Characterization of the activity of fatty-acid epoxide hydrolase in seeds of castor bean (Ricinus communis L.)

Epoxide hydrolase (EC 3.3.2.3) activity was measured with 1-¹⁴C]cis-9,10-epoxystearic acid as the substrate. Homogenates were prepared from the endosperm tissue of germinating seeds of castor bean (Ricinus communis L. zanzibariensis). The activity of fatty-acid epoxide hydrolase was characterized with respect to dependence on time, amount of protein, pH and temperature. Analyses of enzyme distribution in endosperm, cotyledons, root and hypocotyl showed the highest total activity in the endosperm, less in the cotyledons and low activity in the root and hypocotyl. The specific activity was similar for cotyledons and endosperm. Analysis of the temporal expression of the enzyme in the endosperm during germination revealed high activity already in the imbibed seed. Activity was maximal between days four to six and then decreased at the end of one week. Subcellular fractionation of endosperm revealed a dual distribution of activity between the glyoxysomal and the cytosolic fractions.