Plasma specific enzymes

Enzyme assays employed in the diagnosis of diseases are one of the most frequently used clinical laboratory procedures. The most commonly used body fluid for this purpose is serum, the fluid that appears after the blood has clotted. The liquid portion of unclotted blood is called plasma. Serum is used for many enzyme assays because the preparation of plasma requires addition of anticoagulants (e.g., chelating agents) that interfere with some assays. Enzymes in circulating plasma are either plasma-specific or nonplasma-specific. Plasma-specific enzymes are normally present in plasma, perform their primary function in blood, and have levels of activity that are usually higher in plasma than in tissue cells. Examples are those enzymes involved in blood clotting (e.g., thrombin), fibrinolysis (e.g., plas-min), and complement activation, as well as cholinesterase... 

Non-plasma-specific enzymes are intracellular enzymes normally present in plasma at minimal levels or at concentrations well below those in tissue cells. Their presence in plasma is normally due to turnover of tissue cells, but they are released into the body fluids in excessive concentrations as a result of cellular damage or impairment of membrane function. Tissue injury and impairment of... 

Factors affecting the presence and removal of non-plasma-specific enzymes from plasma. 

Plasma enzymes may be classified as (1) plasma-specific enzymes, (2) other secreted enzymes (e.g., amylase), and (3) intracellular enzymes. The plasma-specific enzymes include those that are secreted by some organs and have a direct action in... 

Agarwal et al. 1978), the quantification of these specific enzymes may indicate that exposure to endosulfan has occurred. Blood tests, such as decay curves for aminopyrine in plasma, which are semiquantitative indices of liver enzyme induction, have been used successfully in the past to demonstrate enzyme induction in pesticide-exposed workers. Because numerous chemicals found at hazardous waste sites also induce these hepatic enzymes, these measurements are not specific for endosulfan exposure. However, measurements of enzyme activity, together with the detection of the parent compound or its metabolites in tissue or excreta, can be useful indicators of exposure. All of these potential biomarkers require further verification in epidemiological studies. Further studies with focus on the development of methods to separate and measure the estrogenicity of endosulfan in in vitro assays would be valuable since these assays are more sensitive and discriminative than other conventional biomarkers. Preliminary results have been presented by Sonnenschein et al. (1995). 

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-... 

Methods. A plasma matrix-independent method has been obtained by adding specific enzyme inhibitors to all the various plasma matrices, and an insignificant matrix effect has been obtained using the following criteria ... 

The plasma fucosyltransferase to Gal produces a-L-Fuc-( 1—>-2)-Gal linkages, and is the H-dependent, blood-group-specific enzyme. Its selective inhibition by thiol-blocking agents has permitted its discrimination245 from other fucosyltransferases. 

Many diseases that cause tissue damage result in an increased release of intracellular enzymes into the plasma. The activities of many of these enzymes are routinely determined for diagnostic purposes in diseases of the heart, liver, skeletal muscle, and other tissues. The level of specific enzyme activity in the plasma frequently correlates with the extent of tissue damage. Thus, determining the degree of elevation of a particular enzyme activity in the plasma is often useful in evaluating the prognosis for the patient. 

Flydrolysis is an important metabolic reaction for drugs whose structures contain ester and amide groups. All types of ester and amide can be metabolized by this route. Ester hydrolysis is often catalysed by specific esterases in the liver, kidney and other tissues as well as non-specific esterases such as acetylcholinesterases and pseudocholinesterases in the plasma. Amide hydrolysis is also catalysed by non-specific esterases in the plasma as well as amidases in the liver. More specific enzyme systems are able to hydrolyse sulphate and glucur-onate conjugates as well as hydrate epoxides, glycosides and other moieties. 

Because the intermediary metabolism of various organs is virtually the same, organ-specific enzymes are very rare. One example usually cited is the acid phosphatase of the prostate. However, the enzyme complement of the various organs may differ with respect to relative activities of the enzymes, the time dependence of their appearance in plasma, and the pattern of their isoenzymes (see below). Table 5.2 presents a list of enzymes commonly used for organ- and disease-specific diagnoses. 

