Early scientists classified the proteins found in plasma into three groups, fibrinogen, albumin, and globulins, on the basis of their relative solubility in solutions to which water-miscible organic solvents such as ethanol or salting out agents such as ammonium sulfate had been added. Using electrophoresis in a cellulose acetate matrix, clinical scientists subsequently were able to separate the salt-soluble serum protein fraction into five major components designated albumin and the α1-, α2-, β-, and γ-globulins (Figure 1). Plasma proteins tend to be rich in disulfide bonds and frequently contain bound carbo hydrate (glycoproteins) or lipid (lipoproteins). The relative dimensions and molecular masses of several plasma proteins are shown in Figure 2.

Fig1. Technique of cellulose acetate zone electrophoresis. (A)A small amount of serum or other fluid is applied to a cellulose acetate strip. (B) Electrophoresis in electrolyte buffer is performed. (C) Staining enables separated bands of protein to be visualized. (D) Densitometer scanning reveals the relative mobilities of albumin, α1-globulin, β2-globulin, β-globulin, and γ-globulin. (Reproduced with permission from Parslow TG, Stites DP, Terr AI, et al: Medical Immunology, 10th ed. New York, NY: McGraw Hill; 2001.)

Fig2. Relative dimensions and approximate molecular masses of protein molecules in the blood.

Plasma Proteins Help Determine the Distribution of Fluid Between Blood & Tissues

The aggregate concentration of the proteins present in human plasma typically falls in the range of 7 to 7.5 g/dL. The resulting osmotic pressure (oncotic pressure) is approximately 25 mm Hg. Since the hydrostatic pressure in the arterioles is approximately 37 mm Hg, with an interstitial (tissue) pressure of 1 mm Hg opposing it, a net outward force of about 11 mm Hg drives fluid from the plasma into the interstitial spaces. By contrast, the hydrostatic pressure in venules is about 17 mm Hg; thus, a net force of about 9 mm Hg drives water from tissues back into the circulation. The earlier pressures are often referred to as Starling forces. If the concentration of plasma proteins is markedly diminished (eg, due to severe protein malnutrition), fluid will cease flowing back into the intravascular compartment and begin to accumulate in the extravascular tissue spaces, a condition known as edema.

Most Plasma Proteins Are Synthesized in the Liver

Roughly 70 to 80% of all plasma proteins are synthesized in the liver. These include albumin, fibrinogen, transferrin, and most components of the complement and blood coagulation cascades. Two prominent exceptions are von Willebrand factor, which is synthesized in the vascular endothelium, and the γ-globulins, which are synthesized in the lymphocytes. Most plasma proteins are covalently modified by the addition of either N- or O-linked oligo saccharide chains, or both. Albumin is the major exception. These oligosaccharide chains fulfill a variety of functions. Loss of terminal sialic acid residues accelerates clearance of plasma glycoproteins from the circulation.

As is the case for other proteins destined for secretion from a cell, the genes for plasma proteins code for an amino-terminal signal sequence that targets them to the endoplasmic reticulum. As this leader sequence emerges from the ribosome, it binds to a transmembrane protein complex in the endoplasmic reticulum called the signal recognition particle. The emerging polypeptide chain is pulled through the signal recognition particle into the lumen of the endoplasmic reticulum and its leader sequence cleaved off by an associated signal peptidase. The newly synthesized proteins then move from the rough endo plasmic membrane via the smooth endoplasmic membrane and Golgi apparatus intro secretory vesicles, during which time they are subject to various posttranslational modifications (proteolysis, glycosylation, phosphorylation, etc). Ultimately, the mature proteins are released via the secretory vesicles into the plasma.

Many Plasma Proteins Exhibit Polymorphism

A polymorphism is a mendelian or monogenic trait that exists in the population in two or more phenotypes, neither of which is rare (ie, it occurs with frequency of at least 1-2%). Most polymorphisms are innocuous. While the ABO blood group substances (see Chapter 53) are perhaps the best-known example of a human polymorphism, other human plasma proteins exhibit polymorphism including α1-antitrypsin, haptoglobin, transferrin, ceruloplasmin, and immunoglobulins.

Each Plasma Protein Has a Characteristic Half-Life in the Circulation

 The half-life of a plasma protein is the time required for 50% of the molecules present at any given moment to be degraded or otherwise cleared from the blood. Clearance permits the continuous replacement of older, possibly damaged, protein molecules by newly-synthesized ones, a process called turn over. During normal turnover, the total concentration of these proteins will remain constant as the countervailing processes of synthesis and clearance reach a steady state.

In certain diseases, the half-life of a protein may be markedly altered. For instance, in some gastrointestinal diseases such as regional ileitis (Crohn disease), considerable amounts of plasma proteins, including albumin, may be lost into the bowel through the inflamed intestinal mucosa. The half-life of albumin in these subjects may be reduced from its normal 20 days to as little as 1 day, a condition referred to as a protein losing gastroenteropathy.

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اخبار العتبة العباسية المقدسة

العتبة العباسية المقدسة تكرم المشاركين في مشروع مقرأة العميد الرقمية

تقديم محاضرة عن عالم الأرقام في القرآن الكريم ضمن حفل إطلاق مشروع مقرأة العميد

ضمن فعاليات إطلاق مقرأة العميد الرقمية.. عرض فيديو توثيقي عن فكرة المشروع

جامعة تكنولوجيا المعلومات: مشروع مقرأة العميد منصة رقمية متكاملة لتعليم القرآن الكريم وتلاوته وحفظه

المزيد

الآخبار الصحية

عرض شائع على الأصابع.. قد يكشف سرطان الرئة مبكرا

7 مشروبات طبيعية لتقليل التوتر والقلق

دراسة: "خطر مبكر" يهدد صحة الأطفال

الجوز أو الفول السوداني.. أيهما يتفوّق في حماية القلب؟

المزيد

الآخبار التكنلوجية

مفاجأة علمية عن الإنجاب.. كم طفلا يطيل عمر المرأة؟

استطلاع يكشف "الأكثر قلقا" من تأثر الوظائف بالذكاء الاصطناعي

هل يمنحك تناول الجزر يوميا سمرة طبيعية؟

إنجاز طبي تاريخي.. جراحة قلب بلا شق في الصدر

المزيد

مواضيع ذات صلة

Intermediate Filaments Differ From Microfilaments & Microtubules

Microtubules Contain α- & β-Tubulins

Muscle Tissue are The Target of Several Genetic Disorders

Skeletal Muscle Contains Slow (Red) & Fast (White) Twitch Fibers

Muscle Contraction Requires Large Quantities of ATP

Cardiac Muscle Resembles Skeletal Muscle in Many Respects

Contraction is Orchestrated by the Second Messenger Ca2+

Muscle Converts Chemical Energy to Mechanical Energy

Cholesterol Side Chain Cleavage Enzyme (p450scc)

Steroidogenic Acute Regulatory Protein (StAR)

Cytochrome P450 Steroid Hydroxylases

Major Protein Components of Muscle Fibers