No abstract

Four glycosyl transferases solubilized by detergents from Saccharomyces cerevisiae membranes have been tested for their specificity towards various polyprenols differing in chain length and saturation.1. The formation of dolichyl diphosphate-(N-acetylglucosamine)l-2 from uridine diphosphate N-acetylglucosamine and dolichyl monophosphate showed an obligatory requirement for a-saturated polyprenols. The apparent affinity for a-saturated polyprenyl phosphates improves with increasing chain length; those shorter than seven isoprene units were inactive.2. The rate of transfer of the chitobiosyl unit from dolichyl diphosphate chitobiose to the hexapeptide Tyr-Asn-Leu-Thr-Ser-Val followed the sequence :3. The enzyme transferring the mannosyl group from guanosine diphosphate mannose to dolichyl monophosphate also showed a clear dependence on a-saturation and chain length. In this case the chain length affects mainly the maximal velocity of the reaction. The highest value was obtained with C55 a-dihydropolyprenyl phosphate (100 nmol x h-' x mg protein-'), whereas the value with C35 a-dihydropolyprenyl phosphate was only 8 nmol x h-' x mg protein-'.4. The mannosyl transfer from dolichyl monophosphate mannose to dinitrophenylated tetrapeptide Asn-Ala-Thr-Vol to form on O-glycosidic bond strongly depended on a-saturation and chain length of the polyprenyl phosphate. The rate of transfer correlated directly with the length of the polyprenyl residue. With c 8 0 a-dihydropolyprenol less than half the rate of the C~O O compound was observed, whereas C55 and C X a-dihydropolyprenol are only 20 and 10 % as active, respectively.The biosynthesis of asparagine-linked oligosaccharide units of glycoproteins most likely occurs in all eucaryotic cells by a rather complex sequence of reactions which has been called the dolichol pathway [l-51. An oligosaccharide consisting of 14 hexose units is first built up sequentially onto dolichyl phosphate followed by a transfer en bloc of the preformed oligosaccharide to the nascent polypeptide chain. Subsequently part of the sugar residues is removed and thereafter sugars which compose the outer branches of complex type oligosaccharides are added [6 -81. In addition dolichyl phosphate has been shown to be involved also in the formation of the O-glycosidic linkage of glycoproteins ; however, up to now this role has only been demonstrated for fungal cells [5].

Apart from the long-chain dolichols (C80-C110), pig liver contains also a family of much shorter polyprenols with dominating C55-polyprenol. This compound was identified as cis/trans-dihydroundecaprenol in which the OH-terminal isoprene residue was saturated. The number of internal trans isoprene residues in this compound was three in comparison with two such residues in long-chain C95-dolichol. Chemical preparation of dihydroundecaprenol with a selectively saturated OH-terminal isoprene residue from fully unsaturated plant undecaprenol is presented.

The capacity of isolated membrane fractions to catalyse transfer of sugars from sugar nucleotides to alpha-saturated and non-saturated forms of phosphorylated C85 and C55 polyprenols and retinyl phosphate was examined. The amount of endogenous lipid acceptor present for various sugars was also measured. It appears that the types and amounts of polyprenyl phosphates present in rough- and smooth-microsomal fractions and Golgi membranes are different and the individual polyprenyl phosphates exhibit specificity as sugar acceptors.