ACID-LABILE SUBUNIT OF HUMAN INSULIN-LIKE GROWTH FACTOR-BINDING PROTEIN COMPLEX - MEASUREMENT, MOLECULAR, AND CLINICAL-EVALUATION

Although the acid-labile subunit (ALS) of the similar to 150-kDa insul in-like growth factor (IGF)-binding protein (IGFBP) complex was descri bed over a decade ago, details of ALS physiology have remained largely uncertain. We evaluated antibodies to synthetic human ALS and constru cted a noncompetitive ALS enzyme-linked immunosorbent assay. Whereas u ncomplexed ALS is directly measured, determination of total levels req uired sample pretreatment with SDS, which was found to optimally disso ciate complexed ALS and significantly enhance ALS immunoreactivity. AL S in random adult sera was approximately 50% uncomplexed, and samples devoid of complexed ALS by immunoaffinity separation contained about 5 4% of the total levels. Serum ALS fractionated by gel filtration high performance Liquid chromatography eluted in a single peak at approxima tely 150 kDa with IGF-I and IGFBP-3, but appeared at about 400-500 kDa after sample acidification and fractionation under acidic condition. The unexpected shift in ALS immunoreactivity remained unchanged when a cid-neutralized or SDS-treated samples were fractionated under neutral pH and was reproducible when the 150-kDa complex was isolated, treate d with acid or SDS, and rechromatographed. ALS in adult sera more tigh tly correlated with IGFBP-3 than IGF-I or ICF-II. The total levels (me an +/- so) were 16.7 +/- 3.7 mg/L in 22 normal subjects, 28.3 +/- 8.1 mg/L in 20 acromegalic patients, and 9.5 +/- 3.8 in 32 GH-deficient ad ults. Little or no ALS was detectable in amniotic fluid, cerebrospinal fluid, seminal plasma, or milk, whereas high levels were present in s ynovial fluid. The development of ALS enzyme-linked immunosorbent assa y should greatly facilitate further investigations of this unique glyc oprotein.