Experiments on the lysine and aspartic acid residues in bacitracin A

Abstract

When bacitracin A or deami-nated bacitracin A was treated with U[image] HC1 at 80[degree] for 43 hours most of the amino acids were liberated in the free state, but 4 peptides containing isoleucine or lysing resisted hydrolysis. The surviving peptides were separated by ionophoresis and chromatography on paper and appeared to be Ileu, Phe, Ileu, Lys, Ileu (Asp) Lys, and Asp. Lys. The last was the most stable to further hydrolysis. The DNP derivative of Ileu, Lys, yielded DNP-isoleucine and e -DNP-lysine on hydrolysis, and the DNP-derivative of Asp. Lys yielded DNP-aspartic acid and alpha-DNP-lysine. It is concluded that an isoleucine residue is linked to the alpha-amino group of the lysine residue in bacitracin A and that an aspartic acid residue is linked to the alpha-amino group. However, certain properties of a substance containing lysine and aspartic acid raise the question whether its structure is more complicated than that of a simple dipeptide. One of the aspartic acid residues in bacitracin A is readily liberated in the free state by hydrolysis. The second residue is linked to lysine by a bond which is unusually resistant to hydrolysis. A fraction which contained mainly the first aspartic acid was separated from a fraction which contained mainly the second. In one experiment the 2 fractions were subjected separately to the action of D-aspartic acid oxidase. In another the aspartic acid in each fraction was isolated and its specific rotation measured. The results indicate that the easily liberated aspartic acid is derived from a residue with the D-configuration and that the aspartic acid residue next to lysine has the L-configuration.