Characterization of Gly-d-Phe, Gly-l-Leu, and d-Phe as affinity ligands to thermolysin

In this study, glycyl-d-phenylalanine (Gly-d-Phe), glycyl-l-leucine (Gly-l-Leu), and d-phenylalanine (d-Phe) were characterized for their abilities as affinity ligands to thermolysin. Each of the ligands was immobilized to the resin. The optimum pH for adsorption of thermolysin is 5.0–6.0 for each of the ligands. By the affinity column chromatography in which 2 mg thermolysin was applied onto 4 ml volume of the resins at pH 5.5, the adsorption ratios based on casein hydrolysis activity were 100% for each of the ligands. However, the adsorption ratios of the resins containing Gly-l-Leu and d-Phe, unlike that of Gly-d-Phe, were progressively decreased with increasing the amounts of thermolysin applied to the column. Measurement of adsorption isotherms showed that the association constant to thermolysin at pH 5.5 of the resins containing Gly-d-Phe was (3.3 ± 0.8) × 105 M−1, while those of Gly-l-Leu and d-Phe were approximately ten times less. This result is coincident with the observations of performances in affinity column chromatography. On the other hand, maximum thermolysin binding capacities were almost the same among the resins examined. These results indicate that Gly-d-Phe is more suitable than Gly-l-Leu and d-Phe as an affinity ligand for purification of thermolysin.