UTILIZATION OF A THERMOSTABLE ALKALINE PROTEASE FROM AN ALKALOPHILIC THERMOPHILE FOR THE RECOVERY OF SILVER FROM USED X-RAY-FILM

To improve the processing capacity for the recovery of silver from used X-ray film, a thermostable alkaline protease (Protease B18') was newly obtained from a thermophilic alkalophile. When the enzyme was used to hydrolyze casein, it had a higher optimum pH and temperature, around 13.0 and 85-degrees-C, respectively, compared with Protease B21-2, which is an alkaline protease from a methophilic alkalophile. The enzyme was most active toward gelatin on film at pH 10 and the time required to decompose the gelatin layer decreased with an increase in enzyme activity. The gelatin layer was efficiently broken down within 2 min at 1,000 U/ml. These results indicate that the utilization of Protease B18' will make it possible to increase the processing capacity without scaling up the plant.