Conversion of a Plasma Group-Specific Component (Vitamin D Binding Protein) to a Macromolecule by a Protein Factor in Tissue Extracts

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Abstract
We have studied the altered electrophoretic mobility from the α2-globulin region to the a1-globulin region of the group-specific component (Gc protein) (vitamin D binding protein) in plasma of aged blood, plasma from cadavers or extracts of blood stains by agar-gel immunoelectrophoresis and polyacrylamide gel electrophoresis. The alteration of migration of Gc protein found in plasma of aged blood is reproduced by incubation of plasma of fresh blood with extracts of platelets. Platelet extracts had no effect on the electrophoretic mobility of 20 other serum proteins examined. The alteration of electrophoretic mobility of Gc protein was not due to digestion by proteolytic enzymes or neuraminidase since polyacrylamide gradient gel electrophoresis and gel filtration studies revealed a higher molecular weight for altered Gc protein (Mr 102, 000) compared to native Gc protein (Mr 60, 000). A factor found in platelet extracts was nondialyzable, heat-labile and sensitive to trypsin and chymotrypsin, and was present in all tissues of human and rat examined. The results indicate that the alteration of the electrophoretic mobility of Gc protein found in plasma of aged blood is due to binding of a protein factor released from platelets and other blood cells during aging. The protein factor was identified as actin by partial purification of the factor from platelet extracts. Polyacrylamide gradient gel electrophoresis of fractions containing the factor showed an actin band, which comigrated with purified actin from chicken gizzard. The actin band disappeared on addition of purified Gc protein and a new protein band of Mr 102,000 appeared. The band of Mr 102,000 disappeared with the appearance of another new protein band of Mr 132,000 on further addition of DNase I, which is known to bind actin.