Acid Labile Subunit ELISA

The Insulin-like Growth Factors (IGF) – I and II are bound to specific binding proteins in circulation
(IGFBP). Until today seven different proteins have been identified IGFBP-1 to 7. IGF
bioavailability, transport and storage is regulated or facilitated by these binding proteins which are
expressed differentially according physiological and developmental requirements. The most abundant
IGFBP in circulation is IGFBP-3. Together with IGFBP-5 it is able to form the so called ternary complex
with IGF and the acid-labile subunit (ALS). In the circulation nearly all IGF is bound in this ternary
complex and thus not able to cross the endothelial barrier. Only very small amounts of IGF or IGFBP-3
exist outside this complex. The acid-labile subunit is an important part of the IGF-storage
mechanism in circulation. In ALS deficiency or in ALS knock-out mice the concentration of IGF and
IGFBP-3 in the circulation is significantly decreased resulting in impaired growth.

The acid-labile Subunit, is a synthesized as propeptide of 605 amino acids. The signal peptide,
necessary for ALS secretion (AA 1-27) cleaved off enduring the transport process (Swiss-Prot P35858
Version 82). The mature protein consists of 578 amino acids and contains about 20 leucin rich
sequence repeats. Beside the leucin-rich repeats several potential N-linked glycosylation sides have
been described. Miller BS et al. were able to demonstrate that incomplete glycolsylation of IGFs, ALS
and IGFBP-3 results in a decreased serum concentration of these proteins. Oral mannose therapy
resulted in a partial normalization of the glycosylation pattern and went along with improved growth.

Mutations in or the complete knock out of the ALS gene result in IGF / IGFBP-3 deficiency and
therewith in distribution of growth. Beside growth also other endocrine axes may be involved. In
primary ALS deficiency hyperinsulinemia could be observed. Further, the HGH-IGF-IGFBPsystem
seems to be of relevance in coronary disease.

The first ALS immunoassay was described by Baxter RC in 1990 [6]. By this in-house
radioimmunoassay it was shown that ALS is present in high concentrations in serum (50µg/mL) of
healthy humans. But not detectable in other body fluids like amniotic fluid, cerebrospinal fluid or
seminal plasma – in spite of the fact that these body fluids contain high level IGFBP-3.