PURIFICATION AND CHARACTERIZATION OF THE ACID-LABILE SUBUNIT OF RAT SERUM INSULIN-LIKE GROWTH-FACTOR BINDING-PROTEIN COMPLEX

Circulating insulin-like growth factor (IGF) binding protein-3 (IGFBP-3), when occupied by IGF-I or IGF-II, combines with an acidlabile glycoprotein subunit (ALS) to form a high molecular weight complex. In this study, ALS from rat serum has been purified and its properties investigated. Purification involved ion-exchange chromatography, and affinity chromatography on an IGF-I-IGFBP-8 column, yielding almost 1 mg ALS from 100 mi serum. Amino-terminal sequencing confirmed the prediction from previous complementary DNA analysis but indicated that the protein may circulate in a truncated form. Rat ALS was almost as potent as human ALS in binding to human IGF-I-IGFBP-3 complex (association constant, 2.3 nM-(1)). A sensitive RIA was developed, with high specificity for rodent ALS. Serum ALS rose from 3 mu g/ml in 2-day-old rats to more than 40 mu g/ml at 10 weeks, with no sex difference. GH-deficient rats showed 60-75% lower values than controls. This study shows rat ALS to have similar binding properties, and age and GH dependence, to human ALS. The new RIA will facilitate studies of IGF and IGFBP regulation in the rat.