Crystalline [A21-desamido]bovine insulin.

[A21-Desamido]insulin is the major product formed during mild acid hydrolysis of bovine insulin at low insulin concentration. The derivative was isolated by standard procedures and its purity established by isoelectric focusing, disc electrophoresis and electrophoresis on cellulose acetate strips. The identity of the acid-transformed derivative was determined as [A21-desamido]insulin by the action of carboxypeptidase A, using conditions under which a C-terminal aspartic acid residue would not be removed. The biological activity of this crystalline derivative was found to be 15.9 units/mg as measured by the mouse convulsion assay.