Abstract. The conversion of D-lactate to pyruvate in isolated membrane preparations of E. coli ML 308-225 markedly stimulates the transport of proline, glutamic acid, aspartic acid, asparagine, tryptophan, lysine, serine, alanine, and glycine. The uptake of histidine, phenylalanine, tyrosine, leucine, isoleucine, and valine by the membranes is also markedly stimulated by this conversion, although these amino acids are taken up much less effectively than those mentioned previously. The uptake of arginine, methionine, cystine, and cysteine is enhanced only about twofold in the presence of D-(-)-lactate, and these amino acids are not concentrated well by the membranes. With the exception of glutamate, aspartate, asparagine, and methionine, which are converted to other metabolites to varying extents in the intramembranal pool, each of the other amino acids was recovered from the membranes as the unchanged amino acid. Succinate, L-(+)-lactate, D,L-a-hydroxybutyrate, and DPNH partially replace D-(-)-lactate but are less effective.Previous work from this laboratory" 2 demonstrated that isolated membrane preparations from E. coli W6, in the absence of soluble proteins and nucleic acids, catalyzed the concentrative uptake of proline, whereas membranes prepared from E. coli W157, a proline transport mutant,3 did not. Proline uptake by these preparations was stimulated by glucose and was inhibited by anaerobiosis and by a variety of compounds known to uncouple oxidative phosphorylation or inhibit electron transport.Studies recently made with membranes prepared from E. coli 1\IL 308-225 have defined the energetics of the proline uptake system in much greater detail.4 D-(-)-Lactate markedly stimulates proline uptake with a 20-to 30-fold increase over baseline levels. Of all the metabolites and cofactors tested, only succinate, L-(+)-lactate, D,L-a-hydroxybutyrate, and DPNH replace D-(-)-lactate to any extent whatsoever. Succinate stimulates proline uptake 8-to 10-fold, and L-(+)-lactate, D,L-a-hydroxybutyrate, and DPNH stimulate only 3-to 4-fold. D-(-)-Lactate-14C or succinate-'4C are converted stoichiometrically to pyruvate or fumarate, respectively, by the membrane preparations. These results indicate that the concentrative uptake of proline involves electron transport and, more specifically, that a membrane-bound lactic dehydrogenase with 10(8
