22] Amino acid decarboxylases of animals

Abstract

Publisher Summary Dihydroxyphenylalanine decarboxylase occurs in a variety of tissues of numerous species, but guinea pig kidneys are particularly rich in this enzyme. A stable dry powder may be prepared from guinea pig or rabbit kidneys. In a Waring blendor, fresh kidneys are mixed with 3 to 4 vol. of ice-cold water for 4 minutes at room temperature. The decarboxylase activities of the dry powders (dissolved in M /15 phosphate buffer, pH 6.80) is about 75% (rabbit) and 45 to 60% (guinea pig) of those observed with phosphate buffer extracts from fresh kidneys of the same batch. The amounts of dry powders used in these stability studies (and suitable for assay studies) were 25 to 60 mg. (guinea pigs) and 90 to 120 rag. (rabbits) per Warburg flask and liberated between 35 and 85 μl. of CO 2 during the first 10 minutes after addition of substrate. The enzyme is not absolutely specific for L-3,4-dihydroxyphenylalanine but also decarboxylates other substituted phenylalanines with at least one hydroxy group in ortho or meta position, such as 2-hydroxy-, 3-hydroxy-, 2,5-dihydroxy-, and 3,5-dihydroxyphenylalanine at rates somewhat slower than those observed with 3,4-dihydroxyphenylalanine. It is interesting that the introduction of a second hydroxy group in meta position into 3-hydroxyphenylalanine, leading to 3,5-dihydroxyphenylalanine, considerably reduces the rate of decarboxylation.