Developmental regulation of glucosidase I, an enzyme involved in the processing of asparagine-linked glycoproteins in rat mammary gland

Glucosidase I involved in the processing of N-linked glycoproteins was purified to homogeneity from the lactating rat mammary gland. The purified enzyme exhibited a single band at 85 kDa on 10% sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing conditions. Polyclonal antibodies raised against the enzyme recognized a similar band on Western blots and also inhibited the enzyme activity. The enzyme levels gradually increased until the midlactation stage and thereafter declined sharply during the period of postlactation. A similar profile of the levels of immunoreactive glucosidase I was observed. These findings suggest that the accumulation of glucosidase I is modulated as a function of gland ontogeny. The results on hormonal regulation of glucosidase I indicate that the synthesis of the enzyme is stimulated by a combination of insulin, hydrocortisone, and prolactin; additionally, epidermal growth factor may play a role in this regulation. The above observation was substantiated by immunoprecipitation of [35S]methionine-labeled microsomal extracts with anti-glucosidase I antibodies. The immunoprecipitation of soluble extracts from [35S]methionine-labeled tissue with anti-rat alpha-lactalbumin antibodies indicates that these hormones not only stimulate the synthesis of alpha-lactalbumin but also play an important role in its glycosylation.