IDENTIFICATION OF A PHENYLALANINE AMMONIA-LYASE INACTIVATING FACTOR IN HARVESTED HEAD LETTUCE (LACTUCA-SATIVA)

Exposing head lettuce (Lactuca sativa L., crisphead or Iceberg type) l eaf tissue to hormonal levels of ethylene (10 mu l l(-1)) at 5 degrees C promotes the de novo synthesis of phenylalanine ammonia-lyase (PAL, EC 4.3.1.5) and an increase in its activity. It also promotes the app earance of the postharvest physiological disorder called russet spotti ng (RS). Discontinuing ethylene exposure after 4 days resulted in a ra pid decline in PAL activity which was delayed by treating excised midr ib leaf tissue with actinomycin D or cycloheximide at 5 degrees C. Onl y cycloheximide delayed the loss of PAL activity in tissue that was tr ansferred from 5 to 15 degrees C. Activity of PAL from Rhodotorula glu tinis was slowly lost during incubation in buffer alone, but there was a logarithmic decline in its activity over time when it was incubated with aliquots of the resuspended 10 000 g pellet from homogenized, le ttuce tissue affected with RS. The in vitro loss in PAL activity was 9 -fold higher in extracts from lettuce showing RS symptoms than from co ntrol lettuce, boiled samples or the buffer control. The PAL-inactivat ing factor isolated from lettuce affected with RS had a pH optimum aro und 8.0. It appears that the rapid loss in PAL activity after the disc ontinuation of exposure to ethylene is dependent on the de novo synthe sis of a PAL-inactivating factor.