N-Linked protein glycosylation was examined in vitro in dispersed rat parotid acinar cells from young adult (3–6 months) and aged (22–24 months) rats. A small decrease in general protein production was observed with cells from aged animals (∼20% lower incorporation of [(14)C]leucine into 10% CCI(3) COOH insoluble protein during continuous pulse labeling). Incorporation of [(3)H]mannose into N-linked glycoproteins by aged cells was further reduced (∼35%). Similarly microsomal membranes from parotid glands of aged animals showed ∼50% reduction in the synthesis of mannosylphosphoryl dolichol, a key intermediate in the dolichol pathway of protein N-glycosylation. Man-P-Dol synthase, the microsomal enzyme responsible for production of this saccharide-lipid, displayed no change in apparent K(m) for GDP-mannose when preparations from aged animals were utilized, but did show ∼50% reduction in V(max). Following β-adrenoreceptor activation, cells from both young adult and aged glands showed increased N-linked protein glycosylation almost to the same extent (∼2-fold). The data suggested that in aged rat parotid cells there is a basal reduction of activity in the pathway responsible for asparagine-linked protein glycosylation, but that following exocytotic stimuli this pathway responds in a manner comparable to cells from young adult glands.
N-linked protein glycosylation in the rat parotid gland during aging