D-Mannose uptake by fenugreek cotyledons

The uptake of D-mannose was studied in detached cotyledons of germinated fenugreek (Trigonella foenum-graecum L.) seeds. Uptake kinetics indicate the involvement of two components, a saturable component operating at low concentrations and a diffusion-like one at high concentrations. Treatment of cotyledons with carbonyl-cyanide-m-chlorophenylhydrazone and p-chloromercuribenzenesulfonic acid reduced D-mannose-uptake rates by about 35% and 35–65%, respectively. No difference in the uptake rates was observed in the presence of D-galactose or 3-O-methylglucose. D-Mannose uptake was not very much affected by pH. The optimum pH for its uptake was 6.5 while at pH 8.5 its uptake was reduced by 22%. D-Mannose addition to fenugreek cotyledons did not induce alkalinization of the medium. Furthermore, low turgor, which enhances proton/sugar cotransport, decreased D-mannose uptake while the uptake of 3-O-methylglucose was increased. The rate of D-mannose uptake by fenugreek cotyledons depended on the hours of imbibition. These changes of uptake were not followed by analogous changes in the turgor pressure (_p) of fenugreek cotyledons, which remained fairly constant. Results indicate that D-mannose is partially taken up by a carrier which has high specificity for D-mannose, but not by a H⁺-sugar cotransport system. It is further concluded that the carrier plays an important role in switching on and off the uptake capacity of fenugreek cotyledons during seedling development.Abbreviations and symbols CCCP carbonylcyanide-m-chlorophenylhydrazone - DTT dithiothreitol - 3-OMG 3-O-methylglucose - PCMBS p-chloromercuribenzensulfonic acid - water potential - _s osmotic potential - _p turgor pressure     

Heterogeneous turnover of terminal and core sugars within the carbohydrate chain of dipeptidylaminopeptidase IV isolated from rat liver plasma membrane

Dipeptidylaminopeptidase IV, a plasma membrane-bound glycoprotein, is characterized by an intramolecular heterogeneous turnover of the protein backbone and carbohydrate chain. The faster turnover of the latter is restricted only to the outer sugars. The inner core sugars D-mannose and N-acetyl-D-glucosamine turn over at the same rate as the protein backbone.