Amine Uncoupling of Energy Transfer in Chloroplasts

Abstract

Abstract Chloroplasts suspended in the presence of NH4Cl have been shown to take up ammonium ion from the medium on illumination. NH4+ uptake approached a maximum with increasing NH4Cl concentration, and was associated with a parallel inhibition of the light-induced pH change. Maximal values for rate and extent of NH4+ uptake did not vary greatly with pH. However, below pH 7 the apparent Km for NH4Cl rose markedly with decreasing pH, and this reflected a calculated Km for NH3 which did not change greatly. Above pH 7, the Km for NH3 increased with increasing pH, and this was associated with a loss of coupling between electron flow and energy conservation. It has been concluded that NH3 is the active species in NH4+ uptake. NH4+ and H+ uptake accompanying ferricyanide reduction in the Hill reaction have been compared with uptake linked to N-methylphenazonium methosulfate-catalyzed electron flow. It was found that between 2 and 3 H+ or NH4+ ions were taken up into the chloroplast per electron transported to ferricyanide. Competitive inhibition of NH4+ uptake under conditions of photophosphorylation has been found. Phlorizin at a concentration which completely inhibited photophosphorylation had little effect on the rate of H+ or NH4+ uptake, but partially reduced the extent of NH4+ uptake. A mechanism for NH4+ uptake has been proposed. It has been suggested that, on acidification of the chloroplast interior following H+ uptake, the equilibration of NH3 across the chloroplast membrane is displaced, and that more NH3 enters to replace that lost by association with H+. The net accumulation of NH4+ leads to an uptake of Cl- by diffusion, and the increased osmolarity within the chloroplasts leads to water movement and swelling. The mechanism proposed also provides a general mechanism for uncoupling by amines.