| Plasma proteins can be broadly classified into two groups: those, including albumin, that are synthesized by the liver, and the immunoglobulins, which are produced by plasma cells of the bone marrow, usually as part of the immune response.
|
| page 26 |  | | page 27 |
| The role of the clinical laboratory |
| The clinical laboratory performs a large number of biochemical analyses on body fluids, which can give answers to specific clinical questions about an individual patient. Such analyses are usually requested to aid in the diagnosis or treatment of specific conditions. The majority of specimens received by the laboratory are blood and urine samples. |
| Whereas some measurements are performed on whole blood, serum or plasma are preferred for most analyses of molecules and ions. In general, the time devoted to the analysis of each sample is relatively short, but the entire process from a request for analysis to receipt of a result involves many steps and can take several hours. Throughout the process, constant checking, attention to detail, and quality assurance are performed to ensure that the produced results are analytically and clinically valid. An outline of the laboratory workflow is shown in Figure 3.3. |
| A 44-year-old woman was admitted to hospital because of weakness, anorexia, recurrent infections, bilateral leg edema, and breathlessness. Her plasma albumin concentration was 19 g/L (normal range 35-45 g/L) and her urinary protein excretion 10 g/24 h (normal value <0.15 g/24 h). There was microscopic haematuria. Renal biopsy confirmed the diagnosis as membranoproliferative glomerulonephritis. |
| Comment. This woman had the classic triad of the nephrotic syndrome: hypoalbuminemia, proteinuria, and edema. The nephritis has resulted in damage to the glomerular basement membrane, with resultant leak of albumin. Continued loss of albumin exceeds the synthetic capacity of the liver, and results in hypoalbuminemia; consequently, the capillary osmotic pressure is significantly reduced. This leads to both peripheral (leg) edema and pulmonary edema (breathlessness). With increasing glomerular damage, proteins of larger molecular mass, such as immunoglobulins and complement (Chapter 36) are lost. |
| A number of plasma proteins have the ability to bind certain ligands with a high affinity and specificity. These
proteins can then act as a reservoir for the ligand and help control its distribution and availability by transporting it to tissues throughout the body. Binding to a protein can also render a toxic substance less harmful to the tissues. Major binding proteins and their ligands are shown in Table 3.1.
|
| Figure 3.3 The function of the clinical laboratory. Flow diagram indicating the steps involved in the generation of results from the clinical laboratory. |
| page 27 | | | page 28 |
|
Table 3-1.
Transport proteins and their ligands. |
| Body_ID: None |
| Transport proteins and their ligands |
| Body_ID: T003001.50 |
| Proteins | Ligands |
| Body_ID: T003001.100 |
| Cation binding |
| Body_ID: T003001.150 |
| Albumin | divalent and trivalent cations, e.g. Cu2+, Fe3+ |
| Body_ID: T003001.200 |
| Ceruloplasmin | Cu2+ |
| Body_ID: T003001.250 |
| Transferrin | Fe3+ |
| Body_ID: T003001.300 |
| Hormone binding |
| Body_ID: T003001.350 |
| Thyroid-binding globulin (TBG) | Thyroxine (T4), Tri-iodothyronine (T3) |
| Body_ID: T003001.400 |
| Cortisol-binding globulin (CBG) | Cortisol |
| Body_ID: T003001.450 |
| Sex hormone-binding globulin (SHBG) | Androgens (testosterone ), estrogens (estradiol) |
| Body_ID: T003001.500 |
| Hemoglobin/protoporphyrin binding |
| Body_ID: T003001.550 |
| Albumin | Heme, bilirubin, biliverdin |
| Body_ID: T003001.600 |
| Haptoglobin | Hemoglobin dimers |
| Body_ID: T003001.650 |
| Fatty acid binding |
| Body_ID: T003001.700 |
| Albumin | Non-esterified fatty acids, steroids |
| Body_ID: T003001.750 |
|
| Body_ID: T003001.800 |
Almost all plasma proteins bind ligands, and this is a major function of many proteins. Albumin can bind many molecules weakly and nonspecifically, but other proteins bind tightly to specific molecules - for example, transferrin is specific for ferric iron (Fe3+).
|
|
