Protein expression in transgenic plants is considered one of the most promising approaches for producing pharmaceutical proteins. As has happened with other recombinant protein production schemes, the downstream processing (dsp) of these proteins produced in plants is key to the technical and economic success of large-scale applications. Since dsp of proteins produced transgenically in plants has not been extensively studied, it is necessary to broaden the investigation in this field in order to more precisely evaluate the commercial feasibility of this route of expression. In this work, we studied the substitution of an IMAC chromatographic step, described in previous work (Azzoni et al., 2002), with ion-exchange chromatography on SP Sepharose Fast Flow resin as the second step in the purification of recombinant aprotinin from transgenic maize seed. The main goal of this second purification step is to separate the recombinant aprotinin from the native corn trypsin inhibitor. Analysis of the adsorption isotherms determined at 25°C under different conditions allowed selection of 0.020 M Tris pH 8.5 as the adsorption buffer. The cation-exchange chromatographic process produced a high-purity aprotinin that was more than ten times more concentrated than that generated using an IMAC step.

Two adsorption techniques were studied as potential methods for the recovery of aprotinin: a biospecific adsorption (using immobilized trypsin and chymotrypsin as ligants) and a pseudobiospecific adsorption of aprotinin-trypsin complex onto IMAC matrix. These studies indicated that ionic strength has an important effect on aprotinin adsorption on immobilized trypsin, and the pH was the most significant variable in aprotinin desorption from both immobilized enzymes. The aprotinin-trypsin complex adsorption onto the IMAC matrix was not as significantly affected by changes in pH as it was for changes in ionic strength. For the desorption step, both variables significantly affected the complex recovery. However, the effect of ionic strength was markedly stronger for this desorption step.