Identification of growth hormone binding protein in rat serum.
The present report describes the initial characterization of a specific, high-affinity growth hormone binding protein (GH-BP) in adult male rat serum. GH-BP activity was measured by incubation of rat serum with [125I]hGH and [125I]rGH and separation of bound from free GH by dextran-coated charcoal. [125I]hGH binding to rat serum was dependent on serum concentration and incubation time, equilibrium being reached within 10 min both at 4 and 37 degrees C. Binding was rapidly and completely reversible and specific for somatogenic (but not lactogenic) hormones. Scatchard analysis yielded a linear plot with an affinity (Ka) of 1.51 +/- 0.63 x 10(8) M-1. Preliminary data obtained in various physiological conditions showed that GH-BP activity in adult male rats was 5.95 +/- 0.20%/0.1 ml serum. Significantly higher values were obtained in sera of female (21.66 + 0.79%/0.1 ml serum) and pregnant rats (23.02 +/- 1.15%/0.1 ml serum). A closer analysis of these binding values by Scatchard analysis revealed that the binding capacity in pregnant rats (50.5 +/- 5.8 pmol/0.1 ml serum) was significantly higher than in adult female estrous rats (19.2 +/- 6.5 pmol/0.1 ml serum), both being much higher than in adult male rats (2.5 +/- 0.6 pmol/0.1 ml serum). The GH-BP activity of 10-day-old rats was only approximately 63% of the adult male rat value. The presence of high-affinity GH-specific binding protein in rat serum suggests a probable action in regulation of GH activity. The detailed physiological role of rat serum GH-BP is currently being investigated.
Amit
T
T
Rappaport Family Research Institute, Faculty of Medicine, Technion, Haifa, Israel.
Barkey
R J
RJ
Bick
T
T
Hertz
P
P
Youdim
M B
MB
Hockberg
Z
Z
D000818:Animals; D002352:Carrier Proteins / Q000097:blood* / Q000378:metabolism; D005260:Female; D013006:Growth Hormone / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D011247:Pregnancy; D051381:Rats; D011919:Rats, Inbred Strains; D013696:Temperature
D002352:Carrier Proteins; D013006:Growth Hormone; C051815:somatotropin-binding protein