Circulating levels and molecular distribution of the acid-labile (alpha) subunit of the high molecular weight insulin-like growth factor-binding protein complex.
A RIA is described for the acid-labile (alpha) subunit of the high mol wt insulin-like growth factor (IGF)-binding protein complex, a glycoprotein of approximately 85,000 daltons (approximately 85K) which combines with the GH-dependent binding protein (BP-53 or IGFBP-3) and IGF-I or IGF-II to form the complex. The assay shows relative specificity for higher primate species. Whereas amniotic fluid, cerebrospinal fluid, and seminal plasma contain virtually no immunoreactive alpha-subunit, the protein is easily detectable in 0.5-microL serum samples. Serum alpha-subunit levels are markedly age dependent, rising over 5-fold from birth to puberty, then remaining relatively constant throughout adulthood. In 170 children, there was a strong association between alpha-subunit and IGFBP-3 levels. Mean alpha-subunit levels (+/- SD) in adults were 24.2 +/- 4.7 mg/L in 93 normal subjects, 54.1 +/- 15.5 mg/L in 12 acromegalics, 6.5 +/- 4.8 mg/L in 10 GH-deficient subjects, and 31.5 +/- 5.7 mg/L in 18 third term pregnant women. In serum fractionated by gel chromatography, alpha-subunit appeared as a broad 100-150K peak. After depleting serum samples of IGFBP-3 by immunoaffinity chromatography, approximately one third of alpha-subunit remained, in a peak of about 100K, suggesting that two thirds of the total alpha-subunit in serum is present in the approximately 150K complex, and one third is uncomplexed. Development of this RIA for alpha-subunit will allow further study of regulation of the GH-dependent complex.
Department of Endocrinology, Royal Prince Alfred Hospital, Camperdown, New South Wales, Australia.
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Baxter
D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D002352:Carrier Proteins / Q000097:blood* / Q000276:immunology; D002648:Child; D002675:Child, Preschool; D002846:Chromatography, Affinity; D005260:Female; D006023:Glycoproteins / Q000097:blood* / Q000276:immunology; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D018969:Insulin-Like Growth Factor Binding Proteins; D007334:Insulin-Like Growth Factor I / Q000032:analysis / Q000276:immunology; D007335:Insulin-Like Growth Factor II / Q000032:analysis / Q000276:immunology; D008297:Male; D008875:Middle Aged; D008970:Molecular Weight; D011247:Pregnancy; D011863:Radioimmunoassay
D002352:Carrier Proteins; D006023:Glycoproteins; D018969:Insulin-Like Growth Factor Binding Proteins; D007334:Insulin-Like Growth Factor I; D007335:Insulin-Like Growth Factor II