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Tubular Transport of Amino Acids and Small Peptides

Stefan Silbernagl

10.1002/cphy.cp080241

Source: Supplement 25: Handbook of Physiology, Renal Physiology

Originally published: 1992

Published online: January 2011

Full Article on Wiley Online Library



Abstract

The sections in this article are:

1 The Intact Kidney
1.1 Plasma Concentrations of Amino Acids
1.2 Normal Excretion of Amino Acids
1.3 Variability of Aminoaciduria
2 Interorgan Transport and Renal Metabolism of Amino Acids
3 The Tubule Level
3.1 Sites of Reabsorption
3.2 Internephron Heterogeneity
3.3 Reabsorption Kinetics
4 Transport Steps at the Membrane Level
4.1 Brush Border
4.2 Basolateral Membrane: Exit and Uptake
4.3 Carrier, Pore, or Enzymatic Cycle?
4.4 Secretion of Amino Acids
5 Specificity of The Reabsorption Mechanisms
5.1 Stereospecificity
5.2 Specificities of the Different Reabsorption Systems
6 Hyperaminoaciduria
7 Renal Handling of Oligopeptides
7.1 Prereabsorptive Hydrolysis in the Tubule Lumen
7.2 Glutathione and Other γ‐Glutamyl Peptides
7.3 Reabsorption of Intact Peptides
Figure 1. Figure 1.

Effect of 10 mmol/liter phenylalanine on peritubular cell membrane potential difference measured with conventional (PDpt) and with sodium‐selective (PDNa,pt) microelectrodes in frog kidney. Differential recording (PDNa,pt–PDpt) is middle trace and allows crude estimate of changes in intracellular sodium activity.

Original recording from Messner et al. 163
Figure 2. Figure 2.

Glutamate formation from glutamine, i.e., ammoniagenesis, catalyzed by hippurate‐activated γ‐glutamyltransferase in lumen of proximal convoluted tubule of rat kidney.

From Silbernagl 250
Figure 3. Figure 3.

Fractions of filtered loads of endogenous glycine, glutamate, and valine found by free‐flow micropuncture along superficial proximal convoluted tubules of rat kidney. [3H]inulin used as volume marker; amino acids measured by column chromatographic ultramicromethod. Distance from glomerulus was determined by latex method. Plasma concentrations (μmol/liter±S.D.) were glycine, 282±47; glutamate, 117±30; valine, 278±43.

From Silbernagl 243
Figure 4. Figure 4.

Sodium dependence and saturability of rat renal tubular L‐ornithine resorption in vivo. Peritubular capillaries perfused continuously with solutions containing radiolabeled L‐ornithine (initial concentration, Co = 2, 5, or 10 mmol/liter); droplet of same solution simultaneously microinjected into lumen of proximal convolution. After 45 s, when net flux approached zero, concentration gradient Δc (given as % of co) was measured. Note that net reabsorption fell to zero in absence of luminal sodium.

Redrawn from Ullrich et al. 287
Figure 5. Figure 5.

Predicted concentration profiles for glutamine (Gln) along rat proximal convolution if plasma concentration (A) and leak permeability (B) are varied alternatively. Endogenous profiles are “0.6” in A and “1xP” in B. Note that late proximal tubular fluid to plasma concentration ratio (TF/P) of glutamine is only elevated significantly if plasma concentration is increased 20‐fold (A). Varied permeability of tubular wall results mainly in changed steady state TF/P in second half of proximal convolution (B).

From Silbernagl 244
Figure 6. Figure 6.

Scheme of amino acid (AA) reabsorption from tubule lumen and of peritubular uptake of amino acids. The luminal carriers a, at least for many neutral and for acidic amino acids, are energized by electrochemical Na+ gradient directed into cell (secondary‐active Na+ cotransport). Na+ gradient is established by Na+,K+‐ATPase localized in peritubular membrane. Amino acid exit at this cell side is probably brought about by carriers that facilitate this passive step b. For some amino acids, cellular uptake by Na+ cotransport has been demonstrated also at peritubular side (carriers c). Types b and c carriers may be identical for the same amino acid. Whether passive leak of amino acids is a paracellular event (d) or a transcellular, carrier‐mediated process (c and a used in “wrong” direction), is controversial (see text).

From Silbernagl 248
Figure 7. Figure 7.

