The proportion of glycosylated prolactin in serum is decreased in hyperprolactinemic states

The proportion of serum PRL that is glycosylated has been determined, and the effects of physiological and pathological hyperprolactinaemia on this proportion have been examined. Glycosylated and nonglycosylated PRL were immunoprecipitated from serum and subjected to polyacrylamide gel electrophoresis under reducing and dissociating conditions. Separated proteins were then transferred to nitrocellulose paper by electroblotting and detected immunologically with anti-PRL antiserum and ¹²⁵I-labeled protein-A, followed by autoradiography. The proportion of total monomeric PRL present in the glycosylated form was then estimated by densitometric scanning of autoradiograms. In normal individuals glycosylated PRL was predominant, accounting for about 72% of the circulating monomeric PRL. This proportion was markedly decreased (ranging from undetectable to about 60%) in the serum of women who were pregnant or were lactating postpartum and also in patients with hyperprolactinemia caused by a pituitary tumor. The results suggest that under basal conditions the majority of PRL secreted from the pituitary is glycosylated, but with physiological or pathological hyperprolactinemia the capacity for glycosylation is exceeded. (J Clin Endocrinol Metab 71: 111-115, 1990).