The light chain of bovine enterokinase (EKL) is a serine protease that catalyzes the hydrolysis of peptide bonds in proteins. Enterokinase specifically cleaves after lysine in the consensus sequence Asp-Asp-Asp-Asp-Lys. The gene encoding the light chain of bovine enterokinase was codon-optimized and placed under the control of either the Tetrahymena metallothionein (MTT5) or starvation (SP1) promoter. Transformed cells were induced to express EKL and activity measured directly in spent culture medium (Figure 1). EKL was additionally purified from spent culture medium via ion exchange chromatography (Figure 2).