Development of a radioimmunoassay for human platelet factor 4.

Human platelet factor 4 (PF4), a secreted protein with antiheparin activity, was purified by affinity chromatography on heparinepsilon aminocaproic acid-Sepharose^R and used to prepare antibodies in rabbits. The resulting antibodies reacted in immunodiffusion with purified PF4, platelet lysate, and serum and were monospecific by immunoelectrophoresis. Purified PF4 was radioiodinated with ¹²⁵I by the chloramine-T method and used to develop a radioimmunoassay that was rapid, accurate, and sensitive enough to measure 0.73 ng of PF4. Normal values in platelet poor plasma collected into ethylene-diamine-tetraacetate (EDTA), dibutyryl cyclic adenosine 3′5′ monophosphoric acid (db-cAMP), and prostaglandin E₁ (PGE₁) were 4.7 ± 2.5 ng/ml (mean ± S.D.). Levels in serum from clotted whole blood were 13,200 ± 3,200 ng/ml. This assay should serve as a sensitive indicator of both $\text{in vivo}$ and $\text{in vitro}$ platelet release.