In vitro GAD activity has been characterized
in crude extracts from many plant species and tissues5–7. GAD is
specific for L-glutamate, pyridoxal 59-phosphate-dependent,
inhibited by reagents known to react with sulfhydryl groups, possesses
a calmodulin-binding domain, and exhibits a sharp acidic
pH optimum of ~5.8. GAD genes from Petunia15, tomato16,
tobacco17 and Arabidopsis18,19 have been identified.