Glycosylation of plasma proteins with [U-14C]glucose in vivo

Within 1-12 h after subcutaneous injection of [U-14C]glucose, significant amounts of 14C attached by stable (probably covalent) bonds were found in plasma proteins of rats. More than 30% of 14C of [U-14C]glucose injected per 1 g body weight was incorporated into proteins contained in 1 mL of plasma. Less than 5% of 14C of the [14C]glucose-labelled plasma proteins was soluble in cold dilute perchloric acid, whereas more than 80% of 14C in the [14C]glucose-labelled plasma proteins was soluble in 50% saturated solutions of ammonium sulfate. From [U-14C]glucose-labelled plasma proteins, approximately 50% of the incorporated 14C was recovered in carbohydrate moieties (sialic acid, 8-12%; mannose and galactose, 15-31%; hexosamines, 8-14%) and the rest of the 14C (42-64%) was recovered from protein residue. Gel-filtration and electrophoresis profiles of distribution of 14C in [U-14C]leucine-labelled plasma proteins were very similar to those of [U-14C]glucose-labelled plasma proteins; a relatively high specific radioactivity was observed in fractions having an apparent molecular weight of 105 X 10(3) or its multiples (220 X 10(3) and 520 X 10(3)). However, about 99% of the incorporated 14C was recovered from the protein residue of [14C]leucine-labelled plasma proteins, whereas only traces of 14C were found in the carbohydrate moieties. [U-14C]Alanine is also a relatively poor precursor of 14C for incorporation into carbohydrate moieties of plasma proteins as compared with [U-14C]glucose.