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The binding of rat liver cell multiplication-stimulating activity (MSA) to human placenta and serum proteins
Abstract:
Multiplication-stimulation activity (MSA) from the medium of BRL-3A rat liver cells in culture binds to cell membrane and cytosol receptors from human placenta and to serum proteins. The binding of MSA to placental cell membranes is dependent on time, temperature, pH and divalent ion concentration. MSA bound to placental cytosol receptor and serum is not displaced by insulin, whereas that bound to placental cell membranes is displaced by insulin and insulin-like peptides. The affinity of the three receptors for MSA is similar [approximately 10(8) M(-1)]. An assay using 125I-MSA and placental membrane receptor detects somatomedin-like receoptor activity (SmLRA) in unextracted sera from man and animals. A binding protein in serum that competes for 125I-MSA with receptor could not be completely separated from SmLRA by heating, acidification, charcoal treatment and gel chromatography of the serum. The relative activities of SmLRA and serum binding protein remained constant in three disorders of human growth (acromegaly, growth hormone deficiency and Laron's dwarfism) in which values of SmLRA varied widely. However, the binding protein is only partly responsible for the apparent SmLRA of unextracted serum. It is concluded that MSA is a suitable radioligand for the investigation of somatomedin disorders in man either by receptor assays or by studies of tissue receptors.
Insights
Multiplication-stimulation activity (MSA) binds to human placental receptors and serum proteins. This binding, influenced by various factors, suggests MSA
Area of Science:
- Endocrinology
- Cell Biology
- Biochemistry
Background:
- Multiplication-stimulation activity (MSA) is secreted by BRL-3A rat liver cells.
- MSA interacts with receptors on human placental cells and with serum proteins.
Purpose of the Study:
- To characterize the binding of MSA to placental receptors and serum proteins.
- To investigate the potential of MSA as a radioligand for diagnosing somatomedin disorders.
Main Methods:
- Investigated MSA binding kinetics (time, temperature, pH, ions).
- Examined displacement of MSA by insulin and insulin-like peptides.
- Developed an assay using 125I-MSA and placental membrane receptors to detect somatomedin-like receptor activity (SmLRA) in serum.
- Characterized a serum binding protein that competes with MSA receptors.
Main Results:
- MSA binding to placental membranes is dependent on time, temperature, pH, and divalent ions.
- Insulin displaces MSA from placental membrane receptors but not from cytosol receptors or serum.
- The affinity of MSA for placental membrane, cytosol, and serum receptors is similar (~10^8 M^-1).
- An assay detected SmLRA in human and animal sera, with a competing serum binding protein present.
- SmLRA and binding protein levels remained constant in growth disorders, though SmLRA values varied.
- The binding protein partially accounts for apparent SmLRA in unextracted serum.
Conclusions:
- MSA is a suitable radioligand for investigating somatomedin disorders.
- Receptor assays or tissue receptor studies using MSA can aid in diagnosing human growth disorders.