This study compares a low-Mr heparin preparation with conventional heparin with respect to its interaction with lipoprotein lipase (LPL) in vitro and its effects on the enzyme in vivo. Both heparin preparations were polydisperse in binding to LPL, but on average the low-Mr preparation showed lower affinity. Thus both conventional and low-Mr heparin bound quantitatively to immobilized LPL, and were eluted as broad peaks when a salt gradient was applied, but the peak for low-Mr heparin was shifted towards lower salt concentrations. To displace LPL from immobilized heparin a higher concentration of low-Mr than of conventional heparin was needed. Injection of the low-Mr heparin into intact rats resulted in lower plasma LPL activity than did injection of an equal mass of conventional heparin, but when the liver was excluded from the circulation both heparin preparations resulted in similar plasma LPL activities. In perfused rat hearts, low-Mr heparin had at least the same effect on the release of LPL activity as did conventional heparin. In perfused livers, on the other hand, low-Mr heparin was less effective than conventional heparin in preventing the rapid uptake of exogenous labelled LPL. Hence the apparently lower average affinity of low-Mr heparin for LPL does not result in a demonstrably lower potency to release the enzyme from endothelial binding sites in peripheral tissues, but does result in a substantially decreased effect on the hepatic clearance of the enzyme.
