Decarboxylases of histidine and ornithine in chick embryo.

1. The activities of histidine and ornithine decarboxylases as well as the histamine content of the developing chick embryo were studied.2. Histidine decarboxylase (L-histidine carboxy-lyase; E.C. 4.1.1.22) activity was fairly low with a tendency to increase at later stages of development. This enzyme was preferentially present in the supernatant fraction of the tissue homogenate. alpha-Methyl-histidine, but not alpha-methyl-DOPA, inhibited its activity.3. The histamine content per gramme of embryo was low with a tendency to increase with the age of the embryo.4. Ornithine decarboxylase (L-ornithine carboxy-lyase; E.C. 4.1.1.17) activity was high at the beginning of the stages of development investigated, but later there was a steep fall in activity. A noticeable feature was that while the activity in the residual fraction of the homogenate remained almost constant during development, the activity in the supernatant fraction was high in the early stages, then fell rapidly to nearly zero at later stages.