AMINO-ACID EXCHANGE ACTIVITY OF THE ALANINE TRANSPORTER OF GIARDIA-INTESTINALIS

The influx and efflux of alanine and other amino acids was studied in trophozoites of Giardia intestinalis. Transport of L-[2,3-H-3]alanine was used as the index of influx. On the basis of the competition of L- [2,3-H-3]alanine uptake by analogues of alanine, the substrate specifi city of the alanine transporter was determined. The transporter is an antiport. Influx of alanine or those analogues which inhibited alanine influx caused the efflux of intracellular alanine and a number of ami no acids structurally related to alanine. Amino acids unrelated to ala nine, such as glutamate, effluxed at a slow rate, and the efflux was n ot stimulated by extracellular alanine or alanine analogues. However, there was a subset of intracellular amino acids, the alanine subset co mprising alanine, serine, glycine, and threonine, the efflux of which was stimulated by external alanine or alanine analogues. Direct measur ement by amino acid analysis demonstrated intracellular accumulation o f alanine analogues concomitant with the efflux of the alanine subset. These data indicate unequivocal evidence of exchange of intracellular alanine with extracellular alanine analogues, with a 1:1 molar stoich iometry. This is the first demonstration in G. intestinalis of the ant iport function of an amino acid transporter. (C) 1995 Academic Press, Inc.