Anti-Xa activity of human hepatic triglyceride lipase.

Human hepatic triglyceride lipase (HTGL), purified from plasma obtained after heparin injection, markedly enhanced the anti-Xa clotting activity of normal plasma. This was shown to be caused by direct inhibition of factor Xa clotting activity by HTGL, although the amidolytic activity of factor Xa was unaffected. Preincubation of factor Xa with CaCl2 and phospholipid reduced the rate of inhibition of HTGL, indicating that phospholipid-binding sites may be involved. Heparin, and low-affinity heparin, reduced the anti-Xa activity of HTGL, suggesting that heparin and factor Xa compete for the same binding sites on the lipase molecule. These results suggest that at least part of the enhanced anti-Xa clotting activity observed after injection of heparin and heparin analogues is caused by the release of HTGL. This release could contribute toward the anticoagulant and antithrombotic actions of these drugs.