A stereoselective anomaly in dicarboxylic amino acid transport.

Using L-cysteate and tcysteinesulfinate as model substrates, we characterize here a transport system, both in cultured rat hepatocytes and human skin fibroblasts, serving for the anions glutamate and aspartate, but not for the dipolar species glutamic and aspartic acids.This system appears to be accompanied by a second, lower affinity system for the anionic forms, which is also Na'-dependent; this lower affinity system applies at least to glutamate.These systems show the usual degree of preference for L-over D-glutamate and, in the fibroblast, for L-over DL-a-aminoadipate.D-ASpartate proved nearly as inhibitory to the uptake of L- cysteate or t-aspartate, however, as did L-aspartate itself, a comparison recalling a similar stereoselective anomaly discovered by Pall in Neurospora (Pall, M. (1976) Biochim Biophys.Acta 211, 513-520).We conclude that this anomaly arises from the ability of the two substrate carboxylate groups to bond in the spatial order either a# for the L-isomer or &a for the D-isomer and also to bond in the order a,y for L-glutamate, but scarcely in the order y,a for D-glutamate.A major lack of inhibition by D-cysteate, which might be expected to bind like aspartate in the inverted order, shows, however, that the two anionic groups are not recognized in identical manners by the two corresponding subsites, Precedent for a chemical difference in these two subsites is available from transport systems for neutral aand p-amino acids.A strong transport inhibition of the hepatocyte system by 3-aminoglutarate shows that an a,a relation between the amino group and either of the carboxylate groups of the anionic amino acid is not required.The above anomaly in stereoselectivity is compared with a corresponding one, applying to the reactions of aspartic acid and asparagine, versus glutamic acid and glutamine, with System L for neutral amino acid transport in the Ehrlich cell.A weak pHdependent inhibition of the uptake of anionic amino acids by cysteine can be associated with its unique mode of conversion to an anionic species.