation correlates with two defects in the N-linked oligosaccharide biosynthetic pathway. First, ConA 1-1 has a defect in activity of polyprenol reductase, an enzyme involved in synthesis of dolichol. Metabolic incorporation of [3H]mevalonate showed that ConA 1-1 synthesizes equal amounts of dolichol and polyprenol, whereas WT cells make predominantly dolichol. Second, we found that ConA 1-1 synthesizes and accumulates an oligosaccharide lipid (OSL) precursor that is smaller in size than that from WT cells. The glycan of OSL in WT cells is apparently Man9GlcNAc2, whereas that from ConA 1-1 is Man7GlcNAc2. The smaller OSL glycan in the ConA 1-1 explains how some procyclin polypeptides bear a Man4GlcNAc2 modified with a terminal N-acetyllactosamine group, which is poorly recognized by ConA.

Synthesis of novel acceptor substrates for the dolichyl phosphate mannose synthase from yeast

Dolichols are polyisoprenoid lipid components of mammalian membranes consisting of an average of 20 head-to-tail linked isoprene units of which the first isoprene is fully saturated. The unusual size of these lipids is intriguing and poses questions about the role of dolichol structure in biological processes. In order to probe structure and function we have synthesised potential dolichyl analogues that retain only the first two isoprene units and carry a second functional group within the terminal lipid chain. Such analogues were evaluated as substrates for a key enzyme in the dolichyl-dependent pathway of glycan biosynthesis, dolichyl phosphate mannose (Dol-P-Man) synthase. It was shown that some functional groups, including labels such as biotin, could be tolerated. When the synthetic analogues were attached to a solid support they were still substrates for the Dol-P-Man system and thus allowed the enzymatic solid-phase synthesis of glycolipids.

Enzymatic synthesis of beta-mannosyl phosphates on solid support

Synthetic bifunctional analogues 4a, b and 14 of dolichol phosphate 1 were attached to solid support and were shown to be substrates for Dol-P-Man synthase.

A novel mono-branched lipid phosphate acts as a substrate for dolichyl phosphate mannose synthetase

Dolichyl phosphate mannose synthetase (GDP-mannose: dolichyl-phosphate O-beta-D-mannosyltransferase; EC 2.4.1.83) is an enzyme that is involved in glycoconjugate biosynthesis and possesses a putatively conserved dolichol binding site. In order to probe the interaction between the enzyme and the dolichol chain, lipid phosphates varying in length and extent of branching have been tested as substrates in crude microsomal preparations from Saccharomyces cerevisiae. It was found that phytanyl (3,7,11,15-tetramethylhexadecanyl) phosphate was utilized at 60-70% of the efficiency of the natural dolichyl lipid in transfer of [3,4,-3H]mannose from GDP-Man to organic soluble material, whereas addition of S-3-methyloctadecanyl phosphate, which is of similar length to the phytanyl analogue but with only one branch, resulted in approximately 25% of the incorporation of the natural substrate. Incubations with the unbranched tetradecanyl phosphate and with the short, doubly branched R- and S-dihydrocitronellyl (3,7-dimethyloctanyl) phosphates exhibited levels of activity similar to incubations with no exogenous acceptor. These results were qualitatively confirmed with experiments on Escherichia coli harbouring the S. cerevisiae DPM1 gene. The [3H]mannosylated lipid-linked material from microsomal incubations was purified by anion-exchange chromatography. The major saccharide component recovered after hydrolysis was determined to be mannose, but a mannose-containing disaccharide was also present. It is concluded that branching of lipid phosphates is essential for substrates of dolichyl phosphate mannose synthetase and that significant transfer of mannose occurs even if only branching at C-3 is present.

Dolichyl phosphate-dependent glycosyltransferases utilize truncated cofactors

Synthetic truncated dolichyl phosphates of chain lengths from four to thirteen isoprene units (Jaenicke L. and Siegmund H.-U., Chem. Phys. Lipids 51 (1989) 159-170) were assayed for their cofactor activity in the enzymatic transfer of hexoses and hexosamines. The enzymes were microsomal preparations from the green alga Volvox carteri, baker's yeast, and mammalian liver cells. Under saturating conditions, the acceptor activities of the truncated dolichyl phosphates increased from zero to full strength as compared to the mixture of long-chain dolichyl phosphates from natural sources with growing chain length from five to nine isoprene units. Km determinations confirmed the results. Of the geometric isomers of dolichyl 7-phosphate (35 carbon atoms), the 14-trans compound has unchanged acceptor activity; all-trans dolichyl 7-phosphate, however, was almost inactive. The data suggest that hydrophobicity may be an important, but not the only criterion for the binding of the isoprene moiety to the active sites of the transferase enzyme(s) and that the geometry of more than only one double bond in the dolichols is recognized.

