Summary
To determine a thrombin-binding site on GPIbα on platelet membrane, we have examined
the binding activities of tryptic or chymotryptic fragments of purified GPIbα to a
monoclonal antibody against GPIb (TM60) and thrombin using (immuno) affinity chromatography.
When purified GPIba was digested with trypsin, two fragments (94-kDa, and 43-kDa)
were obtained. The 43-kDa fragment was shown to bind to both affinity columns of TM60-
and thrombin-Affi-Gel, while the 94-kDa fragment did not bind to either Affi-Gel columns.
When trypsin fragments were incubated with TM60 and then applied to the column of
thrombin-Affi-Gel, neither fragments were bound to the column. When the same experiment
was performed using chymotrypsin, three fragments (94-kDa, 45-kDa and 39-kDa) were
observed. On TM60- and thrombin-Affi-Gel columns, the smaller fragments (45-kDa and
39-kDa) were bound to the column. After incubation of these fragments with TM60, neither
bound to the thrombin column. These results indicate (i) that the epitope for TM60
is located near, or on the thrombin-binding site of GPIba, and (ii) that the thrombin-binding
site is located on the tail portion of GPIbα, especially on a chymotrypsin cleavage
site.
Key words
Human platelet thrombin-binding site - Glycoprotein Ib - Monoclonal antibody
