Changes in stability of isocitrate dehydrogenase (NADP+) during germination of castor bean seeds

In crude extract of castor bean endosperm, isocitrate dehydrogenase (NADP+) (EC 1.1.1.42) was stable at 57 degrees C at the beginning of seed germination as well as in maturing and dry seeds. The enzyme gradually became less thermostable as germination proceeded and became unstable after 4 days. Extract from 5-day-old endosperm reduced the thermostability of the thermostable enzyme. The destabilizing factor accumulated in the endosperm as germination progressed and was identified as ricinoleate. Ricinoleate destabilized the purified enzyme which was stabilized by isocitrate and Mg2+, but ricinoleate did not affect the activity of NADP+-isocitrate dehydrogenase itself. Stearate, oleate, palmitate and myristate were similar to ricinoleate in their effect on the thermostability of the enzyme. The thermolabile enzyme in the crude extract of 5-day-old endosperm was readily inactivated by trypsin and in low concentrations of buffer. The thermostable enzyme in the crude extract of 2-day-old endosperm was not affected by these treatments. The thermostable enzyme treated with ricinoleate showed the same instabilities as the thermolabile enzyme. The role of ricinoleate in the germinating castor bean endosperm is discussed.