BIOCHEMICAL INVESTIGATION OF
BLOOD PLASMA PROTEINS
Total protein content of normal plasma is 6 to 8 g/ 100 ml
(65-85 g/l). As a first step of study, the plasma proteins may be separated
into Albumin (35-50g/l), Globulins (25-35 g/l) and fibrinogen (2-4 g/l).
The albumin : globulin ratio is usually between 1.2:1 to 1.5:1.
Almost all plasma proteins, except immunoglobulins are synthesized in
liver.
In clinical laboratory, total proteins in serum or plasma of patients
are estimated by Biuret method.
In clinical laboratory, electrophoresis is employed regularly
for separation of serum proteins.
The term electrophoresis refers to the movement of charged particles
through an electrolyte when subjected to an electric field.
The
positively charged particles (cations) move to cathode and negatively charged
ones (anions) to anode. Since proteins exist as charged particles, this method
is widely used for the separation of proteins in biological fluids. The
technique was first used by Tiselius in 1937; named it as moving boundary or
frontal electrophoresis (Nobel Prize in 1948).
Factors Affecting Electrophoresis
The rate of migration (separation of particles) during electrophoresis
will depend on the following factors:
1. Net charge on the particles
2. Mass and shape of the particles.
3. The pH of the medium.
4. Strength of electrical field.
5. Properties of the supporting medium.
6. Temperature.
Types of Electrophoresis
There are mainly two types of electrophoresis—horizontal and
vertical.
Different
types of support media are used in horizontal electrophoresis, e.g. filter
paper, cellulose acetate, agar gel, agarose gel, starch gel, etc. The vertical
electrophoresis mainly uses polyacrylamide gel. The nature of the supporting
medium will also influence the mobility.
Electrophoresis Apparatus
The
electrophoresis system basically consists of the electrophoresis tank to hold the buffer and fitted with
the electrodes, as well as a power pack to supply electricity at constant
current and voltage. When the electrophoresis is carried out, the buffer
is chosen in such a way so as to ensure
effective separatum of the mixture of proteins. The pH and ionic strength and
nature of the buffer may be varied according to the proteins to be separated,
e.g. serum proteins are separated at a pH of 8.6 using barbitone
buffer. At this pH all serum proteins will have a net negative charge and will
migrate towards the anode.
Support Medium for Electrophoresis
Filter Paper
Cellulose Acetate Membrane
Agar or Agarose
Starch Gel
Polyacrylamide gel electrophoresis (PAGE)
Visualisation of Protein Bands
After the
electrophoretic run is completed, the proteins are fixed to the solid support
using a fixative such as acetone or methanol. Then it is stained by using dyes
(Amido
Schwartz, naphthalene black, Ponceau S or Coomassie Blue) and then destained by
using dilute acetic acid. The electrophoretogram can be scanned using a densitometer and each band
quantitated. In the densitometer, light is passed through the agar gel plate;
the absorption of light will be proportional to the quantity of protein present
on a band. Another method is that the stain may be eluted from the support and
each fraction quantitated colourimetrically.
Normal
Values and Interpretations
In agar gel electrophoresis, normal serum will be separated
into five bands. Their relative concentrations are given
below:
Albumin : 55-65%
Alpha-1 -globulin : 2-4%
Alpha-2-globulin : 6-12%
Beta-globulin : 8-12%
Gamma-globulin : 12-22%
ALBUMIN
The name is derived from the white precipitate formed when egg is boiled
(Latin, albus = white). Albumin constitutes the major part of plasma
proteins. It has one polypeptide chain with 585 amino acids and 17 disulfide bonds. It has a molecular weight of 69,000. It is
synthesised by hepatocytes; therefore, albumin level in blood is
decreased in liver cirrhosis. Estimation of albumin is a liver function test.
Half-life of albumin is about 20 days. Liver produces about
Functions of Albumin
1. It contributes to the colloid osmotic pressure of plasma.
The total osmolality of serum is 278-
2.
