Department of Botany, The University, Sheffield S10 2TN, Great Britain
Abstract:
1. 1. It has been proposed that Mg2+, inorganic pyrophosphate and a protein fraction which exhibits fructose-1,6-diphosphatase activity may interact to regulate photosynthesis by isolated chloroplasts.
2. 2. Evidence is presented which confirms the interaction and regulation but shows that these effects are indirectly attributable to pyrophosphatase activity rather than fructose-1,6-diphosphatase.
3. 3. When provided with Mg2+ and PPi the pyrophosphatase simply alters the proportions of orthophosphate and PPi in the reaction mixture. As the Pi concentration is increased, it first stimulates and then inhibits, the degree of inhibition being enhanced by additional Mg2+. PPi ameliorates the inhibition, possibly by chelation of Mg2+.
4. 4. It is concluded that the proposed regulation is ultimately governed by the Pi concentration and the known relationship between Pi uptake and triose phosphate export across the chloroplast envelope.