Effects of trypsin and protein modification on the renal transporter ofp-aminohippurate

Summary

Basal-lateral membranous vesicles prepared from rabbit renal cortex exhibited Mg²⁺-stimulated, probenecid-inhibitable transport ofp-aminohippurate (PAH). This uptake could be completely eliminated by incubating the membranes with trypsin at a weight ratio of 1∶700 (trypsin/membrane protein). The loss of PAH uptake activity occurred in two stages. Over the first ten minutes of the vesicles' exposure to trypsin, there was a nearly linear loss, with respect to time, of about 80% of the PAH uptake activity. The remaining 20% of activity was resistant to further trypsin digestion for the next ten minutes, but by twenty-five minutes a total inactivation of the uptake activity occurred. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of normal and trypsin-treated vesicles showed very little degradation of proteins. However, two minor polypeptides (M_r-410,000 and 388,000) were degraded during the first ten minutes of the membranes' exposure to trypsin. After twenty minutes of exposure, two other poypeptides (M_r=94,500 and 87,500) were degraded. Chymotrypsin and clostripain also caused a loss of PAH transport activity. However, compared to the effects of trypsin, the effects of these two proteases were less complete, slower in onset, and for clostripain, a much higher concentration of enzyme was required. Other functions or properties of the vesicles including morphological appearance, degree of vesiculation, glucose space or Na⁺-dependentl-glutamate transport and Na⁺, K⁺-ATPase activity were not altered by the concentration of trypsin which abolished 80% of the transport of PAH. Thus, it is possible that one or more of the degraded polypeptides detected by polyacrylamide gel electrophoresis comprises the PAH transporter. Furthermore, modification of the vesicles with phenylglyoxal led to a 38% loss of PAH uptake activity. This suggests that arginine residues may play an important role in the transport system.