Phospholipase C mediated inhibition of factor VII requires triglyceride-rich lipoproteins

The reduction of plasma factor VII (FVII) activity by phospholipase C (PLC), in vitro, has been proposed as a possible indication of a risk of cardiovascular disease. The ability of PLC to reduce FVII activity was found to require calcium ions and the presence of triglyceride-rich lipoproteins (e.g. chylomicra and very-low density lipoproteins) rather than high or low density lipoproteins. The PLC-mediated reduction of FVII activity was prevented by pre-incubation of PLC with chylomicra, before adding FVII, and this suggests that PLC may act on triglyceride-rich lipoproteins already bound to FVII in order to reduce FVII activity. At optimal PLC concentration, the extent of the reduction in FVII activity was proportional to the concentration of chylomicra. The detergent, Tween, prevented any loss of FVII activity, in both plasma and purified systems, if it was present at the beginning of the incubation with PLC. Addition of Tween, but not EDTA, after inhibition of FVII activity had occurred, caused a partial restoration of FVII activity. It is concluded that PLC reduces FVII activity by modifying triglyceride-rich lipoproteins to a form which binds to FVII, independently of calcium ions, and which inhibits procoagulant activity. The detection of PLC-sensitive FVII activity may therefore have no greater significance than the measurement of plasma triglyceride levels in predicting a risk of cardiovascular disease.