The Analysis Of Certain Enzymes Aids Diagnosis

Of the thousands of different enzymes present in the human body, those that fulfill functions indispensable to cell vitality are present throughout the body tissues. Other enzymes or isozymes are expressed only in specific cell types, during certain periods of development, or in response to specific physiologic or pathophysio-logic changes. Analysis of the presence and distribution of enzymes and isozymes—whose expression is normally tissue-, time-, or circumstance-specific—often aids diagnosis.

200 250 300 350

Wavelength (nm)

Figure 7-9. Absorption spectra of NAD+ and NADH. Densities are for a 44 mg/L solution in a cell with a 1 cm light path. NADP+ and NADPH have spectrums analogous to NAD+ and NADH, respectively.

200 250 300 350

Wavelength (nm)

Figure 7-9. Absorption spectra of NAD+ and NADH. Densities are for a 44 mg/L solution in a cell with a 1 cm light path. NADP+ and NADPH have spectrums analogous to NAD+ and NADH, respectively.

HEXOKINASE

Glucose

ATP, Mg2"

ADP, Mg2 Glucose 6-phosphate

NADP+

GLUCOSE-6-PHOSPHATE DEHYDROGENASE

6-Phosphogluconolactone

Figure 7-10. Coupled enzyme assay for hexokinase activity. The production of glucose 6-phosphate by hexokinase is coupled to the oxidation of this product by glucose-6-phosphate dehydrogenase in the presence of added enzyme and NADP+. When an excess of glucose-6-phosphate dehydrogenase is present, the rate of formation of NADPH, which can be measured at 340 nm, is governed by the rate of formation of glucose 6-phosphate by hexokinase.

Nonfunctional Plasma Enzymes Aid Diagnosis & Prognosis

Certain enzymes, proenzymes, and their substrates are present at all times in the circulation of normal individuals and perform a physiologic function in the blood. Examples of these functional plasma enzymes include lipoprotein lipase, pseudocholinesterase, and the proenzymes of blood coagulation and blood clot dissolution (Chapters 9 and 51). The majority of these enzymes are synthesized in and secreted by the liver.

Plasma also contains numerous other enzymes that perform no known physiologic function in blood. These apparently nonfunctional plasma enzymes arise from the routine normal destruction of erythrocytes, leukocytes, and other cells. Tissue damage or necrosis resulting from injury or disease is generally accompanied by increases in the levels of several nonfunctional plasma enzymes. Table 7-2 lists several enzymes used in diagnostic enzymology.

Isozymes of Lactate Dehydrogenase Are Used to Detect Myocardial Infarctions

L-Lactate dehydrogenase is a tetrameric enzyme whose four subunits occur in two isoforms, designated H (for

Table 7-2. Principal serum enzymes used in clinical diagnosis. Many of the enzymes are not specific for the disease listed.

heart) and M (for muscle). The subunits can combine as shown below to yield catalytically active isozymes of L-lactate dehydrogenase:

Table 7-2. Principal serum enzymes used in clinical diagnosis. Many of the enzymes are not specific for the disease listed.

Serum Enzyme

Major Diagnostic Use

Aminotransferases Aspartate aminotransferase (AST, or SGOT) Alanine aminotransferase (ALT, or SGPT)

Myocardial infarction Viral hepatitis

Amylase

Acute pancreatitis

Ceruloplasmin

Hepatolenticular degeneration (Wilson's disease)

Creatine kinase

Muscle disorders and myocardial infarction

y-Glutamyl transpeptidase

Various liver diseases

Lactate dehydrogenase (isozymes)

Myocardial infarction

Lipase

Acute pancreatitis

Phosphatase, acid

Metastatic carcinoma of the prostate

Phosphatase, alkaline (isozymes)

Various bone disorders, obstructive liver diseases

Lactate Dehydrogenase Isozyme

Subunits

HHHH

12 HHHM

13 HHMM

14 HMMM

15 MMMM

Distinct genes whose expression is differentially regulated in various tissues encode the H and M subunits. Since heart expresses the H subunit almost exclusively, isozyme I1 predominates in this tissue. By contrast, isozyme I5 predominates in liver. Small quantities of lactate dehydrogenase are normally present in plasma. Following a myocardial infarction or in liver disease, the damaged tissues release characteristic lactate dehy-drogenase isoforms into the blood. The resulting elevation in the levels of the I1 or I5 isozymes is detected by separating the different oligomers of lactate dehydroge-nase by electrophoresis and assaying their catalytic activity (Figure 7-11).

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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