Activation of lipoprotein lipase. Evaluation of calcium, magnesium, and ammonium as cofactors.

Lipoprotein lipase (LPL) from rat heart acetone powders has been reported to depend on the presence of NH4 + , calcium, or other divalent cations for optimal activity. In addition, the enzyme will not hydrolyze an artificial triglyceride emulsion unless it is converted to an active substrate by the addition of very low density lipoproteins, high density lipoproteins (HDL), or certain peptides contained in these complexes. We observed earlier that HDL that had been extensively dialyzed against 0.005M ethylenediaminetetraacetic acid (EDTA), which probably removed most of the divalent cations, produced little lipolytic activity in LPL from rat heart acetone powders. This observation led us to evaluate the cation requirements for LPL. LPL was prepared in soluble form from rat heart acetone powders. HDL was isolated from rat serum by ultracentrifugation. The assay system contained a phospholipid-stabilized triglyceride emulsion as substrate. The addition of the whole rat serum to the LPL assay system produced high LPL activity. The addition of dialyzed HDL isolated from the same volume of serum produced low LPL activity. The addition of calcium in physiological concentration to the assay containing HDL increased LPL activity markedly. The high enzyme activity produced by both serum and HDL plus calcium was inhibited by ethyleneglycol bis(/3aminoethylether)-N,N'-tetraacetic acid (EGTA), a specific calcium chelating agent. The addition of NH4 + or magnesium at several concentrations did not replace calcium in stimulating LPL activity. We conclude that calcium has a cofactor function in the LPL lipolytic reaction and that the optimal enzyme reaction rate is attained in the presence of calcium concentrations found in serum.