Regulatory Isoenzymes of Aspartate Kinase and the Control of Lysine and Threonine Biosynthesis in Carrot Cell Suspension Culture

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Abstract
Aspartate kinase (EC 2.7.2.4) from carrot (Daucus carota L.) cell suspension culture was partially resolved into lysine-sensitive and threonine-sensitive components by gel filtration chromatography. The yield of lysine-sensitive aspartate kinase changed independently of the yield of the threonine-sensitive activity during the 4 wk growth cycle of the culture, and this provides additional evidence for the existence of 2 independently regulated isoenzymes. Exogenously supplied lysine and threonine specifically inhibited the in vivo formation of lysine and threonine, respectively, from radioactive acetate.