Enzyme isoenzyme

The second main class of blood constituents used as genetic markers are the polymorphic enzymes. The enzymes of interest to the forensic serologist are primarily located within the red blood cell and are commonly referred to as isoenzymes. These can briefly be described as those enzymatically active proteins which catalyze the same biochemical reactions and occur in the same species but differ in certain of their physicochemical properties. (This description does not exclude the tissue isoenzymes that occur within the same organism however, our consideration deals only with those of the red blood cell in particular.) The occurrence of multi-molecular forms of the same enzyme (isoenzymes) has been known for several decades however, it was not until the Metropolitan Police Laboratory of Scotland Yard adapted electrophoretic techniques to dried blood analysis that these systems were catapulted to the prominence they presently receive (.2). For many of the forensic serologists in the United States, the use of electrophoresis and isoenzyme determination is a recently-inherited capability shared by only a few laboratories. 

The blood plasma of a healthy individual typically contains 60-80 g/L of protein. This protein can be separated into five classes designated a through "y. The separation is based on the overall surface charge on each of the types of protein. (See A Clinical Perspective Enzymes, Isoenzymes, and Myocardial Infarction in Chapter 20 for a discussion of the separation of proteins based on surface charge.)... 

A Clinical Perspective Enzymes, Isoenzymes, and Myocardial Infarction... 

The role of LDH and other enzymes in disease diagnosis is discussed in A Clinical Perspective Enzymes, Isoenzymes, and Myocardial Infarction. 

Enzyme decay profile is concave to the X-axis (Henley and Sadana 1986 Ulbrich and Schellenberger 1986 Greco et al. 1993). Such curve can be approximated as a series of straight lines (two or more) with different decreasing slopes. This type of profile can be explained in terms of a parallel mechanism of inactivation that assumes the existence of different molecular variants of the enzyme (isoenzymes) exhibiting the same catalytic behavior but differing in their stability (Henley and... 

Malate Enzyme. Malate enzyme, as well as the other enzymes of the pathway, is known to exist in multiple forms. In apple fruit, Dilley (1966) isolated and purified a form which ran as a single band on acrylamide gels. In corn root tissue, we were able to locate a single isoenzyme (by starch gel electrophoresis) and reported evidence that it was a soluble or nonparticulate protein (Danner and Ting, 1967). Hence, it was concluded that dark CO2 metabolism, insofar as it is mediated by this particular malate enzyme isoenzyme, is in the cytosol. 

Assay of Enzymes In body fluids, enzyme levels aie measured to help in diagnosis and for monitoiing treatment of disease. Some enzymes or isoenzymes are predominant only in a particular tissue. When such tissues are damaged because of a disease, these enzymes or isoenzymes are Hberated and there is an increase in the level of the enzyme in the semm. Enzyme levels are deterrnined by the kinetic methods described, ie, the assays are set up so that the enzyme concentration is rate-limiting. The continuous flow analyzers, introduced in the early 1960s, solved the problem of the high workload of clinical laboratories. In this method, reaction velocity is measured rapidly the change in absorbance may be very small, but within the capabiUty of advanced kinetic analyzers. 

Enzymes, measured in clinical laboratories, for which kits are available include y-glutamyl transferase (GGT), alanine transferase [9000-86-6] (ALT), aldolase, a-amylase [9000-90-2] aspartate aminotransferase [9000-97-9], creatine kinase and its isoenzymes, galactose-l-phosphate uridyl transferase, Hpase, malate dehydrogenase [9001 -64-3], 5 -nucleotidase, phosphohexose isomerase, and pymvate kinase [9001-59-6]. One example is the measurement of aspartate aminotransferase, where the reaction is followed by monitoring the loss of NADH ... 

The gene for myophosphoiylase has been assigned to chromosome 1 lql3. The enzyme is a dimer of two identical 97 IcDa subunits and is the sole isoform present in skeletal muscle. Heart and brain also contain this isoform in addition to a distinct brain isoenzyme and a hybrid muscle/brain isoform. Smooth muscle also contains a phosphorylase isoform distinct from the muscle isoenzyme. If regenerating muscle fibers are present they also contain phosphorylase activity due to the presence, in fetal and developing muscle, of an isoform said to be identical with brain phosphorylase. 