Another
major function of albumin is to transport
various hydrophobic substances.
3.
Being a watery medium,
blood cannot solubilise lipid components. Bilirubin and
non-esterified fatty acids are specifically transported by
albumin. Drugs (sulpha, aspirin, salicylates, dicoumarol,
phenytoin), steroid hormones, thyroxine, calcium, copper
and heavy metals are non-specifically carried by albumin.
Only the unbound fraction of drugs is biologically active.
The histidine residue at position 3 of albumin binds
copper.
4.
All proteins have buffering capacity. Because of its high concentration in blood, albumin
has maximum buffering capacity. Albumin has a total of 16 histidine residues
which contribute this buffering action.
5.
All tissue cells
can take up albumin by pinocytosis. It is then broken down to amino acid level. So albumin may be considered
as the transport form of essential amino acids from liver to extrahepatic
cells.
Clinical Applications
1. Albumin-fatty acid
complex cannot cross blood-brain barrier and hence fatty acids cannot be taken
up by brain. The bilirubin from albumin maybe competitively replaced by
aspirin and such other drugs. In newborns, bilirubin is already high, and
if such drugs are given, there is a probability that free bilirubin is
deposited in brain leading to kernicterus and mental retardation.
2.
When
two drugs having high affinity to albumin are administered together, there may
be competition for the available sites, with consequent displacement of one
drug. Such an effect may lead to clinically significant drug interactions, e.g.
phenytoin -dicoumarol interaction.
3.
Protein-bound
calcium is lowered in hypoalbuminemia. Thus, even though total calcium level
in blood is lowered, ionised calcium level may be normal, and so tetany may not
occur. Calcium is lowered by 0.8 mg/dl for a fall of 1 g/dl of Albumin.
4.
Human
albumin is therapeutically useful to treat burns, hemorrhage and shock..
Normal Value and Interpretation: Normal level of
Albumin is 3.5-5 g/dl. Lowered level of albumin (hypoalbuminemia) has important
clinical significance.
Hypoalbuminemia
1. In cirrhosis of liver and in chronic liver
failure, albumin
synthesis is decreased and so blood level is lowered.
2.
In
malnutrition and in
malabsorption syndrome the availability of amino acids is reduced and so
albumin synthesis is affected.
3.
In
nephrotic syndrome,
kidney glomerular filtration is defective so that albumin is excreted in large
quantities. Increased loss, to a certain extent, is compensated by increased
synthesis; but blood level of albumin is decreased.
4.
Presence
of albumin in urine (albuminuria) is
always pathological. Large quantities (several grams per day) of albumin is
lost in urine in nephrotic syndrome. Small quantities are lost in urine in
acute nephritis, and other inflammatory conditions of urinary tract. Detection of
albumin in urine is done by heat and acetic acid test. In microalbuminuria or minimal
albuminuria or pauci-albuminuria, small quantity of albumin (50-300 mg/ d) is
seen in urine (Paucity = small in quantity). However, microalbuminuria is also clinically important, as it is a predictor
of future renal diseases.
5.
In
burns, albumin is lost through the unprotected skin surface.
6.
In
protein losing enteropathy large quantities of albumin is lost through
intestines.
7.
Hypoalbuminemia
will result in tissue edema. It
may be seen in malnutrition, where albumin synthesis is depressed {generalized
edema), in nephrotic syndrome, where albumin is lost through urine {facial
edema) or in cirrhosis of liver (mainly ascites). In chronic
congestive cardiac failure, venous congestion will cause increased hydrostatic
pressure and decreased return of water into capillaries and so pitting edema
of feet may result.
8.
Albumin
is a negative acute phase protein; level of albumin falls mildly in
presence of inflammatory cytokines such as interleukin-6.
9.
Analbuminemia
{absence of albumin) is a
very rare genetically determined condition.
Albumin-Globulin Ratio: In all the above mentioned conditions of
hypoalbuminemia, there will be a compensatory increase in globulins which are
synthesised by the reticuloendothelial system. Albumin-globulin ratio (A/G
ratio) is thus altered or even reversed. This again leads to edema.
