Blood plasma enzymes

When most lipids circulate in the body, they do so in the form of lipoprotein complexes. Simple, unesterified fatty acids are merely bound to serum albumin and other proteins in blood plasma, but phospholipids, triacylglycerols, cholesterol, and cholesterol esters are all transported in the form of lipoproteins. At various sites in the body, lipoproteins interact with specific receptors and enzymes that transfer or modify their lipid cargoes. It is now customary to classify lipoproteins according to their densities (Table 25.1). The densities are... 

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. 

A major contribution of the free-radical scavenging activity in blood plasma is attributable to the macro-molecular proteins (Wayner et al., 1985) of which albumin is a primary component and trapping agertt (Holt et al., 1984). Serum sulphydryl levels, primarily albumin-related, are decreased in subjects with rheumatoid complicated coalworkers pneumoconiosis, indicative of exacerbated inflammatory R.OM production (Thomas and Evans, 1975). Experimental asbestos inhalation in rats leads to an adaptive but evidendy insufficient response by an increase in endogenous antioxidant enzymes (Janssen etal., 1990). Protection of the vascular endothelium against iron-mediated ROM generation and injury is afforded by the iron sequestiant protein ferritin (Balia et al., 1992). 

It soon became apparent that the biologically active forms of Vitamin Bj.2 contained the unique Co—C-a-bond, and the instability of these covalent compounds to visible light facilitated observations on the occurrence of functional corrinoids in a number of enzymes. Deoxyadenosyl-cobalamin was found to be the most abundant corrinoid in bacteria (24) and in mammalian liver (25). Methylcobalamin was found in Escherichia coli (26), calf liver and human blood plasma (27), and also in a number of Clostridia (28). 

Interestingly, carotenoids more abundant in the blood plasma than zeaxanthin, such as lycopene, P-carotene, and P-cryptoxanthin, do not accumulate in the retina. RPE cells express p,p-carotene 15,15 -monooxygenase (BCO), formerly known as P-carotene 15,l5 -dioxygcnase, an enzyme that catalyzes the oxidative cleavage of P-carotene into two molecules of all-trans-retinal (Aleman et al., 2001 Bhatti et al., 2003 Chichili et al., 2005 Leuenberger et al., 2001 Lindqvist and Andersson, 2002). Therefore it may be suggested that p -carotene transported into RPE-cells is efficiently cleaved into retinal molecules. BCO cleaves also P-cryptoxanthin (Lindqvist and Andersson, 2002), and its absence in the retina may also be explained by its efficient cleavage to retinoids. However, lycopene, often the most abundant carotenoid in human plasma, cannot serve as a substrate for BCO, and yet it is not detectable in the neural retina (Khachik et al., 2002). 

Enzyme-Linked Immunosorbent Assay (ELISA) An immunological technique used to quantify the amount of antigen or antibody in a sample such as blood plasma or serum. 

Enzymes occupy an important place in analytical biochemistry and many investigations require their detection and quantitation. Studies of the enzyme content of blood plasma are particularly useful in clinical biochemistry both in the monitoring of normal metabolic processes and in the detection of abnormal levels of enzyme production or release. Enzyme assays also provide convenient methods for assessing the quality of foodstuffs and checking the efficiency of sterilization and pasteurization processes. 

Physostigmine is an interesting alkaloid because it is generally recognized that it acts by inhibiting a specific enzyme, cholinesterase. This enzyme has been found to be at characteristic levels in the corpuscles and blood plasma of different individuals (p. 79), and it would be expected that the action of the alkaloid in different individ-... 

Coupled enzyme assays have been developed for the determination of substances as diverse as glucose, uric acid, and cholesterol, the principal application being quantitation in biological fluids such as blood, plasma, and urine. Typical examples are illustrated by Eqs. (9)-(12). 

