PYRIDOXAL‐5′‐PHOSPHATE AS A COFACTOR FOR RAT BRAIN HISTIDINE DECARBOXYLASE1

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Abstract
—: The cofactor requirements of brain histidine decarboxylase activity have been studied. Preincubation with carbonyl reagents caused inhibition of the activity ranging from 90% for 10−2m‐semicarbazide, 10−3m‐phenylhydrazide and 10−3m‐hydroxylamine to 50% for isonicotinic acid hydrazide. Sodium borohydride, a reducing agent, also caused complete inhibition of activity. The histidine decarboxylase activity was maximal at 10−4m‐pyridoxal‐P concentration and was inhibited at higher concentrations of the cofactor.The cofactor‐apoenzyme mode of binding was studied by dialysing brain homogenates against several media. Neither the dialysis against buffers alone nor against buffers containing semicarbazide nor cysteine plus EDTA caused a total loss of activity. A 50% of the activity dialysed easily while the other 50% remained ‘tightly’ bound to the apoenzyme. The dialysable and non dialysable activity is evenly distributed between the soluble and particulate activity.