A new enzyme for the interconversion of pyruvate and phosphopyruvate and its role in the C4 dicarboxylic acid pathway of photosynthesis.

1. An enzyme was isolated from leaves of tropical grasses that catalyses the reversible conversion of pyruvate, ATP and orthophosphate into phosphopyruvate, AMP and pyrophosphate. A requirement for Mg(2+) could not be replaced by Mn(2+) or Ca(2+). 2. By replacing orthophosphate with [(32)P]orthophosphate or with arsenate, evidence was provided that the orthophosphate consumed appears in pyrophosphate. 3. Without Mg(2+) or 2-mercaptoethanol the enzyme was rapidly and irreversibly inactivated. EDTA only partially replaced the requirement for the thiol compound. The enzyme was considerably more unstable at 0 degrees or when frozen than at 22 degrees . Even with the best conditions devised the enzyme lost about 25% of its activity every 3hr. 4. The activities of the enzyme in leaves of the tropical grasses sugar cane (Saccharum hybrid var. Pindar), maize (Zea mays) and sorghum (Sorghum vulgare) were comparable with their maximum photosynthesis rates. The enzyme was not detectable in leaf extracts from several other plants. 5. Its role in photosynthesis is discussed.