Pyruvate Orthophosphate Dikinase of C3 Seeds and Leaves as Compared to the Enzyme from Maize

Abstract

Pyruvate orthophosphate dikinase (PPDK) was found in various immature seeds of C(3) plants (wheat, pea, green bean, plum, and castor bean), in some C(3) leaves (tobacco, spinach, sunflower, and wheat), and in C(4) (maize) kernels. The enzyme in the C(3) plants cross-reacts with rabbit antiserum against maize PPDK. Based on protein blot analysis, the apparent subunit size of PPDK from wheat seeds and leaves and from sunflower leaves is about 94 kdaltons, the same as that of the enzyme from maize, but is slightly less (about 90 kdaltons) for the enzyme from spinach and tobacco leaves. The amount of this enzyme per mg of soluble protein in C(3) seeds and leaves is much less than in C(4) leaves. PPDK is present in kernels of the C(4) plant, Zea mays in amounts comparable to those in C(4) leaves.Regulatory properties of the enzyme from C(3) tissues (wheat) are similar to those of the enzyme from C(4) leaves with respect to in vivo light activation and dark inactivation (in leaves) and in vivo cold lability (seeds and leaves).Following incorporation of (14)CO(2) by illuminated wheat pericarp and adjoining tissue for a few seconds, the labeled metabolites were predominantly products resulting from carboxylation of phosphoenolpyruvate, with lesser labeling of compounds formed by carboxylation of ribulose 1,5-bisphosphate and operation of the reductive pentose phosphate cycle of photosynthesis. PPDK may be involved in mechanisms of amino acid interconversions during seed development.