Photophosphorylation by Chloroplasts: Effects of Low Concentrations of Ammonia and Methylamine

Photophosphorylation, Uncoupling, Amines, Proton Motive Force, Chloroplasts, Chemiosmotic Theory Intact chloroplasts exposed to hypotonic assay conditions are capable o f photophosphorylating exogenous ADP. The rate of phosphorylation by these unbroken plastids is increased by 10—50% upon the addition of low concentrations (< mM) of NH 3 or CH3N H 2. Stimulation o f phosphory­ lation is abolished by washing chloroplasts with MgCl2. Evidence is presented that washing re­ moves a factor responsible for amine-induced increase o f ATP production and that this factor is associated with the thylakoid membrane. Addition of CH3N H 2 increased the proton permeability of the thylakoid membrane of unbroken and washed chloroplasts during the light/dark transition. Hence, differences of the membrane permeability for protons between the two preparations seem not to be responsible for an increase o f ATP production upon the addition of amines. Stimulation of photophosphorylation by methylamine is observed even at light itensities which do not saturate the proton motive force, which in turn is reduced upon the addition o f the uncoupler. Apparently, phosphorylation can be stimulated, although the limiting driving force is diminished. It is con­ cluded that phosphorylation by unbroken chloroplasts under low light illumination is limited kinetically, not energetically. Consequences of these findings for observation made with intact chloroplasts are discussed.