Protection of phenylalanine ammonia-lyase from proteolytic attack.

Phenylalanine ammonia-lyase contained within permeabilized cells of Rhodosporidium toruloides was protected from proteolytic attack by trypsin, chymotrypsin and duodenal juice. The inactivation by the proteases was biphasic. The enzyme contained within the yeast cells had a similar Km for phenylalanine and Ki for cinnamic acid to the protein in free solution. Phenylalanine ammonia-lyase present in the yeast depleted duodenal juice of free phenylalanine, while the enzyme in free solution did not. The possibility of using permeabilized cells of R. toruloides as a vehicle for protecting orally ingested therapeutic enzymes from proteolytic inactivation is discussed.