Heparin-releasable and nonreleasable lipoprotein lipase in the perfused rat heart.

Lipoprotein lipase released from the rat heart during a 30-s perfusion with heparin was compared to the lipase remaining in the heart tissue. The perfusate, containing the heparin-releasable enzyme, as well as the heart tissue extract ("residue"), was purified on heparin-Sepharose affinity columns. Both purified fractions showed pronounced inhibition by 1 M NaCl and by antiserum to heart lipoprotein lipase, thus displaying mainly lipoprotein lipase activities. However, their apparent Km values for triglyceride differed significantly (perfusate, 0.4 mM; residue, 4.0 mM). Also, the pattern of the immunotitration curves for the two fractions differed, the perfusate being more susceptible to antibody inhibition than the residue. Addition of heparin (0.5 unit/ml) inhibited the perfusate activity up to 60%, whereas the residual activity was actually stimulated by 15%. Based on these findings, we propose that the heart tissue contains a less active, low affinity enzyme form, possibly representing the precursor of the high affinity, functional, endothelial-bound lipoprotein lipase.