Lab. 3. The key roles which plasma proteins play in bodily function, together with the relative...

Total Proteins &

Albumin Analysis

Lab. 3

INTRODUCTION The key roles which plasma proteins play in

bodily function, together with the relative ease of assaying them, makes their determination a valuable diagnostic tool as well as a way to monitor clinical progress.

In very general terms, variations in plasma protein concentrations can be due to any of three changes:

in the rate of protein synthesis,

the rate of removal,

and in the volume of distribution.

M. Zaharna Clin. Chem. Lab. 2009

PROTEINS: COMMON PROPERTIES

In spite of functional differences between the various serum proteins, they have certain common biophysical and biochemical properties. These include: a basic composition of carbon, hydrogen, nitrogen and

oxygen; a backbone of covalent peptide bonds which join the amino

acid units together; and absorption maxima in the ultraviolet region.

Based on these properties, laboratory methods have been developed to determine the concentration of proteins in serum often with the assumption that: each of the several hundred individual proteins present in

serum reacts similarly in chemical reactions.


SERUM TOTAL PROTEIN Serum total protein, also called plasma total protein

or total protein, is a biochemical test for measuring the total amount of protein in blood plasma or serum.

Protein in the plasma is made up of albumin and globulins. The globulin in turn is made up of α1, α2, β, and γ globulins.

These fractions can be quantitated using protein electrophoresis, but the total protein test is a faster and cheaper test that estimates the total of all fractions together.

The traditional method for measuring total protein uses the biuret reagent, but other chemical methods are also available.


METHODS OF TOTAL PROTEIN ANALYSIS

Method 1: Kjeldahl; quantitative, protein nitrogen determination

Method 2: Ultraviolet absorption; quantitative, absorption at 210 nm

Method 3: refractometry; quantitative, RI

Method 4: Biuret; quantitative, increased absorption at 540 nm;


SPECIMEN

Serum and plasma may be used, and all usually yield comparable results, though, because of the presence of fibrinogen, plasma levels for total protein are 2 to 4 g/L higher than serum levels.

A fasting specimen is not required but may be desirable to decrease lipemia.

Total protein is stable in serum and plasma for 1 week at room temperature, and for at least 2 months at –20° C


The concentration of proteins in plasma is affected by posture: an increase in concentration of 10-20% occurs within 30 minutes of becoming upright after a period of recumbency.

Also, if a tourniquet is applied before veinpuncture, a significant rise in protein concentration can occur within a few minutes.

In both cases, the change in protein concentration is caused by increased diffusion of fluid from the vascular into the interstitial compartment.

These effects must be borne in mind when blood is being drawn for the determination of protein concentration. M. Zaharna Clin. Chem. Lab. 2009

Hypoproteinemia Malnutrition and/or malabsorption Excessive loss as in renal disease, GI leakage, excessive bleeding, severe burns Excessive catabolism Liver disease


Hyperproteinemia• Dehydration• Monoclonal increase• Polyclonal increase

Only disorders affecting the concentration of albumin and/or the immunoglobulins will give rise to abnormal total protein levels.

Other serum proteins are never present in high enough concentrations for changes to have a significant overall effect.

THE BIURET METHOD

The Biuret reagent is made of (NaOH) and copper (II) sulfate (CuSO4), together with potassium sodium tartrate (KNaC4H4O6). A blue reagent which turns violet in the

presence of proteins, The Sodium hydroxide does not

participate in the reaction at all, but is merely there to provide an alkaline medium so that the reaction can take place.


PRINCIPLE: BIURET METHOD

Peptide bonds of proteins react with tartrate-complexed cupric ions in alkaline solutions to form a colored product.

In a positive test, a copper(II) ion is reduced to copper(I), which forms a complex with the nitrogens and carbons of the peptide bonds in an alkaline solution.

A violet color indicates the presence of proteins. It is possible to use the Biuret reaction to determine the

concentration of proteins because (for most proteins) peptide bonds occur with approximately the same frequency per gram of material.

The intensity of the color, and hence the absorption at 540 nm, is directly proportional to the protein concentration, and can be determined spectrophotometrically at 540 nm.


REFERENCE RANGE Reference range for total proteins is 66.6 to

81.4 g/L Results for males are approximately 1 g/L

higher than results for females; this difference is probably not of clinical significance.

In newborns, the mean serum protein concentration is 57 g/L, increasing to 60 g/L by 6 months and to adult levels by about 3 years of age.


ALBUMIN Albumin is the most abundant circulating

plasma protein (40–60 % of the total) Playing important roles in the maintenance

of the colloid osmotic pressure of the blood, in transport of various ions, acids, and hormones.

It is a globular protein with a molecular weight of approximately 66,000 D and is unique among major plasma proteins in containing no carbohydrate.

It has a relatively low content of tryptophan and is an anion at pH 7.4.

These properties have been exploited in the estimation of albumin in body fluids. M. Zaharna Clin. Chem. Lab. 2009

ANALYSIS METHODS Method 1: Precipitation; quantitative

Salt fractionation, Solvent fractionation, Acid fractionation

Principle of analysis: Changes of net charge of protein result in precipitation

Method 2: Tryptophan content; quantitative Principle of analysis: Glyoxylic acid + tryptophan in globulin Purple

chromogen (Amax, 540 nm); Total protein – globulin = albumin.

Method 3: Electrophoresis; quantitative Principle of analysis: Albumin is separated from other

proteins in electrical field; percent staining of albumin fraction multiplied by total protein value


ANALYSIS METHODS

Method 4: ImmunochemicalRadial immunodiffusion; Turbidimetry;

Nephelometry; Radioimmunoassay; Enzyme immunoassay;

Method 5: Dye binding, quantitativeMethyl orange; BCG (bromcresol green);

BCP (bromcresol purple); Method 6: Dye binding;

semiquantitativeBromphenol blue in test strip changes

color from yellow to blue in presence of albumin most commonly used test for urine protein


SPECIMEN Serum is the specimen of choice Fasting is not required, although it

may be desirable since marked lipemia interferes in the BCG assay.

Venostasis should be avoided when collecting samples since hemoconcentration increases the apparent concentrations of albumin and other plasma proteins.


INTERFERING FACTORS

Albumin is decreased in:Pregnancy (last trimester, owing to increased

plasma volume)Oral birth control (estrogens) and other drugs Prolonged bed rest IV fluids, rapid hydration, overhydration

Albumin Reference Interval for Serum

Age Men (g/L) Women (g/L)

21–44 33.3–61.2 27.8–56.5


CLINICAL SIGNIFICANCE Plasma albumin levels, although important

for management and follow-up, have very little value in clinical diagnosis.

Hyperalbuminemia is usually attributable to:

dehydration or hemoconcentration.

Hypoalbuminemia is usually the result of

hemodilution, a rate of synthesis less than the albumin loss, diseases that cause a large albumin loss from

urine, skin, or intestine, and increased catabolism observed in fevers,

untreated diabetes mellitus, and hyperthyroidism.M. Zaharna Clin. Chem. Lab. 2009

DYE-BINDING TECHNIQUES

Serum albumin is most often assayed using dye-binding techniques.

Albumin preferentially binds to anionic dyes that do not attract globulins

Bromcresol purple (BCP) and bromcresol green (BCG) are most commonly used

The amount of light absorbed by the albumin –dye complex is proportional to the amount of albumin present


BROMCRESOL GREEN

PrincipleBCG complexes with albumin, resulting in the

dye having a spectral shift The presence of albumin increases the

absorbance at 546 nm which is determined spectrophotometrically


pH 4.3BCG + Albumin BCG-albumin complex

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