Summary
The kinetics of activation of factor IX and factor X by factor Vila was studied in
the presence of various sources of tissue factor: (1) a surface membrane of human
ovarian carcinoma cell line, OC-2008 (2) the cell lysate (of 002008) and (3) reconstituted
purified human tissue factor. The rates of activation of factors IX and X were monitored
in activation peptide release assays using tritiated substrates. The results indicate
that the apparent K
m values for factor IX and factor X were similar for a given tissue factor, but varied
with tissue factor source. The source of tissue factor greatly influenced the apparent
differences in V
max for factors IX and X. When a surface of monolayer provided tissue factor, the Vmax
of factor IX was only 2-3 fold lower than factor X, but when either a cell lysate
or purified tissue factor was the source of cofactor activity, the difference in V
max rose to about 8-10 fold. Although, the tissue factor apoprotein in the cells was
expressed entirely on the outer surface membrane, the activity of tissue factor on
the intact cell surface was 50 to 100fold lower than in the lysed cell preparation.