Hypoproteinemia: Since albumin is the major protein present in the
blood, any condition causing lowering of albumin will lead to reduced total
proteins in blood (hypoproteinemia). So it is observed in cirrhosis, nephrotic syndrome, malnutrition and
malabsorption syndromes.
Hyper-gamma -globulinemias
1. When albumin level
is decreased,
body tries to compensate it by increasing the production of globulins from reticuloendothelial
system. Thus, all causes for
hypoalbuminemia will result in albumin : globulin ratio reversal,
and corresponding
increase in percentage in
globulins.
2.
In
chronic infections, the
gamma globulins are increased, but the increase is smooth and widebased.
3.
Drastic
increase in globulins are seen in paraproteinemias, when a sharp spike
is noted in electrophoresis. This is termed as M-band. This is due to monoclonal origin of immunoglobulins in multiple myeloma.
Hyper-beta-globulinemia: It is associated with hyperlipoproteinemia,
atherosclerosis and other hyperlipidemic conditions.
Hyper-alpha-globulinemia: In nephrotic syndrome, small molecular weight proteins
(including albumin) leak out through urine. But proteins with larger molecular
weight remains in blood; so there is an increase in alpha globulin fraction,
which contains alpha-2-macroglobulin.
TRANSPORT PROTEINS
Blood is a watery medium; so lipids and lipid soluble substances will
not easily mix in the blood. Hence such molecules are carried by specific
carrier proteins. Albumin is an important transport protein, which
carries bilirubin, free fatty acids, calcium and drugs.
1.
Pre-albumin is
so named because of its faster mobility in electrophoresis than albumin. It is
more appropriately named asTransthyretin or Thyroxin binding pre-albumin (TBPA), because it carries thyroid
hormones, thyroxin (T4) and tri-iodothyronine T3). It can
bind loosely with all substances which are carried by albumin. Its molecular
weight is lesser than that of albumin. It is rich in tryptophan. Its half-life
in plasma is only one day.
2.
Retinol binding protein (RBP)
carries vitamin A. It is a low molecular weight protein, and so is liable to be
lost in urine. To prevent this loss, RBP is attached with pre-albumin; the
complex is big and will not pass through kidney glomeruli. It is a negative acute phase protein.
Zinc is required for RBP synthesis, and so RBP level and vitamin A level may be
lowered in zinc deficiency.
3. Thyroxine
binding globulin
(TBG) is the
specific carrier molecule for thyroxine and tri-iodo thyronine. TBG level is
increased in pregnancy; but decreased in nephrotic syndrome.
4. Transcortin, otherwise
known as Cortisol binding globulin (CBG) is the transport protein for Cortisol
and corticosterone..
5. Haptoglobin (for haemoglobin), Hemopexin (for heme) and
Transferrin (for iron) are importantto prevent loss of iron from body.
6. Cholesterol in blood is carried by lipoproteins, HDL and LDL
varieties.
ACUTE PHASE PROTEINS
The level of certain proteins in blood may increase 50 to 1000-folds in
various inflammatory and neoplastic conditions. Such proteins are acute phase
proteins. Interleukins (ID, especially IL-1 and IL-6, released by macrophages
and lymphocytes, are the primary agents which cause induction and release of
these acute phase proteins. Important acute phase proteins are C-reactive
protein, ceruloplasmin, haptoglobin,a1 -acid glycoprotein, a-1-anti-trypsin and
fibrinogen.
C-reactive Protein (CRP):
It is thus named because it reacts with
C-polysaccharide of capsule of pneumococci. CRP consists of five polypeptide
subunits to form a disc-shaped cyclic polymer. It has a molecular weight of 115-140
kD. It is synthesised in liver. It can stimulate complement activity and
macrophage phagocytosis. When the inflammation has subsided, CRP quickly falls.
CRP level has a positive correlation in predicting the risk of cardiovascular
disease.
