Department of Biochemistry , University of Virginia Health Sciences Center , Charlottesville 22908 .
Biochem J 293 ( Pt 2 ) : 443-50 ( 1993)
Abstract
alpha 2-Macroglobulin ( alpha 2M ) undergoes a major conformational change when reacting with proteinases or primary amines .
This conformational change has been referred to as the ' slow ' to ' fast ' transformation based on the increase in alpha 2M mobility shown by non-denaturing PAGE .
Previous studies demonstrated that many cytokines , including transforming growth factor beta 1 ( TGF-beta 1 ) and interleukin-1 beta , bind preferentially or exclusively to alpha 2M which has undergone conformational change .
In this study , we demonstrate that platelet-derived growth factor-BB ( PDGF-BB ) also binds preferentially to conformationally transformed alpha 2M ( alpha 2M-methylamine , alpha M-trypsin ) in vitro .
Purified 125I-PDGF-BB-alpha 2M-methylamine complex cleared rapidly from the circulation of mice via the alpha 2M receptor/low-density-lipoprotein-receptor-related protein ( alpha M-R/LRP ) .
In order to determine whether PDGF-BB or TGF-beta 1 binds to native alpha 2M , we defined the native conformation by lack of interaction with alpha 2M-R/LRP instead of electrophoretic mobility .
I-PDGF-BB was incubated with 4.3 microM native alpha 2M and 0.47 microM alpha 2M-methylamine .
The 125I-PDGF-BB distributed evenly between slow-form and fast-form alpha 2M without shifting the electrophoretic mobility of either species .
When the mixed preparation was injected intravenously in mice , 125I-PDGF-BB-fast-form-alpha 2M cleared rapidly and selectively from the circulation ; 125I-PDGF-BB which was bound to slow-form alpha 2M was stable in the blood ( apparently not recognized by alpha 2M-R/LRP ) .
Therefore , while conformationally transformed alpha 2M binds PDGF-BB preferentially in vitro , non-alpha 2M-R/LRP-recognized alpha 2M binds PDGF-BB as well .
Binding of 125I-PDGF-BB and I-TGF-beta 1 to alpha 2M was demonstrated in vivo by injecting the free growth factors intravenously into mice .
Plasma samples which were subjected to non-denaturing PAGE and autoradiography demonstrated binding of both growth factors exclusively to the slow-form of alpha 2M .
Therefore , under normal physiological conditions , native alpha 2M ( non-alpha 2M-R/LRP-recognized ) is the primary form of the proteinase inhibitor functioning as a carrier of PDGF-BB and TGF-beta 1 in the blood .