Amino Acid Uptake into Cultivated Mesophyll Cells from Asparagus officinalis L

The uptake of threonine, aspartic acid, and isoleucine into cultivated asparagus cells was examined under culture conditions.Transport was linear with time and with the number of cells present in the assay. The upake of the three amino acids obeyed Michaelis-Menten kinetics with apparent affinities in the range of 0.1 to 0.02 millimolar. Azide and 2,4-dinitrophenol caused inhibition of threonine uptake. Reciprocal competitive inhibitions were found between threonine, aspartic acid, and isoleucine. Methionine and lysine competitively inhibited the uptake of these three amino acids. These results and the comparison of apparent affinities and inhibition constants are in favor of only one uptake system for threonine, aspartic acid, isoleucine, methionine, and lysine with different affinities for the different amino acids. All other amino acids with the exception of histidine, arginine, asparagine, and hydroxyproline inhibited threonine uptake. It is most likely that at least one component of the uptake system is shared by all amino acids.