Different phospholipid transfer protein complexes contribute to the variation in plasma PLTP specific activity

Phospholipid transfer protein (PLTP) facilitates the transfer of phospholipids among lipoproteins. Over half of the PLTP in human plasma has been found to have little phospholipid transfer activity (inactive PLTP). We recently observed that plasma PLTP specific activity is inversely correlated with high-density lipoprotein (HDL) level and particle size in healthy adults. The purpose of this study was to evaluate the factors that contribute to the variation in plasma PLTP specific activity. Analysis of the specific activity of PLTP complexes in nine plasma samples from healthy adults revealed two clusters of inactive PLTP complexes with mean molecular weights (MW) of 342 kDa and 146 kDa. The large and small inactive PLTP complexes represented 52 ± 8% (range 39–63%) and 8 ± 8% (range 1–28%) of the plasma PLTP, respectively. Active PLTP complexes had a mean MW of 207 kDa and constituted 40 ± 6% (range 33–50%) of the plasma PLTP. The specific activity of active PLTP varied from 16 to 32 μmol/μg/h. These data demonstrate for the first time the existence of small inactive plasma PLTP complexes. Variation in the amount of the two clusters of inactive PLTP complexes and the specific activity of the active PLTP contribute to the variation in plasma PLTP specific activity.

Research Highlights
► Inactive PLTP resides in large (mean 342 kDa) and small (mean 146 kDa) complexes.
► Large and small inactive PLTP complexes constitute 52 ± 8% and 8 ± 8% of plasma PLTP, respectively.
► Active PLTP complexes (mean 207 kDa) constitute 33–50% of plasma PLTP.
► The specific activity (SA) of active PLTP complexes varies among plasma samples.
► Variation in plasma active PLTP SA and inactive PLTP contribute to variation in SA.