Immunoblot studies of the IGF-related acid-labile subunit

Insulin-like growth factors I and II (IGF-I and II) are present in serum primarily within a ternary complex consisting of IGF, IGF-binding protein- 3 (IGF-3) and acid-labile subunit (ALS). Relatively little is known about ALS as compared to the other components of the complex. We report immunoblot studies of ALS using a new rabbit antiserum to human ALS_1-34. The antiserum shows high specificity for ALS, labelling only the intact 82-88 kDa doublet in whole serum. Treatment with endoglycosidase-F leads to only a partial deglycosylation of ALS in whole serum, while purified ALS is reduced to Mr~58 kDa. Acidification of both whole serum and purified ALS leads to a complete loss of ALS ability to bind to cross-linked IGFBP- 3:[¹²⁵I]IGF-II tracer; however, immunoblot studies show no change in the apparent Mr of the major ALS band. Immunoblot studies of human serum shows that intact ALS is decreased in growth-hormone (GH) deficiency, increases with GH treatment, is elevated in GH excess and is unchanged with IGF-I treatment. These data provide new information regarding the characteristics of ALS and demonstrate the research utility of a highly-specific antiserum for this protein.