Distinct human prolactin (hPRL) and growth hormone (hGH) behavior under bacteriophage lambda PL promoter control: Temperature plays a major role in protein yields

When producing recombinant protein for therapy, it is desirable not only to obtain substantial amounts of the protein, but also to make sure that potential contaminants such as inducing agents are not present in the final product. To prevent this, one can use expression systems in which the promoter (λPL) is activated by a temperature shift that denatures a repressor (e.g., cIts). In this manner, hGH was successfully expressed and secreted in Escherichia coli periplasm, with specific yields well above 1 μg ml−1A600−1, after a temperature shift from 30 to 42 °C. However, attempts to express a related hormone, human prolactin, employing the same protocol were unsuccessful, providing 0.03 μg ml−1A600−1 at the most. A process is described in which this labile protein is obtained from a cIts− strain under optimized temperature condition (37 °C). The highest periplasmic secretions of prolactin ever reported were thus obtained: 0.92 ± 0.10 μg ml−1A600−1 at an optical density of ∼3 A600 units in shake flask cultures and ∼1 μg ml−1A600−1, at an OD of 35 A600 units, via a rapid and flexible batch feed process in laboratory bioreactor. Purified hPRL was monomeric, correctly processed (Mr = 22,906), properly folded and bioactive (51.5 ± 24.1 IU mg−1).