Ceruloplasmin
Ceruloplasmin is blue in colour (Latin, caeruleus=blue). It is an
alpha-2 globulin with molecular weight of 160,000. It is synthesised in liver.
It contains 6 to 8 copper atoms per molecule. Ceruloplasmin is also called
Ferroxidase, an enzyme which helps in the incorporation of iron into
transferrin. Ninety per cent of copper content of plasma is bound with
ceruloplasmin, and 10% with albumin. Copper is bound with albumin loosely, and
so easily exchanged with tissues. Hence transport protein for copper is
Albumin. Ceruloplasmin is an enzyme. It is an important antioxidant in
plasma.
Clinical Application:
Normal blood level of ceruloplasmin is 25-50 mg/dl. It
is estimated either by its oxidative property on phenylene diamine, or by
radial immunodiffusion. This level is reduced in Wilson's hepatolenticular
degeneration. Ceruloplasmin level less than 20 mg/dl is pathognomonic of
Wilson's disease. It is an inherited autosomal recessive condition. Incidence
of the disease is one in 50,000. The defect is associated with chromosome
No.13. The basic defect is a mutation in a gene encoding a copper binding
ATPase in cells, which is required for excretion of copper from cells. So,
copper is not excreted through bile, and hence copper toxicity is seen.
Increased copper content in hepatocyte inhibits the incorporation of copper to
apo-ceruloplasmin. So ceruloplasmin level in blood is decreased. Accumulation
in liver leads to hepatocellular degeneration andcirrhosis. Deposits in brain
basal ganglia leads to lenticular degeneration and neurological symptoms.
Another common finding is copper deposits as green or golden pigmented ring
around cornea; this is called Kayser-Fleischer ring. Copper deposits in
kidney may cause renal failure, and in bone marrow leads to hemolytic anemia.
Treatment consists of a diet containing low copper and injection of
D-penicillamine which excretes copper through urine. Since zinc decreases
copper absorption, zinc is useful in therapy.
Lowered levels of ceruloplasmin is also seen in malnutrition, nephrosis,
and cirrhosis. Ceruloplasmin is an acute phase protein. So its level in blood
may be increased in all inflammatory conditions, collagen disorders and in
malignancies.
Alpha-1 Anti-trypsin (AAT)
AAT is otherwise called a-anti-proteinase or protease inhibitor (Pi). It
inhibits all serine proteases (proteolytic enzymes having a serine in their
active centre), such as plasmin, thrombin, trypsin, chymotrypsin, elastase, and
cathepsin. Serine protease inhibitors are abbreviated as Serpins. Binding of
this inhibitor to protease is very tight; once bound it is not released.
Normally, about 95% of the anti-protease activity in plasma is due to AAT. It
is synthesised in liver. It has a molecular weight of 50,000 and has 3
polypeptide chains. It forms the bulk of moleculesjn serum having a-1 mobility.
It is estimated by radial immuno-diffusion method. Normal serum level is 75-200
mg/dl. Electrophoretically, multiple allelic forms can be separated, the most
common variety is PiMM determined by the genotype MM. More than 75 variants are
known, out of which about 30 genetic variants show decreased or very low serum
concentrations. Gene is located on the small arm of chromosome number 14.
AAT deficiency causes the following conditions:
1. Emphysema: The deficiency
is inherited as a co-dominant trait. The incidence of AAT deficiency is
2.
Cirrhosis:
AAT deficiency
is also seen in persons with PiZZ genes. This genetic make up is associated
with cirrhosis of liver. The ZZ protein has a substitution of glutamic acid by
lysine at position 342. The protein is unsialylated and is not released from
hepatocytes, causing death of
cells with consequent fibrosis and cirrhosis.
3.
In
Nephrotic syndrome, AAT molecules are lost in urine, and so AAT deficiency is produced.
