Pyruvate orthophosphate dikinase has been identified in the green grains of eight cereal grasses, most of which are classified as C3 plants. The wheat (Triticum aestivum L. cv. Lerma Rojo) grain enzyme was further investigated: activity was low in very young grains, increased to a maximum at about 25 days after anthesis, then returned to a low level as the grain matured. It appeared to be located in the aleurone layer. A procedure was developed for obtaining partially purified preparations of pyruvate orthophosphate dikinase from the ears of wheat, oat (Avena sativa L.), barley (Hordeum distichum L.), and rye (Secale cereale L.). These preparations were suitable for measuring activities in both the forward and reverse reaction directions. The affinities of these enzymes for the six substrates (pyruvate, orthophosphate, and ATP in the forward reaction; phosphoenolpyruvate, pyrophosphate, and AMP in the reverse reaction) were determined and found to be similar to the reported affinities of the enzyme from the leaves of the C4 plant Zea mays. A possible role for pyruvate orthophosphate dikinase in cereal grains is considered briefly.
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