ACE is a rather nonspecific peptidase that can cleave C-terminal dipeptides from various peptides (dipeptidyl carboxypeptidase). As kininase 11, it contributes to the inactivation of kinins, such as bradykinin. ACE is also present in blood plasma however, enzyme localized in the luminal side of vascular endothelium is primarily responsible for the formation of angiotensin 11. The lung is rich in ACE, but kidneys, heart, and other organs also contain the enzyme. 

The fibrin thrombus resulting from blood clotting (see p. 290) is dissolved again by plasmin, a serine proteinase found in the blood plasma. For this purpose, the precursor plasminogen first has to be proteolyti-cally activated by enzymes from various tissues. This group includes the plasminogen activator from the kidney (urokinase) and tissue plasminogen activator (t-PA) from vascular endothelia. By contrast, the plasma protein a2-antiplasmin, which binds to active plasmin and thereby inactivates it, inhibits fibrinolysis. 

This enzyme [EC 3.4.17.3] (also referred to as lysine carboxypeptidase, arginine carboxypeptidase, kininase I, or anaphylatoxin inactivator) is a zinc-dependent member of peptidase family M14. The enzyme hydrolyzes the peptide bond at the C-terminus provided that the C-terminal amino acid is either arginine or lysine. The enzyme inactivates bradykinin and anaphylatoxins in blood plasma. 

A blue, copper-containing glycoprotein present in mammalian blood plasma and containing type 1, type 2, and type 3 copper centers. The type 2 and type 3 copper centers are close together, forming a trinuclear copper cluster. Ceruloplasmin has an important role in the transport and storage of copper ions. Thus, it participates in the metabolism of copper-containing enzymes. 

The stability of the oligopeptide side-chains in blood plasma and serum was determined [251]. Based on these results it was possible to control the degradability of HPMA copolymers by a particular enzyme as well as in the in vivo system [169, 252]. 

Most of the signs and symptoms resulting from diazinon poisoning are due to the inhibition of an enzyme called acetylcholinesterase in the nervous system. This enzyme is also found in your red blood cells and a similar enzyme (serum cholinesterase) is found in blood plasma. The most common test for exposure to many organophosphorus insecticides, including diazinon, is to determine the level of cholinesterase activity in the red blood cells or plasma. This test requires only a small amount of blood and is routinely available in your doctor s office. It takes time for this enzyme to completely recover to normal levels following exposure. Therefore, a valid test may be conducted a number of days following the suspected exposure. This test indicates only exposure to an insecticide of this type. It does not specifically show exposure to diazinon. 

Measurement of pH is one of the most important and frequently used procedures in biochemistry. The pH affects the structure and activity of biological macromolecules for example, the catalytic activity of enzymes is strongly dependent on pH (see Fig. 2-21). Measurements of the pH of blood and urine are commonly used in medical diagnoses. The pH of the blood plasma of people... 

The study of enzymes has immense practical importance. In some diseases, especially inheritable genetic disorders, there may be a deficiency or even a total absence of one or more enzymes. For other disease conditions, excessive activity of an enzyme may be the cause. Measurements of the activities of enzymes in blood plasma, erythrocytes, or tissue samples are important in diagnosing certain illnesses. Many drugs exert their biological effects through interactions with enzymes. And enzymes are important practical tools,... 

Relationship between Fructose 1,6-Bisphosphatase and Blood Lactate Levels A congenital defect in the liver enzyme fructose 1,6-bisphosphatase results in abnormally high levels of lactate in the blood plasma. Explain. 

FIGURE 23-22 The composition of blood. Whole blood can be separated into blood plasma and cells by centrifugation. About 10% of blood plasma is solutes, of which about 10% consists of inorganic salts, 20% small organic molecules, and 70% plasma proteins. The major dissolved components are listed. Blood contains many other substances, often in trace amounts. These include other metabolites, enzymes, hormones, vitamins, trace elements, and bile pigments. Measurements of the concentrations of components in blood plasma are important in the diagnosis and treatment of many diseases. 

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. 

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