Alpha-2-Macroglobulin (AMG): AMG is a tetrameric protein with molecular
weight of 725,000. It is the major component of a-2 proteins. Gene is located in the long arm of
chromosome number 12. It is synthesised by hepatocytes and macrophages. AMG
inactivates all proteases, and thus it is an important in vivo
anti-coagulant. Proteases cleave the "bait" region of AMG,
releasing a small unit, to provoke conformational changes in AMG, which then
"traps" the enzyme. So proteolytic enzymes cannot function. AMG-protease
complexes are internalised by a receptor mediated endocytosis by macrophages,
and then degraded. AMG is the carrier of many growth factors such as platelet
derived growth factor (PDGF). Normal serum level is 130^300 mg/dl. AMG
contributes about 1% of all total plasma proteins. Its concentration is
markedly increased (up to 2-3 g/d I) in Nephrotic syndrome, because
other proteins are lost through urine in this condition.
Alpha-1-Acid Glycoprotein: It is otherwise known as Orosomucoid. It has a molecular
weight of 44,000 and has a high content of about 45% of carbohydrates. Its
isoelectric pH is 0.7-3.5. It is synthesised by hepatocytes. It binds
lipophilic substances and various drugs. It binds with progesterone tightly.
Normal serum level is 55-140 mg/dl, and its half-life is five days. It is
increased in pregnancy. It is also an acute phase protein. It is a reliable
indicator of clinical activity of
ulcerative colitis.
NEGATIVE ACUTE
PHASE PROTEINS
During an inflammatory response, some proteins are seen to be decreased
in blood; these are called negative acute phase proteins. Examples are albumin,
transthyretin (prealbumin), retinol binding protein and transferrin.
Transferrin: It is a specific iron binding protein. It
is a negative acute phase protein; so the blood level is decreased in acute
diseases. It has a half-life of 7-10 days and is used as a better index of
protein turnover than albumin.
Evaluation
of proteinuria.
Proteinuria means the excretion of protein in
the urine. A healthy person does not excrete proteins in the urine or the
excretion of proteins is less than 150 mg per day. The proteins most commonly
found in the urine are those derived from the plasma of blood and consist of a
mixture of albumin and globulin. Predominantly albuminuria (excretion
of albumin in urine) is detectable on routine urine analysis during
a medical examination. Albuminuria could be organic (due
to involvement of kidneys or other organs) or functional (due to
physiological or biological stress on kidneys). The functional albuminuria
is usually intermittent and not accompanied by any symptoms or evidence of kidney
disease. Renal function tests and urinary deposits are found to be normal
during the functional albuminuria. It may be connected with posture;
being absent when the person is lying down and present when standing. The functional
albuminuria usually clears up in early adult life and seems to be
associated with the growth and development of kidneys. Any severe stress may
also lead to transient albuminuria. Exposure to severe cold and
excessive exercise or physical activity may cause functional or transient
proteinuria. However, there is nothing to worry about as the functional
albuminuria is self limiting with respect to the cause. Mild to moderate functional
albuminuria may also be detected during last two months of pregnancy due to
pressure on kidneys.
Organic albuminuria is of three types: 1) Renal
Albuminuria - When the cause is the kidney disease. 2) Pre-renal
Albuminuria - When the kidneys are affected secondarily to some other
disease. Post-renal Albuminuria - When the protein is added to the urine
after it has left the renal tubules.
1. Renal Albuminuria: It is found in all forms of kidney
disease. The cause of renal disorder or kidney disease may be
inflammatory (infectious), degenerative (immunological) or destructive (toxic
or malignant). The plasma globulin and red blood cells (RBCs) may also be
excreted along with albumin during some renal disorders. The urine would
be smoky in color if macroscopic hematuria (blood in urine) is also
associated with proteinuria. The cases of acute glomerulonephritis
may excrete 0.5 to 2.0 percent (0.5 g to 2.0 g/dl) protein in the urine,
whereas the cases affected by chronic glomerulonephritis generally
excrete less than 0.5 percent (0.5 g/dl) protein in the urine. The amount of
protein excreted daily would vary depending on the volume of urine voided
daily. The ratio of albumin to globulin excreted in the urine
may vary from 10:1 to 5:1. A routine and quantitative urine
analysis is required to evaluate the extent of excretion of proteins in the
urine.
