Summary
When bovine thrombin was prepared by one method the N-terminal amino acid was glutamic
acid while with another method it was threonine. In urea solution these N-terminal
amino acids were removed in association with peptides and the N-terminal amino acid
of the main protein was leucine. Urea treated thrombin had the same specific activity
as the original from which it was prepared, and also had the same carbohydrate content.
It was, however, less soluble in water and had a higher viscosity. The sedimentation
constant was concentration dependent. Before treatment with urea the rate of change
of the sedimentation constant with concentration was positive. After urea treatment
it was negative. Extrapolation to zero concentration gave the same value for thrombin
as for urea treated thrombin. The peptides removed from thrombin function to alter
the properties of the protein. They are attached by bonds that are broken in urea
solution. Very likely the original prothrombin molecule is, in part at least, composed
of sub-units consisting of polypeptide chains held together by bonds broken by reagents
such as sodium citrate. In addition to alanine, bovine prothrombin has arginine as
N-terminal amino acid, but the latter was uncovered only in urea solution.