Hypothetical scheme of reabsorption of dibasic amino acids (AA+) like lysine+ or arginine+ across the tubule cell. The luminal uptake is passive; it is electrogenic but Na+‐independent 222. (Electroneutral uptake [205b] and electrogenic Na+‐cotransport [199] may exist, too.) Peritubular exit of lysine+ has been found to be accelerated by Na+, and peritubular uptake of Na+ to be accelerated by preloading the cell with lysine+ 176. Thus peritubular exit of dibasic amino acids may be driven by an electroneutral, secondary‐active Na+ antiport mechanism.

from ref. 253
Figure 8. Figure 8.

Scheme of possible mechanism for tubular brush border Na+,K+‐dependent transport of L‐Glu−. Stoichiometry of this transport mechanism (drawn here 2:1 for Na+/Glu− and 1:1 for K+/Glu−) is controversial (see text).

Adapted from Fukuhara and Turner 88
Figure 9. Figure 9.

General molecular structure of L‐α‐amino acids, D‐α‐amino acids, and L‐β‐amino acids. The α‐carbon atom, which is also numbered 2, of α‐amino acids usually possesses four different ligands and is optically active. Therefore, two stereoisomers exist, the L‐form, which is the naturally occurring isomer in higher animals, and the D‐form, often found in bacteria. Glycine (and a few other non‐protein amino acids) do not have two stereoisomers, because the rest R is a second H ligand in this case.
Figure 10. Figure 10.

Uptake of L‐phenylalanine (circles, squares) and D‐phenylalanine (triangles) by isolated renal brush border microvilli vesicles in presences (circles, triangles) and absence (squares) of sodium (100 mmol/liter). Phenylalanine concentration, 2 mmol/liter. Values are given ±S.E. and are expressed as percentages of uptake observed after 20 min. Note that specific uptake of L‐phenylalanine falls to rates equal to nonspecific D‐phenylalanine uptake in absence of Na+.

Adapted from Evers et al. 73
Figure 11. Figure 11.

Molecular structures of neutral amino acids.
Figure 12. Figure 12.

Molecular structures of acidic amino acids L‐aspartate and L‐glutamate− and of basic amino acids L‐lysine+ and L‐arginine+.
Figure 13. Figure 13.

The aminothiol acid L‐cysteine is easily oxidized to L‐cystine under aerobic conditions, e.g., in tubule lumen. Reversed reaction often is enzymatically catalyzed and takes place, e.g., in tubule cells. L‐cysteine may also react with –SH groups of other compounds (e.g., glutathione), yielding mixed disulfides.
Figure 14. Figure 14.

Conversion of L‐citrulline to L‐arginine in tubule cell. In contrast to liver, only cytosolic part of urea cycle normally takes place in mammalian kidney, from which arginine is exported to other tissues.
Figure 15. Figure 15.

Scheme of possible causes of hyperaminoaciduria. (a) Prerenal hyperaminoaciduria caused by elevated filtered load. Plasma amino acid concentration and/or GFR are so highly elevated that reabsorption becomes saturated, (b) Reabsorption mechanism (brush border or basolateral carrier; carriers a or b in Fig. 6) has a defect, leading to “classic” or “basolateral” renal hyperaminoaciduria. (c) Filtered load of another amino acid is elevated, inhibiting reabsorption of amino acid originally considered, (d) A certain amino acid originally not considered is not reabsorbed because its transport mechanism is defective. This amino acid may now inhibit reabsorption of amino acid originally considered, although its load is not increased and even its transport site may not be defective, (e) Reabsorption is normal, but backleak in the same or in different tubule sections localized downstream is elevated, leading to abnormally high luminal steady‐state concentration, (f) Main driving force, i.e., electrochemical Na+ gradient, is reduced. Not only most amino acids but also glucose, phosphate, and other Na+‐cotransported compounds, as well as Na+ itself, are hyperexcreted. Such unspecific deterioration (ATP depletion, ATPase inhibition, Na+ leakiness, etc.) of cell function might underlay Fanconi's syndrome.

Adapted from Silbernagl 248
Figure 16. Figure 16.

Degradation of L‐alanyl‐L‐valine in lumen of microperfused proximal convolutions of rat kidney in vivo. Despite extremely high initial concentration of dipeptide perfused into lumen at 0 mm (5 mmol/liter), the free constituent amino acids alanine and valine are formed so rapidly (1 mm perfusion distance = 1 s contact time) that their reabsorption is virtually completed still within physiological contact time in proximal tubule. Perfusion rate was 20 nl/min.

