Some investigations on inactivation of phenylalanine ammonia-lyase in cut-injured sweet potato root tissue1

Abstract

In sweet potato roots, activity of the phenylalanine ammonia-lyase (PAL)-inactivating system in crude enzyme solution increased markedly in response to cut injury after a lag period of about 10 hr and reached a maximum after 24 hr of incubation. The results coincided with previous results from experiments using a protein synthetic inhibitor. The inactivating system could be precipitated by centrifugation and was distributed in a different pattern from mitochondrial and microsomal marker enzymes, according to data from cellular fractionation by differential and sucrose density gradient centrifugation. The optimum pH of the inactivation was 6.0. Previous studies showed that PAL content changed in parallel with PAL activity in vivo. However, immunochemical studies indicated that the inactivation was not due to proteolysis. Furthermore, proteinase activity in sweet potato tissue did not change in response to cut injury. These results suggested that PAL was first inactivated by the inactivating system, then the inactivated PAL was rapidly decomposed by the proteinase.