CPQ and cystic fibrosis: Whilst PGP degradation is mediated by the aminopeptidase activity of LTA4H in an extracellular environment, LTB4 is generated intracellularly by the epoxide hydrolase activity of LTA4H. Clearly, only extracellular LTA4H would be liable to neutrophil elastase degradation as seen in the CC-CF children, with the intracellular enzyme still capable of generating LTB4.