Lipoprotein lipase stored in adipocytes and muscle cells is a cryptic enzyme

The status of lipoprotein lipase (LPL) has been examined in different cell types (adipose, skeletal muscle, and heart muscle cells) and different tissues (adipose, muscle, and cardiac tissues) from mouse, rat, and human. Cell and secreted activities were compared in cycloheximide-, heparin-treated cells present in culture. A gross underestimation of cell LPL activity was found; excess of LPL over substrate and/or apolipoprotein C-II was excluded as well as inhibition by cell component(s) or detergent molecules used to disrupt membrane structures in the cell lysates. Unmasking of LPL activity occurred upon dilution: the higher the concentration of LPL, the higher were the dilution factor and the concentration of heparin required to reach a plateau of activity. This maximal value was found to be identical to that determined in the secretion medium, indicating that the cell LPL activity can be determined in toto. The unmasking effect of dilution upon LPL activity was extended to adipose, muscle, and cardiac tissues from rat and to adipose tissues from mouse and human. In agreement with previous results (Vannier et al., 1989, J. Biol. 264: 13199-13205), our results are in favor of LPL as being cryptic within the cell. A model is proposed, in which potentially active LPL molecules are present as aggregates in various membrane compartments. It is concluded that the determination of the pool size of catalytically active cell LPL has to be estimated in vitro under the appropriate conditions described herein.