A procedure for obtaining rat mast-cell histidine decarboxylase in greater than 50% yield in cell-free extracts was developed. The enzyme was found in the supernatant fractions from a 3,500 g and a 105,000 g centrifugation step and was demonstrated to be sensitive to inhibition by alpha-fluoromethylhistidine but not by phenylalanine. Although the enzyme shows a half-life of only 3 h in cell-free extract, the initial high recovery of activity allowed for active-site labelling of the enzyme by [3H]histidine and NaBH4. Labelled protein migrated on non-denaturing polyacrylamide-gradient-gel electrophoresis as a 55,000 Da species.
