Isolation of Escherichia coli synthesized recombinant eukaryotic proteins that contain 6‐N‐acetyllysine

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Abstract
Recombinant porcine (rpST) and bovine somatotropins (rbST) synthesized in Escherichia coli contain the amino acid, ←-N-acetyllysine. This amino acid was initially discovered in place of the normal lysine144 in a modified reversed-phase HPLC (RP-HPLC) species of rpST. Mass spectrometry and amino acid sequencing of a tryptic peptide isolated from this RP-HPLC purified protein were used to identify this altered residue as ←-N-acetyllysine. Ion-exchange chromatography was utilized to prepare low isoelectric point (pI) forms of rpST and rbST, which are enriched in ←-N-acetyllysine. Electrospray mass spectrometry demonstrated that the majority of the protein in these low pI fractions contained species 42 Da larger than normal. Immobilized pH gradient electrophoresis (IPG) of the ion-exchange purified low pI proteins was used to isolate several monoacetylated species of rpST and rbST. The location of the acetylated lysine in each IPG-purified protein was determined by tryptic peptide mapping and amino acid sequencing of the altered tryptic peptides. Amino acid analyses of enzymatic digests of rpST and rbST were also used to confirm the presence of ←-N-acetyllysine in these recombinant proteins. These data demonstrate that a significant portion of rpST and rbST produced in E. coli contain this unusual amino acid.