A sensitive and rapid assay method for cysteic acid decarboxylase was developed which combined the selectivity of ion exchange resin (a complete retention of the substrate, cysteic acid, and exclusion of the product, taurine) with the speed of a vacuum filtration. The synthesis and purification of35S-labeled cysteic acid were described. The validity of the assay was established by the identification of the reaction product as taurine. With this new method, the decarboxylase activity was measured in discrete regions of bovine brain. Putamen had the highest activity, 172 pmol taurine formed/min/mg protein (100), followed by caudate nucleus, 90; cerebral cortex, 82; hypothalamus, 81; cerebellar cortex, 79; cerebellar peduncle, 59; thalamus, 42; brain stem, 25; pons, 10; and corpus callosum, 3. The decarboxylase activity in various mouse tissues was also determined as follows: liver, 403; brain, 145; kidney, 143; spinal cord, 59; lung, 21; and spleen, 10 pmol taurine formed/min/mg. No activity could be detected in skeleton muscle and heart, suggesting a different biosynthetic pathway for taurine synthesis in these tissues. The advantages and disadvantages of the new assay method are also discussed.
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