Carrier-mediated Translocation ofUridine Diphosphate GlucoseintotheLumen ofEndoplasmic Reticulum-derived Vesicles fromRatLiver

Radiolabeled UDPGlcincubated withroughendoplasmic reticulum (RER)-derived microsomes fromratliver became associated withthevesicles. Thismicrosomal uptake ofnucleotide sugar wastimeandtemperature dependent. Analysis of themolecular species containing radiolabel revealed that initial uptake represented entry ofpredominantly intact UDPGIc inthemicrosomes. Conclusive evidence forproper translocation ofUDPGlcacross themicrosomal membrane into the intravesicular spacewasobtained bydemonstrating that UDPGlcwastransported into anosmotically sensitive compartment. Microsomal uptake ofUDPGlcexhibited features characteristic ofcarrier-mediated transport including saturation, specificity, andcountertransport. Inhibition andtransstimulation studies showed thatother uridine-containing nucleotide sugars and5'-UMP weresubstrates ofthepostulated microsomal carrier system forUDPGlc,while cytosine- or guanosine-containing nucleotides andnon-5'-uridine monophosphates were, atbest, verypoorsubstrates. UDPGlc translocation activities werelower insmooth microsomal fractions thanintheRER-derived vesicles, indicating that contamination withGolgi membranes could notberesponsible for microsomal transport ofUDPGlc. Ourfindings suggest that ratliver endoplasmic reticulum possesses acarrier system mediating proper translocation ofUDPGlcand5'-uridine-substituted structural analogues across themembrane.