Characterization of dolichyl diphosphate phosphatase from rat liver

Dolichyl diphosphate phosphatase (DolPPase) has been characterized in rat liver. Subcellular distribution studies indicate that the enzyme is localized in the endoplasmic reticulum. The in vitro enzymatic activity is stimulated by EDTA, due to release of inhibition by trivalent cations found in the assay tubes. All di- and trivalent cations tested were inhibitory, with the trivalent ions Al3+ and Fe3+ showing greater than 70% inhibition at a concentration of 10 microM. The assay requires the presence of a detergent for optimal activity, with Triton X-100 giving maximum activity at 0.1%. The substrate specificity of DolPPase toward polyprenyl diphosphates has been determined and indicates that there is little preference of the enzyme for substrates of different chain length, and either stereochemical orientation or degree of saturation of the alpha-isoprene unit. Km values of 11-14 microM were obtained for all substrates tested. Preliminary studies on the transmembrane topology of the DolPPase using latency assays, indicate that the active site of the enzyme may reside on the cytoplasmic face of the endoplasmic reticulum.

The biosynthesis of dehydrodolichyl phosphates by rat liver microsomes

Using improved conditions with rat liver microsomes in the presence of 20% glycerol and 2% Triton X-100 at pH 8.5 it was shown that dehydrodolichyl diphosphate and dehydrodolichyl phosphate were synthesized from isopentenyl diphosphate and farnesyl diphosphate. Small amounts of geranylgeranyl diphosphate and geranylgeranyl phosphate were also formed. The carbon chain lengths of the dehydrodolichyl diphosphate and dehydrodolichyl phosphate were identical (C80-C85). A kinetic study showed that dehydrodolichyl diphosphate formed from farnesyl diphosphate and isopentenyl diphosphate was subsequently hydrolyzed to dehydrodolichyl phosphate. As the concentration of isopentenyl diphosphate was increased from 1 to 50 microM, the chain-length distribution of dehydrodolichyl products shifted from C75-C80 to C80-C85. Addition of MgCl2 into the assay mixture decreased product formation, but did not affect the chain-length distribution (C80-C85). The shift of the chain-length distribution to the same as that observed in naturally occurring dolichol derivatives (C90-C95) was observed when Triton X-100 was omitted from the assay mixture, although deletion of the detergent decreased the enzyme activity. These results, which provide insight into optimal conditions for enzymatic synthesis of the dolichol chain, are discussed in the context of the in vivo pathway for dolichol biosynthesis.

Cell-cycle dependence of dolichyl phosphate biosynthesis

The cell-cycle dependence of dolichyl phosphate biosynthesis has been investigated in mouse L-1210 cells fractionated by centrifugal elutriation. Dolichyl phosphate levels increased linearly through the cell cycle, reaching a value in late S phase twice that of early G1. The cell-cycle dependences of four dolichyl phosphate metabolizing enzymes have been measured: cis-prenyltransferase, CTP-dependent dolichol kinase, dolichyl phosphatase, and dolichyl pyrophosphatase. The kinase, the cis-prenyltransferase, and the pyrophosphatase showed cell-cycle variations, increasing through G1 to a maximum in S phase while the monophosphatase activity was cell-cycle independent. The rate of accumulation of dolichyl phosphate was not affected by growing the cells in mevalonolactone-supplemented media. The evidence presented is consistent with models in which either the cis-prenyltransferase or the kinase/phosphatase couple (or both) regulates the levels of dolichyl phosphate in the cell.

Changes in hepatic dolichol and dolichyl monophosphate caused by treatment of rats with inducers of the endoplasmic reticulum and peroxisomes and during ontogeny

Rats were treated with various inducers of the endoplasmic reticulum and peroxisomes and the properties and distributions of dolichol and dolichyl phosphate analyzed. The treatment of rats with carcinogenic agents 2-acetylaminofluorene, N-nitrosodiethylamine and 3-methylcholanthrene and with the compounds such as phenobarbital, terpentine, cholestyramine and di(2-ethylhexyl)phthalate have all caused changes in the microsomal or lysosomal contents of dolichol to various extents, but only the latter group influenced dolichyl-P concentration. Shortly after birth, the hepatic content of dolichyl-P reaches the adult level, whereas the level of the free alcohol is low at birth but increases continuously thereafter. Incorporation of [3H]mevalonate into dolichol was also dependent on factors other than de novo synthesis, e.g., the pool size. Rates of glycosylation reactions dependent on dolichyl-P exhibit considerable changes but are independent of the existing levels of lipid intermediate. GDP-mannosyl transferase activity increases greatly with birth, but the enzyme activity returns to the adult level within a day after birth. These results demonstrate that structural and functional modifications induced with drugs can greatly influence the content and distribution of dolichol which are independent of the existing levels of dolichyl-P.