Heparin and other glycosaminoglycans have profound activity in vitro on the
regulation of complement activity. The studies reported here examined the
mechanism whereby heparin enhances C1 esterase inhibitor (C1INH) activity o
n C1 esterase (C1). The interaction of heparin and heparan sulfate with C1I
NH was first examined using surface plasmon resonance. Heparin was immobili
zed on a biosensor chip in two orientations, at its reducing end and in mid
chain, and heparan sulfate was immobilized at its reducing end. Heparin imm
obilized at its reducing end interacted with C1INH, giving an association c
onstant (K-a) value of 1.43 x 10(7) M-1, whereas heparin immobilized in mid
chain afforded a K-a value of 7 x 10(6) M-1. No interaction between C1INH a
nd heparan sulfate could be observed. Next, the augmentation of C1INH by he
parin (M-r (av) 13,000), low-molecular-weight (LMW) heparin (M-r (av) 5000)
, and heparan sulfate (M-r (av) 11,000) was determined. C1INH alone was at
least 10,000 times more active in inhibiting fluid phase C1 compared with e
rythrocyte-bound C1 (EAC1). When C1 was in the fluid phase, both heparin an
d LMW heparin were relatively ineffective at augmenting C1INH activity on C
1. In contrast, when C1 was present as EAC1, heparin augmented C1INH activi
ty at all C1INH concentrations examined and LMW heparin was up to 1.3 times
more effective than heparin. This augmentation only occurred when both C1I
NH and heparin were present together with the EAC1. Hence, although surface
plasmon resonance shows that heparin binds to C1INH, heparin augmentation
of C1INH activity appears to require a terniary complex in which cell bound
C1 interacts with both heparin and C1INH. This is the first report of LMW
heparin augmenting C1INH activity. Heparan sulfate neither interacted with
C1INH nor did it augment C1INH activity. (C) 1999 Academic Press.