Monovalent Cation Act Dehydroge ivation of Plant Pyruvate nase Kinase'

l h e pyruvate dehydrogenase kinase-catalyzed inactivation of the pyruvate dehydrogenase complex was studied using dialyzed, soluble proteins from mitochondria purified from green leaf tissue of Pisum sativum 1. seedlings. At subsaturating ATP concentrations, K+ or NH4+, but not Na+, stimulated the pyruvate dehydrogenase kinase by lowering the K,,,(ATP). Micromolar concentrations of NH4+ were required to produce the same effect as millimolar concentrations of K+. This is apparent from the observations that the activation constant (Kad) for NH4+ was 0.1 mM, whereas the K,,(K+) was 0.7 mM. Maximal pyruvate dehydrogenase kinase velocities attained with NH4+ were higher than those with K+, and, therefore, NH4+ was able to stimulate PDH kinase further in the presence of saturating K+. This result supports our conclusion that photorespiratory NH4+ production in plant mitochondria may be involved in regulating the entry of carbon into the Krebs cycle by way of the pyruvate dehydrogenase complex.