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Liver nuclei, prepared from normal and vitamin A-deficient rats, were incubated in the presence of GDP-(14C)mannose or UDP-N-acetyl(14C)glucosamine and the labelled glycoproteins analysed by SDS PAGE. Fluorographic analysis has shown that (14C) mannose labelling is enhanced by vitamin A deficiency whereas N-acetyl(14C)glucosamine transfer remains approximately at the same level regardless of the vitamin A status; we did not notice any modification when the proteins were monitored by Coomassie blue or by silver nitrate.

Aspergillus niger van Tieghem microsomes contain an enzyme that catalyzes mannose transfer from GDP-mannose to polyprenylphosphate. The studies of the specificity of this enzyme for both the sugar donor (nucleoside diphosphate sugar) and the acceptor (polyprenylphosphates that were made available to the enzyme by means of the fusion of acceptor-loaded liposomes with the microsomal membranes) gave the following results. i) All the polyprenylphosphates from C15 to C120 were acceptors except retinylphosphate. ii) The specificity of the enzyme for both the sugar and the base is very strict.

Summary Microsomal glycoprotein-mannosyltransferase, -galactosyltransferase and -N-acetyl-glucosaminyltransferase utilizing endogenous acceptors were characterized in the epithelial cells of rat small intestine. Galactosyltransferase gave a Km value of 14,6 × 10 −6 M with respect to UDP-galactose. A pH optimum of 6,7 and a temperature optimum of 22°C were observed. The enzyme shows an absolute requirement for Mn ++ , for which the optimal concentration is 0,01 M. On the other hand, mannosyltransferase and N-acetyl-glucosaminyltransferase did not require divalent cations and were not inhibited by EDTA 10 −3 M. They have a pH optimum of 6,2 and a temperature optimum respectively of 37°C and 28°C. The Km value for the glycosyl donors, GDP-mannose and UDP-N-acetylglucosamine, were 6 × 10 −5 and 4,35 × 10 −4 M. Each of these enzymes is strongly bound to the membranous structure; Triton X-100 causes a 2 to 3 fold enhancement of incorporation of mannose, galactose and N-acetylglucosamine when a suitable detergent: protein ratio is applied. ATP, UTP and GTP had no significant effect on incorporation. A discontinuity in heat denaturation curves reveals at least two enzymatic activities for each glycosyltransferase.

Resume L'incorporation in vivo de d -[1-14C]glucosamine ou de d -[1-14C]galactose ou de d -[1-14C]mannose permet de marquer un constituant glycoproteique de la muqueuse intestinale de rat. Extrait par trypsination moderee, ce constituant est purifie par filtration en gel de Sephadex G-200, suivie de chromatographie sur hydroxylapatite et d'electrophorese preparative sur bloc d'acetate de cellulose. Cette glycoproteine, qui migre comme une α2-globuline, a une constante de sedimentation de 14 S; son poids moleculaire apparent, estime par filtration sur Sepharose 6 B, est voisin de 560 000. Sa composition en acides amines et sa teneur en oses neutres (galactose plus mannose:7.8%), en 6-desoxyhexose (fucose:2.6%), en osamines (glucosamine plus des traces de galactosamine:7.7%), en acide sialique (3.3%) sont determinees. Elle ne contient ni sulfate, ni acides uroniques. Elle ne presente aucune reaction d'identite immunologique avec les proteines seriques. Cette etude confirme l'existence d'une biosynthese glycoproteique active et donne une methode d'obtention d'un accepteur exogene specifique pour les glycosyltransferases mises en evidence dans un precedent travail (J. Frot-Coutaz and R. Got, Biochimie, 53 (1971) 595) au niveau de la muqueuse intestinale de rat.

The present studies are concerned with the subcellular localisation of an active multiglycosyltransferase system involved in the transfer of galactose, N-acetylglucosamine and mannose to various glycoprotein acceptors. Smooth microsomes, obtained according to the method of Dallner, were the main loci of the three glycosyltransferases. After placing smooth microsomes in the bottom of a sucrose gradient, we recovered galactosyltransferase and mannosyltransferase in a fraction (density 1.12) containing Golgi apparatus and endoplasmic reticulum and N-acetylglucosaminyltransferase in a non-identified fraction with a density from 1.14 to 1.16; these results are obtained when performing all enzyme assays on endogenous acceptors. When exogenous acceptors was used, galactosyltransferase N-acetylglucosaminlytransferase and mannosyltransferase appear to be present in fractions having density of 1.14–1.16, 1.18 and 1.12, respectively.

Nuclei and non-nuclear membranes were tested for their ability to transfer in vitro (14C)mannose from GDP-(14C)mannose to endogenous glycoprotein acceptors in the presence and in the absence of exogenous retinyl-phosphate. Electrophoretic analysis shows that retinylphosphate is responsible for the labeling of a few endogenous acceptors only in the non-nuclear membranes; in the nuclei the mannosylation reaction is not retinylphosphate dependent and the electrophoretic profile of the labeled protein acceptors is different from that of the non-nuclear membranes.