Quantification of human plasma phospholipid transfer protein (PLTP): relationship between PLTP mass and phospholipid transfer activity

A sensitive sandwich-type enzyme-linked immunosorbent assay (ELISA) for human plasma phospholipid transfer protein (PLTP) has been developed using a monoclonal capture antibody and a polyclonal detection antibody. The ELISA allows for the accurate quantification of PLTP in the range of 25–250 ng PLTP/assay. Using the ELISA, the mean plasma PLTP concentration in a Finnish population sample (n=159) was determined to be 15.6±5.1 mg/l, the values ranging from 2.30 to 33.4 mg/l. PLTP mass correlated positively with HDL-cholesterol (r=0.36, P<0.001), apoA-I (r=0.37, P<0.001), apoA-II (r=0.20, P<0.05), Lp(A-I) (r=0.26, P=0.001) and Lp(A-I/A-II) particles (r=0.34, P<0.001), and negatively with body mass index (BMI) (r=−0.28, P<0.001) and serum triacylglycerol (TG) concentration (r=−0.34, P<0.001). PLTP mass did not correlate with phospholipid transfer activity as measured with a radiometric assay. The specific activity of PLTP, i.e. phospholipid transfer activity divided by PLTP mass, correlated positively with plasma TG concentration (r=0.568, P<0.001), BMI (r=0.45, P<0.001), apoB (r=0.45, P<0.001), total cholesterol (r=0.42, P<0.001), LDL-cholesterol (r=0.34, P<0.001) and age (r=0.36, P<0.001), and negatively with HDL-cholesterol (r=−0.33, P<0.001), Lp(A-I) (r=−0.21, P<0.01) as well as Lp(A-I/A-II) particles (r=−0.32, P<0.001). When both PLTP mass and phospholipid transfer activity were adjusted for plasma TG concentration, a significant positive correlation was revealed (partial correlation, r=0.31, P<0.001). The results suggest that PLTP mass and phospholipid transfer activity are strongly modulated by plasma lipoprotein composition: PLTP mass correlates positively with parameters reflecting plasma high density lipoprotein (HDL) levels, but the protein appears to be most active in subjects displaying high TG concentration.