Carbamazepine induces CYP3A, and the metabolism of risperidone, which mainly involves CYP2D6, may also involve CYP3A. Carbamazepine can therefore reduce risperidone plasma concentrations (SEDA-22, 71). However, since carbamazepine alters the biotransformation of many agents, non-specific enzyme induction has been suggested for the risperidone and carbamazepine interaction (240). 

At One time it was thought that women taking oral contraceptives were at risk for B deficiency. This notion seem-S to have been in error. The error was due to a misinterpretation of the tryptophan load lest. As mentioned earlier, a deficiency in vitamin B(,can induce the accumulation of specific intermediates of the tryptophan catabolic pathway and enhanced excretion in the urine. Oral contraceptives can also induce ar increase in the formation and excretion of specific intermediates by stimulating the activity of specific enzymes of the tryptophan catabolic pathway, This stimulation was responsible for the false indications of deficiency. Independently of the tryptophan load test, there continues to be some evidence for risk associated with the use of oral contraceptives. Oral contraceptive use may result in lowered levels of plasma vitamin Bf, Tlicsc lowered levels may result in a vitamin deficiency when coupled with pregnancy and lactation. 

Metabolic Roles. Ascorbic acid is an electron donor required for a variety of oxidative processes. It is readily regenerated by glutathione, NAD, and NADP and thus has a long biological half-life. Currently, there are eight known human enzymes that require ascorbic acid, and they are listed in Table 8.5. The precise metabolic roles have not been completely elucidated, but it appears that in the met-alloenzymes, ascorbate reduces the active metal site. In addition to these specific enzymes, ascorbic acid seems to function as a free-radical scavenger in the aqueous phase of plasma and cells. 

The present review will focus on the analysis of l-DOPA and L-tyrosine by HPLC in plasma. Both are involved in the first step of melanogenesis controlled by a melanocyte-specific enzyme, tyrosinase. The plasma L-DOPA/L-tyrosine ratio has been evaluated as a tumor marker in melanoma during a 10-year collaboration between the Biochemistry Laboratory, the Dermatology Department of Saint-Louis Hospital (AP-HP) in Paris (France) and the Dermatology Department of the National Center of Oncology in Sofia (Bulgaria). 

Figure 4.1 Early phases of the melanogenesis pathway. The first steps are critically regulated by the melanocyte-specific enzyme tyrosinase. l-DOPA is directly formed from L-tyrosine (1) and/or indirectly via L-DOPAquinone (2). Adapted from Melanoma Research, 9, Letellier S, Gamier JP, Spy J, Stoitchkov K, Le Bricon T, Baccard M, Revol M, Kernels Y, Bousquet B. Development of metastases in mahgnant melanoma is associated with an increase of plasma L-DOPA/L-tyrosine ratio, pages 389-394, 1999, with permission from Lippincott Williams Wilkins...

CJ Wang, Z Tian, V Reyes, CC Lin. A specific enzyme immunoassay (EIA) with selective extraction for quantitation of a topical anti-inflammatory agent, SCH40120, in human plasma. J Pharm Biomed Anal 13 121, 1995. 

For damage to the heart and pancreas, specific enzymes may be measured in the plasma or serum such as creatine kinase and amylase, respectively. 

The non-microsomal oxidation is catalysed by enzymes that are free in the cellular plasma or by relatively non-specific enzymes present in the mitochondrial fraction of the homogenized tissue of liver cells. Two of these enzymes which are particularly important are monoaminooxidase, catalysing the metabolism of primary amines and controlling the level of biogenic amines, and diaminooxidase, catalysing selectively the oxidation of one out of two amino groups of diamines. Aliphatic alcohols are oxidized by the alcohol dehydrogenase to aldehydes. This reaction is reversible and the same enzyme participates in the reduction of aldehydes and ketones to alcohols [7]. 

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