2. Pre-renal Albuminuria: It is found in a variety of
conditions exerting stress on the kidneys. The pre-renal albuminuria
usually disappears when the primary disease is cured. Impairment
of renal circulation due to dehydration, diarrhea or vomiting, blood loss due
to accidental injuries or anemia are the most common conditions, which could
lead to pre-renal albuminuria.
3. Post-renal Albuminuria: The proteinuria or
albuminuria is termed as post-renal albuminuria if protein is possibly
added to the urine as it passes along the urinary tract after leaving the
urinary tubules of the kidneys. The major causes of the post-renal
albuminuria are the lesions of the renal pelvis or urinary bladder. Lesions
of the prostate (in male patients) and urethra also lead to post-renal
albuminuria. Admixture of discharges from the vagina (in female patients)
and semen (in male patients) may also give positive tests for protein.
People with diabetes, hypertension, or certain family
backgrounds are at risk for proteinuria. In the United States, diabetes is the
leading cause of ESRD.1 In both type 1 and type 2 diabetes,
albumin in the urine is one of the first signs of deteriorating kidney
function. As kidney function declines, the amount of albumin in the urine
increases.
Another risk factor for developing proteinuria is
hypertension, or high blood pressure. Proteinuria in a person with high blood
pressure is an indicator of declining kidney function. If the hypertension is
not controlled, the person can progress to full kidney failure.
African Americans are more likely than Caucasians to
have high blood pressure and to develop kidney problems from it, even when
their blood pressure is only mildly elevated. In fact, African Americans are
six times more likely than Caucasians to develop hypertension-related kidney
failure.2
Other groups at risk for proteinuria are American
Indians, Hispanics/Latinos, Pacific Islander Americans, older adults, and overweight
people. These at-risk groups and people who have a family history of kidney
disease should have their urine tested regularly.
Proteinuria has no signs or symptoms in the early
stages. Large amounts of protein in the urine may cause it to look foamy in the
toilet. Also, because protein has left the body, the blood can no longer soak
up enough fluid, so swelling in the hands, feet, abdomen, or face may occur.
This swelling is called edema. These are signs of large protein loss and
indicate that kidney disease has progressed. Laboratory testing is the only way
to find out whether protein is in a person’s urine before extensive kidney
damage occurs.
Several health organizations recommend regular urine
checks for people at risk for CKD. A 1996 study sponsored by the National
Institutes of Health determined that proteinuria is the best predictor of
progressive kidney failure in people with type 2 diabetes. The American
Diabetes Association recommends regular urine testing for proteinuria for
people with type 1 or type 2 diabetes. The National Kidney Foundation
recommends that routine checkups include testing for excess protein in the
urine, especially for people in high-risk groups.
The evaluation of proteinuria is shown schematically in and is typically initiated
after colorimetric detection of proteinuria by dipstick examination. The
dipstick measurement detects mostly albumin and gives false-positive results
when pH > 7.0 and the urine is very concentrated or contaminated with blood.
A very dilute urine may obscure significant proteinuria on dipstick
examination, and proteinuria that is not predominantly albumin will be missed.
This is particularly important for the detection of Bence Jones proteins in the
urine of patients with multiple myeloma. Tests to measure total urine
concentration accurately rely on precipitation with sulfosalicylic or
trichloracetic acids. Currently, ultrasensitive dipsticks are available to
measure microalbuminuria (30 to 300 mg/d), an early marker of glomerular
disease that has been shown to predict glomerular injury in early diabetic
nephropathy.
Until recently, an accurate protein
measurement required a 24-hour urine collection. In a 24-hour collection, the
patient urinates into a container, which is kept refrigerated between trips to
the bathroom. The patient is instructed to begin collecting urine after the
first trip to the bathroom in the morning. Every drop of urine for the rest of
the day is to be collected in the container. The next morning, the patient adds
the first urination after waking and the collection is complete.
