The N-terminal and sulphur-containing residues of bacitracin A

Abstract

The isoleucine in hydrolysates of bacitracin A has been analysed on columns of Dowex 50. After hydrolysis with 6N hydrochloric acid for 24 hours, about 2 molecules of isoleucine and 0.5 molecules of alloisoleucine were found to be present for each molecule of leucine. The amount of isoleucine was somewhat larger after hydrolysis for 90 hours. The nin-hydrin color density given by bacitracin A indicated that a free amino group other than the amino group of ornithine was present in the molecule. In hydrolysates of dinitrophenol-bacitracin A, DNP-isoleucine (and/or DNP-alloisoleucine) was the only identified product that could have been derived from an N-terminal residue. Deaminated bacitracin A, obtained by treating bacitracin A with nitrous acid, yielded no alloisoleucine on hydrolysis. Ornithine was also absent from the hydrolysate and cysteine had been partly oxidized to cysteic acid. The remaining amino acids appeared to be unchanged. Hydrogenolysis of bacitracin A with Raney nickel resulted in the formation of an N -terminal alanine residue and three volatile bases, identified as isoleucinol, 1-amino-2-methyl-n-butane and ethylamine respectively. Using commercial bacitracin, 1-amino 2-methyl-n-butane was isolated as chloroplatinate and ethylamine as a picrate. The isoleucinol obtained by hydrogenolysis of commercial bacitracin was oxidized to isoleucine and analysed on columns of Dowex 50. Isoleucine was the major component. These results are thought to be consistent with the view that bacitracin A contains three isoleucine residues, that an N-terminal isoleucine residue is condensed with the cysteine residue to form a thiazoline ring, and that racemization of this isoleucine residue is responsible for the alloisoleucine that appears in hydrolysates.