Electrogenic transport of 5-oxoproline in rabbit renal brush-border membrane vesicles. Effect of intravesicular potassium

The Na+-dependent transport of 5-oxoproline into rabbit renal brush-border vesicles was stimulated by a K+ diffusion potential (interior-negative) induced by valinomycin. Na+ salts of two anions of different epithelial permeabilities also affected 5-oxoproline transport. These results show that the Na+-dependent 5-oxoproline transport in renal brush-border vesicles is an electrogenic process which results in a net transfer of positive charge. Maximum transport of 5-oxoproline occurred at an extravesicular pH of 6.0 to 8.0 and over that pH range, 5-oxoproline exists completely as an anion with a negative charge. The simplest stoichiometry consistent with this process is, therefore, the cotransport of one 5-oxoproline anion with two sodium ions. The presence of K+ inside the vesicles stimulated the Na+-dependent transport of 5-oxoproline. This stimulatory effect was specific for K+ and required the presence of Na+. The presence of Na+ gradient was not mandatory for the K+ action. The stimulation by the intravesicular K+ was seen in the presence as well as in the absence of a K+ gradient. Therefore, the increased influx of 5-oxoproline was not coupled to the simultaneous efflux of K+. The presence of K+ in the extravesicular medium alone did not affect the Na+-dependent transport of 5-oxoproline, showing that the site of K+ action was intravesicular. Glutamate did not interact with the Na+-dependent 5-oxoproline transport even in the presence of an outward K+ gradient.

Transport and utilization of methionine sulfoxide in the rabbit

Methionine sulfoxide is transported into purified intestinal and renal brush border membrane vesicles from rabbit by an Na+-dependent mechanism and is accumulated inside the vesicles against the concentration gradient. Both in intestine and kidney, the rate of transport is enhanced with increasing concentrations of Na+ in the external medium. Increasing the Na+ gradient reduces the apparent Kt for methionine sulfoxide without causing any change in Vmax. With an outward K+ gradient (vesicle greater than medium), valinomycin stimulates the Na+-gradient-dependent transport of methionine sulfoxide in the kidney, showing the electrogenicity of the transport process. A number of amino acids inhibit methionine sulfoxide transport in both the intestine and kidney. An enzymatic activity capable of reducing methionine sulfoxide to methionine is present in the intestinal mucosa, renal cortex and liver. The activity is highest in renal cortex and lowest in intestine. The methionine sulfoxide-reducing activity is stimulated by NADH, NADPH, glutathione and dithiothreitol and the potency of the stimulation is in the order: dithiothreitol greater than NADPH greater than glutathione greater than NADH.

Prolidase contamination in commercial bovine serum albumin

Bovine serum albumin from a number of commercial sources were screened for the presence or absence of peptidase contamination. Peptidase activity was monitored using various peptides as substrates. Two commercial preparations were found to have peptidase activity, and the enzyme was identified, on the basis of its substrate specificity, as prolidase (EC 3.4.14.9). The contaminating activity was in the order of 3-4 units/g albumin.

Peptide transport in rabbit kidney. Studies with L-carnosine

L-Carnosine was shown to be transported into rabbit renal brush-border membrane vesicles by an Na+ -independent mechanism. The transport was competitively inhibited by glycyl-L-proline. Various di- and tripeptides inhibited L-carnosine transport, whereas free amino acids did not. Inhibition studies showed that blocking the free amino and carboxyl groups of the peptide reduced its affinity for the transport carrier. Under the conditions in which there was no detectable hydrolysis of L-carnosine in the medium, intravesicular contents showed a 30% hydrolysis of the peptide within the vesicles. Disruption of membrane vesicles with deoxycholate resulted in a 3-fold increase in L-carnosine hydrolyzing activity over untreated intact vesicles. Based on these observations, a model for peptide transport is proposed in which transport of the intact peptide across the membrane is followed by its partial or complete hydrolysis by a membrane peptidase whose active site is on the cytoplasmic side of the membrane.

Peptide transport in intestinal and renal brush border membrane vesicles

A longstanding question about the possible dependence of transmembrane peptide transport on sodium has now been resolved. Recent studies with purified intestinal brush border membrane vesicles have shown that peptide transport across this membrane is Na+-independent and occurs by a non-concentrative mechanism. Similar studies with renal brush border membrane vesicles have established for the first time the presence of a peptide transport system in mammalian kidney. The essential characteristics of peptide transport in these two tissues are the same. However, it still remains to be seen whether a new mechanism other than the Na+-gradient, hitherto unrecognized, is involved in energizing the active transport of peptides in vivo in mammalian intestine and kidney.

Evidence for a dipeptide transport system in renal brush border membranes from rabbit

Papain treatment of renal brush border vesicles was carried out as a successful first step towards the purification of the membrane components involved in dipeptide transport. The treated vesicles exhibited increased specific transport activity of glycyl-L-proline. In contrast, the specific transport activity of L-alanine in the treated vesicles was less than that in the control vesicles. Papain treatment resulted in the solubilization of 38% of protein, 55% of alkaline phosphatase, 90% of gamma-glutamyltransferase and 95% of leucine aminopeptidase. There was no change in the intravesicular volume nor was there any increase in vesicular permeability. Glycyl-L-proline transport was Na+-independent in the control and papain-treated vesicles. Diamide reduced the Na+-dependent L-alanine transport while glycyl-L-proline transport remained unaffected in the presence of Na+. Many dipeptides inhibited glycyl-L-proline transport both in the presence and absence of Na+. The inhibition by dipeptides was greater than the inhibition by equivalent concentrations of free amino acids. These data demonstrate that renal brush border vesicles can efficiently handle dipeptides by a mechanism completely different from that of amino acid transport.

