SPHINGOSINE-CONTAINING PHOSPHOLIPID-VESICLES SUPPORT HUMAN FACTOR-VIIAUTOACTIVATION IN THE ABSENCE OF TISSUE FACTOR

We studied the effects of lipid membrane composition on factor VII aut oactivation and observed that factor VII was activated in the presence of phospholipid vesicles containing sphingosine. The time course for the factor VII activation was sigmoidal and the duration of the initia l lag phase was decreased by the addition of exogenous factor VIIa. Ki netic studies revealed that factor VII activation in the presence of s phingosine-containing phospholipids can be defined by a second-order r eaction mechanism with an apparent second-order rate constant of 1.1 x 10(4) M(-1)S(-1). The sphingosine-mediated factor VII autoactivation rate was dependent on the concentration of calcium ions and sphingosin e content of the vesicles. Neither bovine serum albumin-conjugated sph ingosine nor sphingosine analogues (ceramide, sphingomyelin) affected the factor VII autoactivation rate.