Identification of a phenylalanine ammonia-lyase inactivating factor in harvested head lettuce (Lactuca sativa)

Exposing head lettuce (Lactuca sativa L., crisphead or Iceberg type) leaf 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 appearance of the postharvest physiological disorder called russet spotting (RS). Discontinuing ethylene exposure after 4 days resulted in a rapid decline in PAL activity which was delayed by treating excised midrib leaf tissue with actinomycin D or cycloheximide at 5 degrees C. Only cycloheximide delayed the loss of PAL activity in tissue that was transferred from 5 to 15 degrees C. Activity of PAL from Rhodotorula glutinis 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, lettuce tissue affected with RS. The in vitro loss in PAL activity was 9-fold higher in extracts from lettuce showing RS symptoms than from control lettuce, boiled samples or the buffer control. The PAL-inactivating factor isolated from lettuce affected with RS had a pH optimum around 8.0. It appears that the rapid loss in PAL activity after the discontinuation of exposure to ethylene is dependent on the de novo synthesis of a PAL-inactivating factor.