Isoenzymes (isozymes) Enzymes that are very similar to one another in size and structure but with differences in catalytic ability. 

Succinyl-CoA is converted to succinate by the enzyme succinate thiokinase (succinyl-CoA synthetase). This is the only example in the citric acid cycle of substrate-level phosphorylation. Tissues in which glu-coneogenesis occurs (the hver and kidney) contain two isoenzymes of succinate thiokinase, one specific for GDP and the other for ADP. The GTP formed is used for the decarboxylation of oxaloacetate to phos-phoenolpymvate in gluconeogenesis and provides a regulatory hnk between citric acid cycle activity and the withdrawal of oxaloacetate for gluconeogenesis. Nongluconeogenic tissues have only the isoenzyme that uses ADP. 

Alkaline phosphatase assays based on 3-glycerophosphate now appears to be obsolete, and methods buffered by either glycine or barbital are also obsolete as these buffers inhibit ALP or are poor buffers. Serum alkaline phosphatase is known to be composed of several isoenzymes which presumably arise from bone, liver, intestine, and placenta. The placental alkaline phosphatase is known to be much more resistant to heat denaturation than the other isoenzymes, and this resistance provides a simple test for it (5). The other enzymes can be separated through the differential inhibition by phenylalanine, by electrophoresis and by specific antibodies. However, the clinical usefulness of the results obtained is in doubt (23). 

In liver cells the activity of GOT is higher than that of GPT, but most of the GPT activity is located in the cytoplasm and therefore leaks more readily into the blood stream with minor or reversible cell damage. Enzymes located in the mitochondria, such as one of the GOT isoenzymes appear in serum only when there has been more severe liver cell injury including cell death. 

HT is metabolised primarily by MAO to 5-hydroxyindoleacetic acid (5-HIAA) (Fig. 9.4). In vitro, 5-HT is the preferred substrate for the MAOa, rather than the MAOb isoenzyme (see Chapter 8) and this appears to be the case in vivo since MAOa, but not MAOb, knock-out mice have increased concentrations of 5-HT in the brain. Obviously, because of its indole nucleus, 5-HT is not a substrate for the enzyme COMT which metabolises the catechol derivatives, dopamine and noradrenaline. However, other metabolic products of 5-HT are theoretically possible and one, 5-hydroxytryptophol,... 

The psubunit has been purified from PGl by ourselves and others and is a heat stable, acidic, heavily glycosylated protein with an apparent molecular mass of 37-39 kD (19, 26). No enzymatic activity has been identified for the protein. The psubunit can be extracted from the cell walls of both green and ripe tomato fruit by high salt buffers (13, 14, 18, 19, 20), and in the latter case is associated with PG2 polypeptide(s) in the form of PGl. Purified psubunit can also associate with and convert PG2 in vitro into an isoenzyme that closely resembles PGl (13, 14, 24). Biochemical studies have shown that in vivo and in vitro formation of PGl by the association of PG2 with the p-subunit alters the biochemical and enzymic properties of the associated catalytic PG2 polypeptide including its pH optima, response to cations and thermal stability (summarized in Table 1). This later property provides a convenient assay for the levels of PGl and PG2 in total cell wall protein extracts. 

HPAEC analyses were carried out to determine the oligomeric products released from various pectic substrates after depolymerization by the PL isoenzymes. Action pattern analyses for the concerted action of PL isoenzymes utilized 68% esterified pectin as substrate. One-ml reaction mixtures in a buffer system as detailed in section 2.2. comprising 0.5% (w/v) substrate and 5 U of enzyme were incubated for 30 s to 18 h, and then thermoinactivated. Samples of 750 pi were applied to a Carbopac PA-1 (Dionex) column before the carbohydrates were eluted over a period of 70 min using a gradient of 0.2 M KOH, 0.05 M K-acetate to 0.2 M KOH, 0.7 M K-acetate. Detection employed a Pulsed Electrochemical Detector (PED, Dionex) in the integrated amperometry mode according to the manufacturer s recommendations. 

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