Functional plasma enzymes

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 Upase, 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. 

HDL is synthesized and secreted from both liver and intestine (Figure 25—5). However, apo C and apo E are synthesized in the liver and transferred from fiver HDL to intestinal HDL when the latter enters the plasma. A major function of HDL is to act as a repository for the apo C and apo E required in the metabohsm of chylomicrons and VLDL. Nascent HDL consists of discoid phosphohpid bilayers containing apo A and free cholesterol. These hpoproteins are similar to the particles found in the plasma of patients with a deficiency of the plasma enzyme lecithimcholesterol acyltransferase (LCAT) and in the plasma of patients with obstructive jaundice. LCAT—and the LCAT activator apo A-I— bind to the disk, and the surface phosphohpid and free cholesterol are converted into cholesteryl esters and... 

Adults require 1-2 mg of copper per day, and eliminate excess copper in bile and feces. Most plasma copper is present in ceruloplasmin. In Wilson s disease, the diminished availability of ceruloplasmin interferes with the function of enzymes that rely on ceruloplasmin as a copper donor (e.g. cytochrome oxidase, tyrosinase and superoxide dismutase). In addition, loss of copper-binding capacity in the serum leads to copper deposition in liver, brain and other organs, resulting in tissue damage. The mechanisms of toxicity are not fully understood, but may involve the formation of hydroxyl radicals via the Fenton reaction, which, in turn initiates a cascade of cellular cytotoxic events, including mitochondrial dysfunction, lipid peroxidation, disruption of calcium ion homeostasis, and cell death. 

Reasons for the presence of enzymes in the plasma Enzymes can normally be found in the plasma either because they were specifically secreted to fulfill a function in the blood, or because they were released by dead or damaged cells. Many diseases that cause tissue damage result in an increased release of intracellular enzymes into the plasma. The activities of many of these enzymes (for example, creatine kinase, lactate dehydrogenase, and alanine aminotransferase) are routinely determined for diagnostic purposes in diseases of the heart, liver, skeletal muscle, and other tissues. 

The plasma membrane (Fig. 1-4) is the outer boundary of the cell it is a continuous sheet of lipid molecules (Chap. 6) arranged as a molecular bilayer 4-5 nm thick. In it are embedded various proteins that function as enzymes (Chap. 8), structural elements, and molecular pumps and selective channels that allow entry of certain small molecules into and out of the cell, as well as receptors for hormones and cell growth factors (Chap. 6). 

Because hepatic function is often normal in many patients with liver disease, the plasma activities of numerous cytosolic, mitochondrial, and membrane-associated enzymes are measured as they are increased in many forms of liver disease. As plasma enzyme measurements are discussed in greater detail in Chapter 21, only those factors relevant to understanding of liver disease will be summarized here. 

The plasma membranes of many types of eukaryotic cells also contain receptor proteins that bind specific signaling molecules (e.g., hormones, growth factors, neurotransmlt-ters), leading to various cellular responses. These proteins, which are critical for cell development and functioning, are described in several later chapters. Finally, peripheral cytosolic proteins that are recruited to the membrane surface function as enzymes. Intracellular signal transducers, and structural proteins for stabilizing the membrane. 

Hepatitis describes infiltration of the hepatic tissue by mononuclear cells, which may or may not be associated with hepatocellular changes. There are also different patterns of cellular injury, such as those affecting the hepatocellular organelles— particularly, the microsomes, peroxisomes, and mitochondria. When hepatocellular injury occurs, the aminotransferases are probably the most useful markers, and they may be supplemented by measurements of other plasma enzymes—ALP, GLDH, and plasma bilirubin (see the following sections on laboratory investigations). Several markers of cellular function require tissues for the measurements of altered function (e.g., microsomal cytochrome P450 measurements). 

Poor metabolizers (PMs) for those individuals that have shown no enzymatic activity in diagnostic tests. Therefore, this is a phenotype that refers to people who lack the functional CYP enzyme and slowly metabolize the drug substrates, allowing their concentration to reach high plasma levels. 

Synthesized in the liver and intestine, HDL initially exists as a dense, phospholipid disk composed primarily of apoA-l. The primary function of HDL is to act as a scavenger to remove cholesterol from extrahepatic cells and to facilitate its transport back to the liver. Nascent HDL accepts free, unesterified cholesterol. A plasma enzyme, lecithin-cholesterol acyltransferase, then esterities the cholesterol. This process allows the resulting cholesterol esters to move from the surface to the core and results in the production of spherical HDL3 particles. As cholesterol content is added, HDL3 is converted ... 

Mendell JR, Engel WK, Derrer EC. (1972) Increased plasma enzyme concentrations in rats with functional ischemia of muscle provide a possible model of Duchenne muscular dystrophy. Nature 239, 522-524. 

Transits placenta, suppresses fetal enzyme activity Transits placenta, binds to protein and nucleic acids, interferes with cell division Induces placental enzymes, interferes with normal placental function Transits placenta Transits placenta, reduces maternal plasma enzyme activity... 

In the previous sections, we have described the determinations of clinically important biomarkers such as 02, NO, N02, NOs, cyt c, glucose, etc. For all the measurements, 1—2 mL of the sample is required, making it difficult to prepare the sample (blood plasma, serum, and cell cultures) for measurements. Therefore, in order to reduce the sample volume, we have developed electrochemical assays for the measurement of various biomarkers in a single drop of the biological sample using screen-printed electrode functionalized with enzyme, viz., SOD, CcO, CcR on CNT, and GNP. We previously immobilized SOD on Pt and screen-printed electrodes. The sensitivities of these two electrodes for nitrite determinations are 0.19 pA cm and... 

Erythrocyte Entrapment of Enzymes. Erythrocytes have been used as carriers for therapeutic enzymes in the treatment of inborn errors (249). Exogenous enzymes encapsulated in erythrocytes may be useful both for dehvery of a given enzyme to the site of its intended function and for the degradation of pathologically elevated, diffusible substances in the plasma. In the use of this approach, it is important to determine that the enzyme is completely internalized without adsorption to the erythrocyte membrane. Since exposed protein on the erythrocyte surface may ehcit an immune response following repeated sensitization with enzyme loaded erythrocytes, an immunologic assessment of each potential system in animal models is required prior to human trials (250). 

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