Effects of 1-palmitoyl-sn-glycero-3-phosphorylcholine on mouse liver microsomal sialyltransferase

Effects of glycosylation enzymes from membrane fractions, induced by chronic ethanol administration

To study the effect of chronic ethanol administration on the enzyme activities involved in the glycosylation processes (glycosidases and glycosyltransferases), we used 6-week ethanol-treated female Wistar rats and pair-control rats. Biological material was membrane fractions of microsomes and plasma membrane obtained by subcellular fractionation technique. Ethanol treatment increased with statistical significance the Vmax of N-acetyl-beta-D-glucosaminidase (P less than 0.10), beta-D-glucuronidase and alpha-D-mannosidase (P less than 0.001) activities from microsomal fractions and likewise it increased the Km value of beta-D-glucuronidase (P less than 0.001). In vitro doses of ethanol (0.1-1.7 M) were added to delucidate hypothetical membrane tolerance responses. However in vitro addition of ethanol provoked no differential effects in treated and untreated rats. Glycosyltransferase assays were carried out using [14C]sugar derivatives. We detected several glycosyltransferase activities in both microsome and plasma membrane fractions. Chronic ethanol exposure appeared to affect N-acetyl-neuraminyltransferase activity from both membrane fractions, producing a greatly increased incorporation in the presence of asialofetuin. Glucosyl and mannosyltransferase activities from plasma membrane fractions were also altered by ethanol treatment, producing an increased enzyme activity when reaction was performed in the presence of phosphatidylcholine + dolichol-phosphate liposomes.

Breaks in arrhenius plots of reactions involving membrane-bound and solubilized sialyltransferases, due to temperature dependence of kinetic parameters

Temperature dependence of asialomucin-sialyltransferase (CMP-N-acetylneuraminate:D-galactosyl-glycoprotein N-acetylneuraminyltransferase, EC 2.4.99.1) activity is investigated. Discontinuities in Arrhenius plots are observed, whether the enzyme is membrane-associated or solubilized. These discontinuities cannot be firmly correlated with the phase-transition temperatures of either endogenous or exogenous phospholipids. Arrhenius plots of the kinetic parameters also exhibit sharp discontinuities, so that it is concluded that a significant change in Km and Vmax values occurs with varying temperature. Our results suggest that the biphasic behavior of Arrhenius plots may be attributed to the temperature dependence of the kinetic parameters for both membrane-associated and solubilized sialyltransferase activities.

Temperature dependence of membranous and solubilized sialyltransferase activities in the presence of 1-palmitoyl-sn-glycero-3-phosphorylcholine and fatty acids

The temperature dependence of sialyltransferase (CMP-N-acetylneuraminate: D-galactosyl-glycoprotein N-acetyl-neuraminyltrasferase, EC 2.4.99.1) inhibition is described when 1-palmitoyl-sn-glycero-3-phosphorylcholine, or a saturated fatty acid (lauric, myristic or palmitic acid) or an equimolar mixture of the two components are added. Lysophospholipid and fatty acids have no appreciable effect on the optimal temperature for sialyltransferase activity. In the presence of lysophospholipid, the membranous sialyltransferase activity is decreased for all the temperature range tested. In contrast, the solubilized sialyltransferase activity is decreased for temperatures exceeding 29 degrees C. In the presence of saturated fatty acids, the membranous activity is decreased above a chain-length dependent temperature: 22 degrees, 25 degrees and 30 degrees C for lauric, myristic and palmitic acids, respectively. In contrast, the solubilized activity remains unchanged. In the presence of equimolar mixtures of lysophospholipid and fatty acid, the membranous activity is decreased above the same critical temperature as that described for fatty acids added alone. In contrast, the solubilized activity is decreased above 29 degrees C. From these observations, it is suggested that lysophospholipid inhibits the solubilized enzyme when the temperature exceeds the critical micellar temperature of this lipid. The fatty acids inhibit the microsomal enzyme probably by incorporating into the membrane. It is also suggested that equimolar mixtures of lysophospholipid and fatty acid give rise to molecular analogs of 1,2-dipalmitoyl-sn-glycero-3-phosphorylcholine.

Biochemical evidence for the role of alkyl-lysophospholipids on liver sialyltransferase

Antineoplastic alkyl-lysophospholipids were found to exert a strong inhibitory effect on membranous or solubilized asialomucin-sialyltransferase (CMP-N-acetylneuraminate: D-galactosyl-glycoprotein N-acetylneuraminyltransferase, EC 2.4.99.1) activity. This inhibitory effect was dependent on the presence of the choline moiety in position 3 of the glycerol molecule, as well as on the presence of long ether-linked aliphatic side chain in position 1 and the absence of any large substituent in position 2. As an example, 1-octadecyl-2-O-methyl-glycero-3-phosphorylcholine acted as a mixed-type inhibitor. Such an inhibitory process on sialyltransferase activity might be an additional factor in the tumor cell destructive effect of alkyl-lysophospholipids.