Biosynthesis of glycoproteins in the aortic wall. Study of intima microsomic mannosyl-transferase activity

Subcellular localisation of cerebral fucosyltransferase

GDP-fucose: asialofetuin fucosyltransferase from sheep brain was fractionated on a sucrose gradient into two activity peaks. Using purification on Ficoll adapted from the proposed method [(1980) J. Neurochem. 35, 281-296], double localisation of cerebral fucosyltransferase was confirmed and the subcellular active fractions identified as light microsomes and mitochondria.

Effect of phospholipids on the regulation of a soluble galactosyltransferase in aortic wall

A galactosyltransferase activity is located in the cell-sap of aortic intima-media cells. This enzymatic system calatyzes [14C]galactose transfer from UDP-[14C]galactose into endogenous and exogenous proteinic acceptors. Labelled products are isolated from the proteinic fraction obtained in 20% trichloroacetic acid pellet or from organic solvent extractions. Maximal [14C]galactose incorporation occurs at pH 7.8 in Tris-HCl buffer in the presence of 0.1 mM MnCl2 at 30 degrees C. The enzymatic activity is modified by phospholipids, particularly by phosphatidic acid and lysophosphatidylcholine, which behave as mixed inhibitors, while L-alpha-phosphatidylserine interacts as a competitive inhibitor. The effect of phospholipids is not stereospecific but appeared to be closely related to their polar headgroups, especially the acidic headgroups of phosphatidylcholine and phosphatidic acid. The chain length and the unsaturation degree of fatty acids involved in phospholipid structures are not a main factor of regulation. The lysophosphatidylcholine effect could be explained by its solubilization properties, as non-ionic detergents interact in the same way with galactosyltransferase activity. Exogenous phospholipids probably interact with the enzymatic environment by their own molecular arrangement and so could exert a control on galactosyltransferase activity or lead to a conformation change of this enzyme.

Effect of phospholipids on membrane-bound and solubilized mannosyltransferase activity from aortic wall

Phospholipids interact on the membrane-bound and solubilized mannosyltransferase activity. The biosynthesis of Dol-P-Man is strongly inhibited by phosphatidic acid and lysophosphatidylcholine. The effect of phospholipids is not related to stereospecificity. Chemical properties of phospholipids (ester or ether bond, length of fatty acids and polarity of head groups) are not an essential factor for inhibition. The different parameters involved in enzymatic reaction of glycosylation are not modified by phospholipids, in particular the integrity of GDP-[14C]mannose. The inhibitory effect of lysophosphatidylcholine and phosphatidic acid on mannosyltransferase activities is related to their possible formation of micellar structures which definitely induce a conformation change of this enzyme.

Glycosaminoglycan biosynthesis in arterial wall. Hexosaminyltransferase and glucuronyltransferase in cell membranes of aortic media-intima

Hexosaminyltransferase and glucuronyltransferase are well known for their role in the biosynthesis of proteoglycans. These two enzymes are characterized in rough and smooth membranes, obtained following subcellular fractionation of aortic media-intima. They require the presence of Mn2+ or Mg2+ for activity. The optimum concentration for these two cations is 5 mM. The optimum pH for hexosaminyltransferase and glucuronyltransferase is approximately 6.8 in Tris buffer, and their optimum temperature is 30 degrees C. Hexosaminyltransferase has an apparent Km of 0.27 nM. Glucuronyltransferase has an apparent Km of 0.21 nM. Uptake of labeled sugars by endogenous proteoglycans is inhibited by puromycin. Hexosaminyltransferase and glucuronyltransferase are present in both rough and smooth submicrosomal fractions. The different endogenous glycosaminoglycans are labeled during our incubation experiments. The greatest incorporation is noted for hyaluronic acid and heparan sulfate; the least is seen for chondroitin sulfate. The results obtained in vitro for incorporation of labeled precursors into endogenous proteoglycans are consistent with those observed during the study in vivo of the turnover of these macromolecules.

Collagen biosynthesis. Characterization of subcellular fractions from embyonic chick fibroblasts and the intracellular localization of protocollagen prolyl and protocollagen lysyl hydroxylases

1. Subcellular fractions of freshly isolated matrix-free embryonic chick tendon and sternal cartilage cells have been characterized by chemical analysis, electron microscopy and the location of specific marker enzymes. These data indicate the fractions to be of a high degree of purity comparable with those obtained from other tissues, e.g. liver and kidney. 2. When homogenates were assayed for protocollagen prolyl hydroxylase and protocollagen lysyl hydroxylase activities, addition of Triton X-100 (0.1%, w/v) was found to stimulate enzyme activities by up to 60% suggesting that the enzymes were probably membrane-bound. 3. Assay of subcellular fractions obtained by differential centrifugation for protocollagen prolyl hydroxylase activity indicated the specific activity to be highest in the microsomal fraction. Similar results were obtained for protocollagen lysyl hydroxylase activity. 4. Submicrosomal fractions obtained by discontinuous sucrose-gradient centrifugation were assayed for the two enzymes and protocollagen prolyl hydroxylase and protocollagen lysyl hydroxylase were found to be associated almost exclusively with the rough endoplasmic reticulum fraction in both tendon and cartilage cells.