Physiological Functions of Albumin and Globulin
Albumin and globulin are the two major protein groups in blood plasma that serve distinct but complementary roles: albumin primarily maintains oncotic pressure and transports substances, while globulins function mainly in immune defense and specialized transport.
Albumin Functions
Oncotic Pressure Maintenance
- Albumin is the main determinant of plasma oncotic pressure, regulating fluid distribution between blood vessels and body tissues 1, 2.
- It accounts for approximately 35-50 mg/mL in normal plasma conditions and represents the most abundant plasma protein 1.
- The protein has a half-life of approximately 14-20 days in healthy individuals 1, 3.
Transport and Binding Capacity
- Albumin displays extraordinary ligand binding capacity, serving as the primary carrier for fatty acids and numerous endogenous and exogenous compounds 2.
- It binds and transports various substances including hormones, drugs, and other molecules throughout the bloodstream 4.
- The protein affects pharmacokinetics of many drugs and provides metabolic modification of some ligands 2.
Additional Protective Functions
- Albumin accounts for most of the antioxidant capacity of human plasma, neutralizing free radicals 4, 2.
- It renders potential toxins harmless and displays pseudo-enzymatic properties 2.
Synthesis and Production
- The liver synthesizes all plasma albumin, as demonstrated through studies using labeled amino acids 5.
- Albumin comprises over 90% of the ten major plasma proteins (along with immunoglobulins and fibrinogen), with the remaining 10% consisting of more than 1500 less abundant proteins 1.
Globulin Functions
Immune System Components
- Globulins, particularly immunoglobulins (antibodies), provide the primary immune defense mechanism in blood plasma 1.
- Immune globulin preparations contain 15-18% protein obtained through cold ethanol fractionation of large blood plasma pools 1.
- These antibody-containing preparations are used for passive immunization against various infectious diseases including measles and hepatitis A 1.
Specialized Globulin Types
- Specific immune globulins are obtained from donor pools preselected for high antibody content against specific antigens (e.g., hepatitis B, varicella-zoster, rabies, tetanus) 1.
- Intravenous immune globulin (IGIV) is used primarily for replacement therapy in primary antibody-deficiency disorders and treatment of conditions like Kawasaki disease and immune thrombocytopenic purpura 1.
- Alpha and beta globulin fractions include various transport proteins and clotting factors like fibrinogen 5.
Production Characteristics
- Unlike albumin, gamma globulins are not synthesized by the liver but are produced by immune cells 5.
- Alpha globulins demonstrate more rapid turnover compared to other major plasma protein fractions 5.
Clinical Significance
Protein Distribution
- Together, albumin and the major globulins account for approximately 60-80 mg of soluble proteins per mL of plasma 1.
- The protein composition in plasma is dynamic and dependent on both genetic and environmental factors, as well as longitudinal changes 1.