Enzymatic amidation of recombinant (Leu²⁷) growth hormone releasing hormone-Gly⁴⁵

Abstract

By chemoenzymatic synthesis the gene for a (Leu27) analogue of human growth hormone releasing hormone-Gly45 [(Leu27)GHRH-Gly45] was constructed, cloned and expressed in Escherichia coli as a fusion protein with beta-galactosidase under the control of the lac promoter and operator. Upon induction with isopropyl-D-thio-beta-galactopyranoside the fusion protein accumulated to a yield of 15-20% of the total cellular protein. After cyanogen bromide cleavage of the fusion protein the precursor peptide (Leu27)hGHRH-Gly45 was separated by extraction and purified by ion exchange and h.p.l.c.-RP18 chromatography. The purified peptide was analysed by sequencing, isoelectric focusing, amino acid analysis and amino acid analysis after V8 protease digestion. The carboxy-terminal glycine was subsequently amidated by PAM (peptidylglycine-alpha-amidating-monooxygenase), an enzyme which was isolated and characterized from fresh bovine pituitaries. Correct amidation of the penultimate amino acid, leucine, was verified by peptide sequencing with an authentic leucine amide reference.