Transport of glycyl-L-proline into intestinal and renal brush border vesicles from rabbit

Transport of labeled glycyl-L-proline has been shown to occur with highly purified brush border membrane vesicles from the epithelial cells of rabbit small intestine and renal cortex. With 1-min incubation, transport occurs mainly as the intact dipeptide since less than 10% of the dipeptide in the medium is hydrolyzed within the period. The properties of the dipeptide transport system are similar in both small intestinal and renal brush border membrane vesicles. The steady state transport varies inversely with medium osmolarity. Extrapolation to infinite medium osmolarity indicates that transport occurs predominantly into an osmotically reactive intravesicular space rather than binding to the membranes. The affinity constants (Kt) for glycyl-L-proline transport in small intestinal and renal brush border membrane vesicles are comparable (0.9 mM in intestine and 1.1 mM in kidney). Under conditions in which presence of a Na+ gradient between external and intravesicular media stimulated L-alanine transport, glycyl-L-proline transport remains unaffected. Other dipeptides strongly inhibit the transport of glycyl-L-proline but amino acids have no effect. The selective inhibition of glycyl-L-proline transport by other dipeptides is observed in the presence as well as in the absence of a Na+ gradient. Harmaline inhibits Na+-stimulated L-alanine transport but it has no effect on glycyl-L-proline transport even in the presence of Na+. In these respects, dipeptide transport seems to differ from amino acid transport. It is proposed that the Na+ gradient hypothesis of sugar and amino acid transport is not applicable for dipeptide transport. These data provide additional evidence for the distinct nature of amino acid and dipeptide transport systems.

Impaired intestinal absorption of dipeptide in tropical sprue patients in India

1. Intestinal absorption of glycylglycine was studied in four control subjects and six patients with tropical sprue by using a direct technique of intestinal perfusion. 2. The patients with tropical sprue showed significant impairment in the absorption of the dipeptide.

Interaction of amino acids with glycl-L-leucine hydrolysis and transport in monkey small intestine

1. There are two saturable transport processes in the monkey small intestine for glycyl-L-leucine, one with Vmax. 1 mumol min-1 g-1 wet weight of tissue and an affinity constant (kt) of 5 mmol/l, and the other with Vmax 3.9 mumol min-1 g-1 wet weight of tissue and kt 33 mmol/l. 2. Glycyl-L-leucine uptake is inhibited by a wide variety of amino acids, although to a variable extent. The inhibition was shown to be competitive with leucine used as the representative amino acid. Phenylalanine, methionine, alanine and leucine are the most potent in their inhibitory action. 3. The effect of various amino acids on the hydrolysis of glycyl-L-leucine by particulate and cytosol fractions of monkey small intestine was studied. All the amino acids, except glycine, proline, alanine and glutamic acid, inhibit both the particulate and cytosol glycyl-L-leucine hydrolase activities. In general, the cytosol enzyme is more susceptible to amino acid inhibition than the particulate enzyme. 4. There is no correlation between the effects of amino acids on glycyl-L-leucine uptake and hydrolysis of glycyl-L-leucine by either particulate or cytosol fraction.

Intestinal perfusion studies in tropical sprue. 1. Amino acid and dipeptide absorption

Intestinal absorption of glycine 20 mmol/1, glycyl-glycine 10 mmol/1 plus L-leucine 10 mmol/1, and glycyl-L-leucine 10 mmol/1 has been studied by intestinal perfusion in 11 patients with tropical sprue and 10 control subjects. The patients with sprue had a significant reduction in the rate of absorption of glycine from a 20 mmol/1 solution, but there were no significant differences in the absorption of the other substances. The failure to demonstrate any difference in the absorption of these substances is probably related to their low concentration relative to the maximum absorptive capacity of the intestine. In both groups of subjects the kinetic advantage of glycyl-glycine absorption as compared with glycine absorption was maintained. When the dipeptides were perfused, free amino acids appeared in the perfusate presumably by "back diffusion" from the mucosal cells. In the case of glycyl-L-leucine considerably more glycine and leucine were found in the perfusate in patients with sprue than in the control subjects. There was no correlation between peptide absorption and the concentration of total glycly-glycine hydrolase and glycyl-L-leucine hydrolase, measured as combined brush border and cytosol enzymes. The concentrations of these enzymes were similar in both groups of subjects.

Intestinal absorption in normal Indian and English people

The absorption of glycine, glycylglycine, water, and electrolytes was studied by intestinal perfusion in normal Indian and English people. Compared with the English people the Indians showed impaired absorption of all four substances. In the Indians the absorption of glycine and glycylglycine was impaired to the same extent, so that the kinetic advantage of glycylglycine as compared with glycine was preserved. The reduced absorption in the Indians may be the functional counterpart of the minor morphological changes seen in the jejunal mucosa of people living in southern India.