Containers for a 24-hour urine collection.
In recent years, researchers have found that a single
urine sample can provide the needed information. In the newer technique, the
amount of albumin in the urine sample is compared with the amount of
creatinine, a waste product of normal muscle breakdown. The measurement is
called a urine albumin-to-creatinine ratio (UACR). A urine sample containing
more than 30 milligrams of albumin for each gram of creatinine (30 mg/g) is a
warning that there may be a problem. If the laboratory test exceeds 30 mg/g,
another UACR test should be done 1 to 2 weeks later. If the second test also
shows high levels of protein, the person has persistent proteinuria, a sign of
declining kidney function, and should have additional tests to evaluate kidney
function.
Cup for a single urine sample.
The magnitude of proteinuria and the protein composition in the urine
depend upon the mechanism of renal injury leading to protein losses. Large
amounts of plasma proteins normally course through the glomerular capillaries
but do not enter the urinary space. Both charge and size selectivity prevent
virtually all of albumin, globulin, and other large-molecular-weight proteins
from crossing the glomerular wall. However, if this barrier is disrupted, there
can be leakage of plasma proteins into the urine. Smaller proteins (<20 kDa)
are freely filtered but are readily reabsorbed by the proximal tubule. Normal
individuals excrete less than 150 mg/d of total protein and only about 30 mg/d
of albumin. The remainder of the protein in the urine is secreted by the
tubules (Tamm-Horsfall, IgA, and urokinase) or represents small amounts of
filtered b2-microglobulin, apoproteins, enzymes, and peptide hormones. Another
mechanism of proteinuria occurs when there is excessive production of an
abnormal protein that exceeds the capacity of the tubule for reabsorption. This
most commonly occurs with plasma cell dyscrasias such as multiple myeloma and
lymphomas that are associated with monoclonal production of immunoglobulin
light chains.
The normal glomerular endothelial cell forms a barrier penetrated by pores
of about 100 nm that holds back cells and other particles but offers little
impediment to passage of most proteins. The glomerular basement membrane traps
most large proteins (>100 kDa), while the foot processes of epithelial cells
(podocytes) cover the urinary side of the glomerular basement membrane and
produce a series of narrow channels (slit diaphragms) to allow molecular
passage of small solutes and water. The channels are coated with anionic
glycoproteins that are rich in glutamate, aspartate, and sialic acid, which are
negatively charged at physiologic pH. This negatively charged barrier impedes
the passage of anionic molecules such as albumin. Some glomerular diseases,
such as minimal change disease, cause fusion of glomerular epithelial cell foot
processes, resulting in predominantly "selective" (Fig. 2) loss of
albumin. Other glomerular diseases can present with disruption of the basement
membrane and slit diaphragms (e.g., by immune complex deposition), resulting in
large amounts of protein losses that include albumin and other plasma proteins.
The fusion of foot processes causes increased pressure across the capillary
basement membrane, resulting in areas with larger pore sizes. The combination
of increased pressure and larger pores results in significant proteinuria
("nonselective").
When the total daily excretion of protein exceeds 3.5 g, there is often
associated hypoalbuminemia, hyperlipidemia, and edema (nephrotic syndrome).
However, total daily urinary protein excretion greater than 3.5 g can occur
without the other features of the nephrotic syndrome in a variety of other
renal diseases . Plasma cell dyscrasias (multiple myeloma) can be associated
with large amounts of excreted light chains in the urine, which may not be
detected by dipstick (which detects mostly albumin). The light chains produced
from these disorders are filtered by the glomerulus and overwhelm the
reabsorptive capacity of the proximal tubule. A sulfosalicylic acid precipitate
that is out of proportion to the dipstick estimate is suggestive of light
chains (Bence Jones protein), and light chains typically redissolve upon warming
of the precipitate. Renal failure from these disorders occurs through a variety
of mechanisms including tubule obstruction (cast nephropathy) and light chain
deposition.