Proteins and enzymes

Published on 01/03/2015 by admin

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Last modified 22/04/2025

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Proteins and enzymes

Plasma proteins

The biochemistry laboratory routinely measures ‘total protein’ and ‘albumin’ concentrations, usually in a serum specimen, and reports the ‘globulin’ fraction as the difference between the first two results. Some proteins (e.g. immunoglobulins) are measured as classes, and immunochemical methods are available for measuring specific proteins and hormones. Enzymes are measured both by determining their activity and by immunochemical methods to assess their mass.

Albumin

Albumin is the major plasma protein and is synthesized and secreted by the liver. It has a biological half-life in plasma of about 20 days and it accounts for about 50% of the total hepatic protein production. Albumin makes the biggest contribution to the plasma oncotic pressure. If the albumin concentration falls very low, oedema is the result (Fig 25.1). There are four main reasons for the occurrence of a low plasma albumin concentration:

Although serum albumin measurements have previously been used to monitor a patient’s response to long-term nutritional support, they are unreliable and insensitive.

Specific proteins

Measurement of a number of specific proteins gives useful information in the diagnosis and management of disease (Table 25.1). Characteristic changes in the concentration of certain plasma proteins are seen following surgery or trauma, or during infection or tumour growth. The proteins involved are called acute phase reactants (pp. 110–111). These acute phase proteins may be used to monitor progress of the condition or its treatment.

Table 25.1

Specific proteins that are measured in serum

Protein name Function Reason for assay
α1-antitrypsin Protease inhibitor Reduced in α1-antitrypsin deficiency
β2-microglobulin A subunit of the HLA antigen Raised in renal tubular dysfunction on all cell membranes
Caeruloplasmin Oxidizing enzyme Reduced in Wilson’s disease
C-reactive protein (CRP) Involved in immune response Increased in acute illness, especially infection
Ferritin Binds iron in tissues Gives an indication of body iron stores
Haptoglobin Binds haemoglobin Reduced in haemolytic conditions
Thyroid-binding globulin (TBG) Thyroid hormone binding Investigation of thyroid disease
Sex hormone binding globulin Binds testosterone and oestradiol Investigation of androgen excess and/or insulin resistance
Transferrin Iron transport Assessment of iron status and/or response to nutritional support

Enzymes

Serum enzymes in disease

Enzymes may be classified in two groups. Some, such as the enzymes of the coagulation cascade, have a defined function in blood. Others appear in the blood incidentally and their measurement is of value in diagnosis. Damage to the tissues of origin, or proliferation of the cells from which these enzymes arise, will lead to an increase in the activity of these enzymes in plasma (Fig 25.2). It should be noted that increases in serum enzyme activity are only roughly proportional to the extent of tissue damage.

Enzymes that have been shown to have a diagnostic value are:

A further enzyme of practical interest is cholinesterase. Cholinesterase, normally involved in the process of neuromuscular conduction, incidentally hydrolyses suxamethonium (succinylcholine), a muscle-relaxing drug used in anaesthesia. Patients with abnormal cholinesterase may fail to hydrolyse the drug normally and as a result suffer prolonged paralysis after anaesthesia. This is called scoline apnoea. Cholinesterase measurements are also useful in the diagnosis of poisoning with pesticides that are cholinesterase inhibitors.

Isoenzyme determination

Some enzymes are present in the plasma in two or more molecular forms. These variants are known as isoenzymes and, although they have different structures, they perform the same catalytic function. Different isoenzymes may arise from different tissues and their specific detection may give clues to the site of pathology. Alkaline phosphatase isoenzymes may distinguish between bone and liver disease, especially in patients in whom metastases of bone or liver are suspected. A specific isoenzyme of creatine kinase (CK-MB) is useful in the early detection of myocardial infarction. Heart muscle contains proportionally more of this isoenzyme than skeletal muscle, and raised levels of CK-MB indicate that a myocardial infarction has occurred.

Case history 19

Eight months after an attack of acute glomerulonephritis, a 38-year-old woman was hospitalized for investigation of progressive bilateral leg oedema. On examination, she was normotensive and exhibited pitting oedema of both ankles and dullness over her lung bases. Her face was pale and puffy and she admitted to frequent minor intercurrent infections.

Comment on page 166.

image Clinical note

The only non-specific index of the presence of disease, comparable to C-reactive protein (CRP), is the erythrocyte sedimentation rate (ESR). This, in part, reflects the intensity of the acute phase response. The rate of change of ESR is much less than that of CRP and it reflects the clinical status of the patient less well. It is also affected by the number and morphology of the red cells. CRP can now be measured with relative ease and its determination is preferred over ESR in most pathological conditions. ESR as a non-specific indicator of disease may be helpful, for example in the investigation of paraproteinaemias which do not necessarily provoke an acute phase response.

Proteins and enzymes

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