Regulatory isoenzymes of aspartate kinase and the control of lysine and threonine biosynthesis in carrot cell suspension culture.

Aspartate kinase (EC 2.7.2.4) from carrot (Daucus carota L.) cell suspension culture has been 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-week growth cycle of the culture, and this provides additional evidence for the existence of two independently regulated isoenzymes. Exogenously supplied lysine and threonine specifically inhibited the in vivo formation of lysine and threonine, respectively, from radioactive acetate.