The effect on amino acid transport of trypsin treatment of rat renal brush border membranes

Inhibition by trypsin of the high-affinity acidic amino acid transport system in C6 glioma cells

The high-affinity uptake of the acidic amino acid D-aspartate was inhibited in a dose- and time-dependent manner, when C6 cells were exposed to trypsin. The protease decreased the maximal velocity for uptake but not its Km, consistent with a reduction in the number of competent carriers at the plasma membrane. Cellular energy production and [K+]i were unaffected, indicating that the transporter itself was the site of trypsin action. Maximum inhibition of uptake was 50%, which suggests the presence of a heterogeneous population of transporters, only half of which is sensitive to trypsin. These results support our earlier postulate that in glial cells, the high-affinity transporter for acidic amino acids is a transmembrane protein, part of which extends into the external environment.

Enzymatic inhibition of lysine transport across the small intestine in vivo

1. Trypsin, at different concentrations, significantly inhibited lysine absorption (P less than 0.05) in a dose-dependent pattern. 2. Maximum inhibition equivalent to 35% below control value was reached with 10 micrograms/ml (100 BAEE units) trypsin with a non-reversible inhibitory effect. 3. Chymotrypsin at 10 micrograms/ml produced a significant decrease (P less than 0.05) of lysine absorption although it did not exceed 5%. Perfusion of both enzymes did not show an additive inhibitory effect. 4. Lysine absorption showed a 39% decrease with 10 micrograms/ml trypsin and 1 X 10(-4) M ouabain, whereas ouabain alone produced 34% inhibition. 5. Lysine absorption showed a 71% decrease with 10 micrograms/ml trypsin in a sodium-free medium, and 70% inhibition with Na-free medium alone. 6. The inhibition of lysine absorption after trypsin treatment could be due to inhibition of the active component of lysine transport.

L-cystine transport by papain-treated rat renal brush-border membrane vesicles

In papain-treated rat renal brush-border membrane vesicles, cystine uptake was enhanced under sodium gradient conditions. This effect was not observed when sodium was equilibrated across the vesicle membrane or when sodium was completely absent from the incubation medium. The increased rate of cystine uptake occurred within the first two minutes of incubation and coincided with the period of increased flux of sodium known to occur after papain treatment. Under sodium gradient conditions, the Vmax of cystine uptake by treated vesicles was 65% greater while the Km was 25% lower than the value observed in untreated membranes. The increased cystine uptake after papain treatment occurred when medium cystine was in the electroneutral form. In the absence of a sodium gradient, cystine uptake by control membranes was insensitive to changes in membrane potential and this was unaltered after papain treatment. Exposure of the membranes to papain also resulted in a profound decrease in cystine binding which occurs in native membranes incubated with cystine. The fact that cystine uptake is unchanged under sodium equilibration and even enhanced under sodium gradient conditions suggests that the component of cystine binding is not essential for cystine transport and may represent non-specific binding to membrane proteins.

Effectors of amino acid transport processes in animal cell membranes

Various effectors, which act upon ion gradients, protein synthesis, membrane components or cellular functional groups, have been employed to provide insights into the nature of amino acid-membrane transport processes in animal cells. Such effectors, for example, include ions, hormones, metabolites and various organic reagents and their judicious use has allowed the following list of conclusions. Sodium ion has been found to stimulate amino acid transport in a wide variety of cell systems, although depending on the tissue and/or substrate, this ion may have no effect on such transport, or even inhibit it. Amino acid transport can be stimulated in some cell systems by other ions such as K+, Li+, H+ or Cl-. Both H+ and K+ have been found to be inhibitory in other systems. Amino acid transport is dependent in many cell systems upon an inwardly directed Na+ gradient and is stimulated by a membrane potential (negative cell interior). In some cell systems an inwardly directed Cl- and H+ gradient or an outwardly directed K+ gradient can energize transport. Structurally dissimilar effectors such as ouabain, Clostridium enterotoxin, aspirin and amiloride inhibit amino acid transport presumably through dissipation of the Na+ gradient. Inhibition by certain sugars or metabolic intermediates of the tricarboxylic acid cycle may compete with the substrate for the energy of the Na+ gradient or interact with the substrate at the carrier level either allosterically or at a common site. Stimulation of transport by other sugars or intermediates may result from their catabolism to furnish energy for transport. Insulin and glucagon stimulate transport of amino acids in a variety of cell systems by a mechanism which involves protein synthesis. Microtubules may be involved in the regulation of transport by insulin or glucagon. Some reports also suggest that insulin has a direct effect on membranes. In addition, a number of growth hormones and factors have stimulatory effects on amino acid transport which are also mediated by protein synthesis. Steroid hormones have been noted to enhance or diminish transport of amino acids depending on the nature of the hormone. These agents appear to function at the level of protein synthesis. While stimulation may involve increased carrier synthesis, inhibition probably involves synthesis of a labile protein which either decreases the rate of synthesis or increases the rate of degradation of a component of the transport system.(ABSTRACT TRUNCATED AT 400 WORDS)

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

Basal-lateral membranous vesicles prepared from rabbit renal cortex exhibited Mg2+-stimulated, probenecid-inhibitable transport of p-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 (Mr - 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 polypeptides (Mr = 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+-dependent L-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.(ABSTRACT TRUNCATED AT 250 WORDS)

Leakiness of brush-border vesicles

From the water content of pelleted brush-border vesicles and from a comparison of the aqueous volume within the pellet that is available to [3H]inulin (58%), inulin [14C]carboxylic acid (34%, both approx. 5000 daltons), [3H]raffinose (97%, 540 daltons) and [3H]glucose (94%, 180 daltons) it is concluded that only 1 in 4 to 6 of the brush-border vesicles is sealed. The implication of this finding for labelling and transport studies and for vesicle formation is discussed.

The effect of papain upon proline and sodium transport of rat renal brush-border membrane vesicles

Treatment of renal brush-border membrane vesicles with papain resulted in the removal of the activity of maltase, gamma-glutamyl transpeptidase and leucine aminopeptidase by 85, 50 and 75%, respectively. Stripping of these membrane enzyme activities constituted about 2% of the total membrane proteins and resulted in a widespread diminution in the ability of a variety of amino acids and sugars to be taken up by the membrane vesicles which remained osmotically responsive. Kinetic analysis of the uptake of proline, which was shown previously to be transported by both sodium-dependent and sodium-independent systems, revealed that the Vmax for the sodium-dependent system and Km for the sodium-independent system were halved, but other parameters were not affected indicating that the papain treatment altered sodium-gradient-stimulated entry and the affinity of the sodium-gradient-independent system for proline. Experiments on sodium entry and efflux demonstrate a marked enhancement of flux, so that equilibration of the sodium gradient occurred about 5-times more rapidly than in untreated vesicles. This occurred without any change in the osmotic properties of the vesicle with regard to sodium or amino acid uptake. Studies of fluorescence polarization suggest that incubation with papain does not alter the lipid domains of the membrane.