Two types of ammonium uncoupling in pea chloroplasts.

The effect of ammonium on ATP synthesis, electron transfer, and light-induced uptake of hydrogen ions in pea chloroplasts was studied. It is shown that the dependence of these reactions on ammonium concentration could be due to effects of two different uncoupling processes. The first process is induced by low ammonium concentrations (<0.2 mM); the second one is observed in the NH(4)Cl concentration interval of 0.5-5.0 mM. The first type of uncoupling is stimulated by palmitic acid or by N,N'-dicyclohexylcarbodiimide, while the second is stimulated by chloroplast thylakoid swelling caused by energy-dependent osmotic gradients. In the presence of the fluorescent dye sulforhodamine B, which does not penetrate through the cell membrane, this swelling causes the dye to enter the lumens. It is supposed that ammonium activates two different routes of cation leakage from the lumen. The first route involves channel proteins, while the second is a mechanosensitive pore that opens in response to osmotic gradients.