Biosynthèse des glycoconjugués pulmonaires. III. Mannosylation des accepteurs lipidiques et protéiniques

Abstract

Earlier studies on the biosynthesis of mannosyl glycoproteins in microsomes of rat lungs have been extended to show that a mannosyltransferase was able to catalyze the incorporation of mannose from GDP-mannose. In sheep lungs, we have shown that a mannosyltransferase activity is involved at microsomal level This enzymatic system is able to catalyze the incorporation of mannose from GDP-mannose into three different endogenous acceptors. The enzymatic products are as follows: a mannolipid (product A) extracted with CHCl3–CH3OH (2:1), mannose is also incorporated into a product B extracted with CHCl3-CH3OH–H2O (10:10:3), and into another product C insoluble in water and precipitable in trichloracetic acid. The kinetic studies of product biosynthesis suggest that there does not exist a precursor–product relationship. The product A is synthesized in larger quantities than the products B and C. Exogenous dolichoi-monophosphate stimulates the incorporation of [14C]mannose into mannolipid (product A) only. The results indicate that the mannolipid (product A) is not an intermediate in the transfer of mannose from GDP-mannose to the other products (B and C). The product A of the reaction, the mannolipid, has the properties of a polyprenyl-phosphoryl-mannose. This is illustrated by its lability to dilute acid but stability to dilute alkali at low temperature, and by its mobility on thin layer chromatography. However, this product A is not similar to dolichyl-monophosphoryl-mannose from pig liver.