Biosynthesis of glycoproteins in the intestinal Mucosa3-I. Soluble state of five glycosyl-transferases

Characterization and purification of fucosyltransferases from the cytosol of rat colon

The occurrence and baseline characteristics of fucosyltransferases (alpha-1,2, alpha-1,3 and alpha-1,4) in the cytosol (soluble) and pellet (membrane-bound) of rat colon have been studied since the fucosylation process is known to alter in colon pathology. All enzymes studied in the colon pellet had higher activity when compared to the cytosol. The colon pellet alpha-1,3 fucosyltransferase preferred desialylated alpha 1-acid glycoprotein as acceptor substrate. Both soluble and membrane-bound enzymes, alpha-1,2 and alpha-1,3 fucosyltransferases, required Mn2+, Mg2+ and Ca2+ for maximum activity but were inactivated by Cu2+ ions. Both soluble alpha-1,2 and alpha-1,3 fucosyltransferases showed optimal activity at pH 6.0, whereas the optimum for their membrane-bound activities were at pH 5.8 and 6.2, respectively. Furthermore, a soluble alpha-1,3 fucosyltransferase from rat colon was purified and during purification the co-presence of alpha-1,3/4 fucosyltransferase was detected. The acceptor of preference for the purified soluble alpha-1,3 fucosyltransferase was desialylated glycoprotein while low molecular weight substrates were poor acceptors. Both the purified fucosyltransferases were inhibited by N-ethylmaleimide. The M(r) values determined by SDS-PAGE electrophoresis of alpha-1,3/4 fucosyltransferase and of alpha-1,3 fucosyltransferase were 68,780 and 40,680 respectively. In conclusion, based on their properties, the purified soluble colon alpha-1,3 fucosyltransferase appeared to be of plasma-type (or FT-I) while the soluble alpha-1,3/4 fucosyltransferase corresponded to Lewis-type or FT-III.

In vitro synthesis of colominic acid by membrane-bound sialyltransferase of Escherichia coli K-235. Kinetic properties of this enzyme and inhibition by CMP and other cytidine nucleotides

The membrane-bound sialyltransferase obtained from Escherichia coli K-235 grown in a chemically defined medium (ideal for colominic acid production) was studied. The in vivo half-life calculated for this enzyme was 20 h. Kinetic tests revealed (at 33 degrees C and pH 8.3) hyperbolic behaviour with respect to CMP-Neu5Ac (Km250 microM) and a transition temperature at 31.3 degrees C. The enzyme was inhibited by NH4+, some divalent cations and by several agents that react with thiol groups. Detergents and fatty acids also inhibited the sialyltransferase activity. In vitro synthesis of colominic acid is strongly inhibited by CMP by blocking the incorporation of [14C]Neu5Ac into a protein-complex intermediate and therefore into free polymer. CDP and CTP also inhibited (91% and 84%) this enzyme activity whereas cytosine and cytidine had no effect. CMP inhibition corresponded to a competitive model the calculated Ki was 30 microM. Incubations of protein[14C]Neu5Ac with CMP, CDP and CTP led to de novo synthesis of CMP-[14C]Neu5Ac. The presence of colominic acid, which usually displaces the reaction equilibrium towards polymer synthesis, did not affect this de novo CMP-[14C]Neu5Ac formation. CMP also inhibited in vivo colominic acid biosynthesis.

Purification and separation of two soluble fucosyltransferase activities of small intestinal mucosa

1. Rat small intestinal soluble fucosyltransferase is purified more than 2000-fold using chromatographic procedures with DEAE-cellulose, CM-cellulose, GDP-Sepharose and Concanavalin A-Sepharose. 2. Chromatography on Sephadex G15 of the final enzymatic fraction clearly separates two activities: a first peak incorporates fucose on asialoserotransferrin and a second peak on asialofetuin. 3. The use of small saccharidic acceptors (phenylgalactose, lactose, lacto-N-fucopentaose I) and the analysis of fucosylated asialoglycoproteins indicate that the first activity corresponds to an alpha-(3/4)-fucosyltransferase and the second one to an alpha-(1-2)-fucosyltransferase. 4. Protein analysis by polyacrylamide gel electrophoresis in the presence of SDS for each enzyme shows two bands corresponding to a mol. wt of about 65,000 and 70,000. The two enzymes have the same sensitivity to the action of N-ethylmaleimide.

[Kinetic parameters of a soluble purified intestinal fucosyl-transferase]

The soluble fucosyl-transferase of the rat small intestinal mucosa was purified by passing through a Sephadex G-100 column, then resolved in two isoenzymes F1 (pHi = 4,47) and F2 (pHi = 4,96) by isoelectric focusing. The use in the first step of DEAE-cellulose instead of Sephadex G-100 leads to another component F3 (pHi = 8,70). Kinetic parameters (KM et Vm) for the three isoenzymes are given. The enzymatic mechanism seems to be a bi-bi random type for the three isoenzymes, and F3 appears as the most active species.

[Mechanism of lipid inhibitory effect on intestinal fucosyl-transferase in the rat]

High-fat diets decrease microsomic and soluble fucosyl-transferase activities in rat intestinal mucosa. This inhibition is not due to qualitative (pHi) or quantitative (relative activities) changes ion three isoenzymes, nor is it caused by alterations in the kinetic behaviour of these enzymes (Km, V). It is also not due to a direct effect of the fatty acids on the enzyme activities. It seems reasonable to suggest that a decreased biosynthesis of the fucosyl-transferase occurs as a result of the high-fat diets.