Enterokinase (EC 3.4.21.9) is a serine proteinase in the duodenum that exhibits specificity for the sequence (ASP)(4)-Lys. It converts trypsinogen to trypsin. Its high specificity for the recognition site makes enterokinase (EK) a useful tool for in vitro cleavage of fusion proteins. cDNA encoding the catalytic chain of Chinese bovine enterokinase was cloned and its encoding amino acid sequence is identical to the previously reported sequence although there are two one-base mutations which do not change the encoded amino acid. The EK catalytic subunit cDNA was cloned into plasmid pET32a, and fused downstream to the fusion partner thioredoxin (Trx) and the following DDDDK enterokinase recognition sequence. The recombinant bovine enterokinase catalytic subunit was expressed in Escherichia coli BL21(DE3), and most products existed in soluble form. After an in vivo autocatalytic cleavage of the recombinant Trx-EK catalytic domain fusion protein, intact, biologically active EK catalytic subunit was released from the fusion protein. The recombinant intact EK catalytic subunit was purified to hornogencity with a specific activity of 720 AUs/mg protein through ammonium sulfate precipitation, DEAE chromatography, and gel filtration. The purified intact EK catalytic subunit has a K-m of 0.17 mM, and K-cat is 20.8 s(-1). From 100 ml flask culture, 4.3 mg pure active EK catalytic subunits were obtained. (C) 2002 Elsevier Science (USA). All rights reserved.