5-Hydroxytryptophan decarboxylase activity in rat mast cells

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Abstract
Mast cells of rats contain, in addition to histamine and heparin, serotonin. This study was undertaken to determine the ability of mast cells to synthesize serotonin. Preliminary experiments demonstrated a positive correlation between mast cell concentration in different rat tissues and 5-hydroxytryptophan decarboxylase activity. An attempt to demonstrate this activity directly in peritoneal mast cells separated from other cells by centrifugation through a high density sucrose solution was unsuccessful. A new method of separating mast cells was developed employing concentrated bovine albumin solution. The mast cells could be centrifuged through this solution; most other cells remained at the interface. Mast cells separated in purities of 40–80% contained decarboxylase activity at a similar level to that of centrifuged unpurified mast cells. Characteristics of the mast cell decarboxylase system were investigated using peritoneal washings which contained 5% mast cells. The decarboxylase was inhibited by substrate concentrations greater than 1 mg/ml. The activity of the decarboxylase system was enhanced by the addition of pyridoxal-5-phosphate. A pH optimum of 7.5 was found for the decarboxylase activity in the cells.