Amino acid permeability of the chloroplast envelope as measured by light scattering, volumetry and amino acid uptake

Abstract

The amino acid permeability of the envelope of intact, functional spinach (Spinacia oleracea L.) chloroplasts was investigated by light scattering, volumetry and uptake of (14)C-labelled amino acids. The criterion for the functionally of the chloroplasts was their ability to reduce CO2, PGA and oxaloacetate in the light at high rates.Net uptake into the chloroplast interior of neutral amino acids such as alanine, glycine, serine, proline, threonine or valine occurred only at very low rates. The uptake was concentration dependent, indicating unspecific diffusion rather than carrier-mediated transport. The slowness of uptake is emphasized by the capability of neutral amino acids to provide osmotic support for intact chloroplasts during a considerable length of time. Back-exchange experiments also failed to indicate the existence of specific exchange carriers for the transport of neutral amino acids such as alanine or glycine through the envelope of intact chloroplasts. Dicarboxylic amino-acids are known to be taken up by the so-called dicarboxylate translocator. The same carrier was found to catalyze also the transfer of asparagine and glutamine.The data do not support current assumptions concerning fast carrier-mediated transport of neutral amino acids and their role in the transfer of carbon during photosynthesis.