FUNCTIONAL STUDIES OF TISSUE FACTOR

Tissue factor (coagulation factor III) is a membrane glycoprotein
and an essential co-factor for the "extrinsic" initiation of
coagulation. Only in the presence of tissue factor does factor VII
(or VIIa) have significant proteolytic activity towards its
substrates, factors IX and X. Tissue factor apoprotein has no
procoagulant activity, but the activity can e restored by
reassociation of the apoprotein with appropriate lipids.
Procoagulant activity appears to be strictly dependent on
association of the apoprotein with a phospholipid membrane.
Existing information regarding the molecular mechanism of tissue
factor's cofactor activity and the regulation of that activity is
limited. The proposed research will investigate the reconstitution
of tissue factor-membrane complexes, and will evaluate the
influence of the lipid environment on the reconstituted actvity.
Assembly of tissue factor apoprotein into lipid vesicles will be
studied using purified phospholipids to determine which variables
in lipid structure (charge and acyl chains) influence physical
association of tissue factor with lipids and which variables affect
expression of procoagulant activity in the assembled complex.
Plasma high density lipoprotein can alter the physical properties
of phospholipid vesicles and has been shown to inhibit tissue
factor activity. Individual apolipoproteins will be purified and
tested for their effects on tissue factor activity. Possible
lipoprotein effects to be specifically addressed include
interference with factors X and VII access to tissue factor-vesicle
complexes, and alterations of the tissue factor conformation in
the lipid bilayer. Other proteins known to bind membranes and
associated with pathological lesions (e.g., C-reactive protein and
serum amyloid component) will also be examined with regard to
effects on tissue factor activity. These studies will contribute to
understanding tissue factor function and its regulation, and
provide information with which to evaluate the importance of
plasmalipoprotein effects on the initiation of coagulation.