Summary
Properties of the thrombin zymogen are quite different when in the form of prothrombin
complex, DEAE-prothrombin (prothrombin) or prethrombin. When removed from the prothrombin
complex, prothrombin spontaneously became refractory to the two-stage analytical reagents.
No new N-terminal amino acids formed in association with this activation. A first
step in prothrombin activation might be related to conformation. Repeatedly one mole
of alanine was found as N-terminal amino acid for prothrombin and it is thus a single
chain protein. Prethrombin did not have this alanine, but lysine and threonine, were
found. Prethrombin, like thrombin, thus had two chains. After the conversion of prethrombin
to thrombin with purified autoprothrombin C, a new N-terminal alanine amino acid was
found attached to a peptide. Additionally peptides with N-terminal serine, lysine,
and glycine were found. Threonine and isoleucine were again found as the N-terminal
amino acids for 3.7 S thrombin and 3.2 S thrombin. As a working hypothesis, a perspective
on the possible structure of prothrombin is outlined on the basis that it contains
two moles of thrombin. Prethrombin probably forms when Ala1-peptide splits from a polypeptide chain which forms a loop held together by a disulfide
bridge. Proteolysis probably also occurs in this loop. The postulated disulfide bridge
would be the one which holds the A and B chains of thrombin together. In the conversion
of prethrombin to thrombin, the first mole of thrombin would be set free. Then the
Ala2-peptide released might correspond to the Ala1-peptide removed when prethrombin originally formed. Ala2-peptide thus would be related to the second mole of thrombin which could arise by
further proteolysis including a split in the loop held together by a disulfide bridge.
It is postulated that acidic peptides are attached to the main prothrombin polypeptide
chain, to prethrombin, and to 3.7 S thrombin as satellite material.