S. Silbernagl, unpublished data
Figure 17. Figure 17.

Hydrolysis of glutathione in lumen of proximal tubule catalyzed by brush border γ‐glutamyltransferase (γ‐GT), and, as second step, by brush border aminopeptidase and cysteinyl‐glycinase. Glutathione can be reabsorbed only in form of free constituent amino acids.
Figure 18. Figure 18.

Two mechanisms of tubular reabsorption of oligopeptides. Top: Luminal hydrolysis of peptides by brush border enzymes with subsequent reabsorption of free constituent amino acids. Bottom: H+ gradient–driven transport of intact dipeptides, which subsequently are hydrolyzed within the cell (e.g., carnosine) or might reach peritubular interstitium as intact molecules (e.g., glycyl‐sarcosine).

From Silbernagl 249


Figure 1.

Effect of 10 mmol/liter phenylalanine on peritubular cell membrane potential difference measured with conventional (PDpt) and with sodium‐selective (PDNa,pt) microelectrodes in frog kidney. Differential recording (PDNa,pt–PDpt) is middle trace and allows crude estimate of changes in intracellular sodium activity.

Original recording from Messner et al. 163


Figure 2.

Glutamate formation from glutamine, i.e., ammoniagenesis, catalyzed by hippurate‐activated γ‐glutamyltransferase in lumen of proximal convoluted tubule of rat kidney.

From Silbernagl 250


Figure 3.

Fractions of filtered loads of endogenous glycine, glutamate, and valine found by free‐flow micropuncture along superficial proximal convoluted tubules of rat kidney. [3H]inulin used as volume marker; amino acids measured by column chromatographic ultramicromethod. Distance from glomerulus was determined by latex method. Plasma concentrations (μmol/liter±S.D.) were glycine, 282±47; glutamate, 117±30; valine, 278±43.

From Silbernagl 243


Figure 4.

Sodium dependence and saturability of rat renal tubular L‐ornithine resorption in vivo. Peritubular capillaries perfused continuously with solutions containing radiolabeled L‐ornithine (initial concentration, Co = 2, 5, or 10 mmol/liter); droplet of same solution simultaneously microinjected into lumen of proximal convolution. After 45 s, when net flux approached zero, concentration gradient Δc (given as % of co) was measured. Note that net reabsorption fell to zero in absence of luminal sodium.

Redrawn from Ullrich et al. 287


Figure 5.

Predicted concentration profiles for glutamine (Gln) along rat proximal convolution if plasma concentration (A) and leak permeability (B) are varied alternatively. Endogenous profiles are “0.6” in A and “1xP” in B. Note that late proximal tubular fluid to plasma concentration ratio (TF/P) of glutamine is only elevated significantly if plasma concentration is increased 20‐fold (A). Varied permeability of tubular wall results mainly in changed steady state TF/P in second half of proximal convolution (B).

From Silbernagl 244


Figure 6.

Scheme of amino acid (AA) reabsorption from tubule lumen and of peritubular uptake of amino acids. The luminal carriers a, at least for many neutral and for acidic amino acids, are energized by electrochemical Na+ gradient directed into cell (secondary‐active Na+ cotransport). Na+ gradient is established by Na+,K+‐ATPase localized in peritubular membrane. Amino acid exit at this cell side is probably brought about by carriers that facilitate this passive step b. For some amino acids, cellular uptake by Na+ cotransport has been demonstrated also at peritubular side (carriers c). Types b and c carriers may be identical for the same amino acid. Whether passive leak of amino acids is a paracellular event (d) or a transcellular, carrier‐mediated process (c and a used in “wrong” direction), is controversial (see text).

From Silbernagl 248


Figure 7.

Hypothetical scheme of reabsorption of dibasic amino acids (AA+) like lysine+ or arginine+ across the tubule cell. The luminal uptake is passive; it is electrogenic but Na+‐independent 222. (Electroneutral uptake [205b] and electrogenic Na+‐cotransport [199] may exist, too.) Peritubular exit of lysine+ has been found to be accelerated by Na+, and peritubular uptake of Na+ to be accelerated by preloading the cell with lysine+ 176. Thus peritubular exit of dibasic amino acids may be driven by an electroneutral, secondary‐active Na+ antiport mechanism.

from ref. 253


Figure 8.

Scheme of possible mechanism for tubular brush border Na+,K+‐dependent transport of L‐Glu−. Stoichiometry of this transport mechanism (drawn here 2:1 for Na+/Glu− and 1:1 for K+/Glu−) is controversial (see text).

Adapted from Fukuhara and Turner 88


Figure 9.

