CHARACTERIZATION OF P-AMINOHIPPURATE TRANSPORT FROM RAT-KIDNEY WHICH IS EXPRESSED AFTER INJECTION OF SIZE-SELECTED MESSENGER-RNA INTO OOCYTES OF XENOPUS-LAEVIS

First, the existence of an endogeneous p-aminohippurate (PAH) transpor ter in oocytes of Xenopus laevis was demonstrated. When, however, the oocytes were injected with mRNA from rat kidney cortex, an expressed p -aminohippuric acid (PAH) uptake was seen which differed from the endo geneous transporter. Both transport systems are saturated at high PAH concentrations, exhibit trans-stimulation by PAH and are partially inh ibited by probenecid. The endogeneous transport has a rather low affin ity for PAH (K(m) = 0.57 mM) and is about 50% inhibited by probenecid (one apparent inhibition site with half maximal inhibition at 0.5 mM). The expressed PAH transport has a high affinity for PAH (K(m) = 60 mu M) and can be inhibited 80% by probenecid (two apparent inhibition sit es with half maximal inhibitions at 1 muM and 2 mM). Expression experi ments with fractionated mRNA revealed that the PAH transport expressed from rat kidney cortex is encoded by an mRNA of 1.8 to 2.5 kb.