fast apple malus x domestica and tobacco nicotiana tobacum leaf polyphenol oxidase activity assay for screening transgenic plants a spectrophotometric assay method for the analysis of polyphenol oxidase ppo in apple and tobacco leaves has been optimized to increase efficiency in the screening of large numbers of transgenic plants crude protein extracts from leaf punches were prepared in a fastprep homogenizer the addition of triton x100 during extraction resulted in 44 and 74 increases in the ppo activity recovered from apple and tobacco respectively the enzyme kinetics differed markedly between apple and tobacco apple leaf ppo was isolated in a latent state and was activated by the addition of sds in contrast tobacco ppo activity was inhibited by sds particularly at acidic ph apple ppo showed a pronounced ph optimum around ph 6 whereas the ph profile for tobacco ppo was much flatter with a broad optimum around ph 4 the calculated km value for apple ppo using 4methylcatechol as substrate was 81 and for tobacco the km was 43 the ppo reaction was strongly inhibited by tropolone a cu competitor and restored by the addition of cu2 several factors affecting variability in leaf ppo activity levels in plants are discussed