General molecular structure of L‐α‐amino acids, D‐α‐amino acids, and L‐β‐amino acids. The α‐carbon atom, which is also numbered 2, of α‐amino acids usually possesses four different ligands and is optically active. Therefore, two stereoisomers exist, the L‐form, which is the naturally occurring isomer in higher animals, and the D‐form, often found in bacteria. Glycine (and a few other non‐protein amino acids) do not have two stereoisomers, because the rest R is a second H ligand in this case.



Figure 10.

Uptake of L‐phenylalanine (circles, squares) and D‐phenylalanine (triangles) by isolated renal brush border microvilli vesicles in presences (circles, triangles) and absence (squares) of sodium (100 mmol/liter). Phenylalanine concentration, 2 mmol/liter. Values are given ±S.E. and are expressed as percentages of uptake observed after 20 min. Note that specific uptake of L‐phenylalanine falls to rates equal to nonspecific D‐phenylalanine uptake in absence of Na+.

Adapted from Evers et al. 73


Figure 11.

Molecular structures of neutral amino acids.



Figure 12.

Molecular structures of acidic amino acids L‐aspartate and L‐glutamate− and of basic amino acids L‐lysine+ and L‐arginine+.



Figure 13.

The aminothiol acid L‐cysteine is easily oxidized to L‐cystine under aerobic conditions, e.g., in tubule lumen. Reversed reaction often is enzymatically catalyzed and takes place, e.g., in tubule cells. L‐cysteine may also react with –SH groups of other compounds (e.g., glutathione), yielding mixed disulfides.



Figure 14.

Conversion of L‐citrulline to L‐arginine in tubule cell. In contrast to liver, only cytosolic part of urea cycle normally takes place in mammalian kidney, from which arginine is exported to other tissues.



Figure 15.

Scheme of possible causes of hyperaminoaciduria. (a) Prerenal hyperaminoaciduria caused by elevated filtered load. Plasma amino acid concentration and/or GFR are so highly elevated that reabsorption becomes saturated, (b) Reabsorption mechanism (brush border or basolateral carrier; carriers a or b in Fig. 6) has a defect, leading to “classic” or “basolateral” renal hyperaminoaciduria. (c) Filtered load of another amino acid is elevated, inhibiting reabsorption of amino acid originally considered, (d) A certain amino acid originally not considered is not reabsorbed because its transport mechanism is defective. This amino acid may now inhibit reabsorption of amino acid originally considered, although its load is not increased and even its transport site may not be defective, (e) Reabsorption is normal, but backleak in the same or in different tubule sections localized downstream is elevated, leading to abnormally high luminal steady‐state concentration, (f) Main driving force, i.e., electrochemical Na+ gradient, is reduced. Not only most amino acids but also glucose, phosphate, and other Na+‐cotransported compounds, as well as Na+ itself, are hyperexcreted. Such unspecific deterioration (ATP depletion, ATPase inhibition, Na+ leakiness, etc.) of cell function might underlay Fanconi's syndrome.

Adapted from Silbernagl 248


Figure 16.

Degradation of L‐alanyl‐L‐valine in lumen of microperfused proximal convolutions of rat kidney in vivo. Despite extremely high initial concentration of dipeptide perfused into lumen at 0 mm (5 mmol/liter), the free constituent amino acids alanine and valine are formed so rapidly (1 mm perfusion distance = 1 s contact time) that their reabsorption is virtually completed still within physiological contact time in proximal tubule. Perfusion rate was 20 nl/min.

S. Silbernagl, unpublished data


Figure 17.

Hydrolysis of glutathione in lumen of proximal tubule catalyzed by brush border γ‐glutamyltransferase (γ‐GT), and, as second step, by brush border aminopeptidase and cysteinyl‐glycinase. Glutathione can be reabsorbed only in form of free constituent amino acids.



Figure 18.

Two mechanisms of tubular reabsorption of oligopeptides. Top: Luminal hydrolysis of peptides by brush border enzymes with subsequent reabsorption of free constituent amino acids. Bottom: H+ gradient–driven transport of intact dipeptides, which subsequently are hydrolyzed within the cell (e.g., carnosine) or might reach peritubular interstitium as intact molecules (e.g., glycyl‐sarcosine).

From Silbernagl 249
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Article Title: Tubular Transport of Amino Acids and Small Peptides
 

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Stefan Silbernagl. Tubular Transport of Amino Acids and Small Peptides. Compr Physiol 2011, Supplement 25: Handbook of Physiology, Renal Physiology: 1937-1976. First published in print 1992. doi: 10.1002/cphy.cp080241