Quantitation of the active and low-active forms of human plasma phospholipid transfer protein by ELISA

Human plasma contains two forms of phospholipid transfer protein (PLTP), one catalytically active [high-activity PLTP (HA-PLTP)] and the other a low-activity (LA-PLTP) form. We present here a PLTP ELISA that allows not only for accurate measurement of PLTP concentration in plasma but also of the distribution of both LA- and HA-PLTP. To achieve similar immunoreactivity of the two PLTP forms, a denaturing sample pretreatment with 0.5% SDS was required. Distribution of LA- and HA-PLTP in plasma was assessed using size-exclusion chromatography, Heparin-Sepharose chromatography, anti-PLTP immunoaffinity chromatography, and dextran sulfate-CaCl2 precipitation. All four methods demonstrated that approximately 60% of plasma PLTP represents LA-PLTP and 40% represents HA-PLTP. According to the modified ELISA, the total serum PLTP concentration in a random Finnish population sample (n = 80) was 5.81 +/- 1.33 mg/l (mean +/- SD) (range, 2.78-10.06 mg/l) and the mean activity was 5.84 +/- 1.39 micromol/ml/h (range, 3.21-11.15 micromol/ml/h). To quantitate both forms of PLTP in sera from this sample, we combined dextran sulfate-CaCl2 precipitation with the modified PLTP ELISA. The HA-PLTP mass (mean, 1.87 +/- 0.85 mg/l) correlated significantly with serum PLTP activity, whereas that of LA-PLTP (mean, 3.94 +/- 1.4 mg/l) showed no correlation with phospholipid transfer activity.