Engineering of a Pichia pastoris expression system for secretion of high amounts of intact human parathyroid hormone

Human parathyroid hormone (hPTH) is involved in calcium metabolism, and the unique ability of this hormone to stimulate bone growth makes it a promising agent in the treatment of osteoporosis. We have engineered the methylotrophic yeast Pichia pastoris for the production of over 300 mg intact hPTH per liter growth medium. The presence of 10 mM EDTA in the culture medium was essential to obtain this high hormone yield, indicating that metallopeptidases are mainly responsible for the otherwise instability of hPTH. Furthermore, the secretion process of hPTH was considerably improved by coexpression of Saccharomyces cerevisiae protein disulphide isomerase (ScPDI). Since hPTH does not contain any cystein residues, this effect may be indirect or ascribed to the chaperone activity of PDI. Contrary to the situation in S. cerevisiae, use of a protease-deficient host strain provided no additional advantage. The hormone secreted by P. pastoris was not subjected to proteolytic processing by Kex2p in the two internal tribasic sites, nor were any C-terminal truncated hPTH forms detected. However, the P. pastoris hPTH producing transformants cosecreted ubiquitin to the culture medium, possibly as a result of a stress-related response.