Enzyme assay definitions

Definitive diagnosis of a UCD can be made by specific enzyme assays. A deficiency of OTC,... 

A second, cytosolic CPS activity (CPSII) occurs in mammals as part of the CAD trifunctional protein that catalyzes the first three steps of pyrimidine synthesis (CPSII, asparate tran-scarbamoylase, and dihydroorotase). The activities of these three enzymes—CPSII, aspartate transcarbamoylase, and dihydroorotase—result in the production of orotic acid from ammonium, bicarbonate, and ATP. CPSII has no role in ureagenesis, but orotic aciduria results from hepatocellular accumulation of carbamyl phosphate and helps distinguish CPSI deficiency from other UCDs. Defects in CPSI classically present with neonatal acute hyperammonemic encephalopathy. The plasma citrulline and urine orotic acid concentrations are both low. A definitive diagnosis can be established by enzyme assay of biopsied liver tissue or by mutation analysis. 

A soil enzyme assay might have been used to verify the involvement of soil microorganisms in degradation. Continued efforts in this area may result in a definite diagnostic assay based on enzymatic activities coupled with specific pesticides. 

Some clinicians are critical of the use of amniotic material as described above and prefer to use cells which are more definitely foetal in origin. Therefore, more rapid assay procedures have been developed by increasing the sensitivity of the enzyme assays and reducing the number of tissue culture passages (Brady, 1978). 

The two forms of BH4 deficiency without hyperphenylalaninemia are detectable only by investigations for neurotransmitter metabolites and pterins in CSF. In DRD, a phenylalanine loading test, a trial with L-Dopa, and enzyme activity measurement in cytokine-stimulated fibroblasts are confirmatory for the diagnosis. SR deficiency can be definitely diagnosed only by an enzyme assay of cultured fibroblasts. 

In many situations, the actual molar amount of the enzyme is not known. However, its amount can be expressed in terms of the activity observed. The International Commission on Enzymes defines One International Unit of enzyme as the amount that catalyzes the formation of one micromole of product in one minute. (Because enzymes are very sensitive to factors such as pH, temperature, and ionic strength, the conditions of assay must be specified.) Another definition for units of enzyme activity is the katal. One katal is that amount of enzyme catalyzing the conversion of one mole of substrate to product in one second. Thus, one katal equals 6X10 international units. 

Definitive diagnosis made by assay of appropriate enzyme, often using leukocytes... 

Laboratory confirmation is vital to effective treatment of HSV, especially in individuals in whom a clinical diagnosis cannot be obtained. There are several methods by which a definitive diagnosis may be acquired, and these include virologic typing, serologic diagnosis, rapid point-of-care antigen detection, enzyme-linked immunosorbent assay (ELISA), immunoblot, and DNA polymerase chain reaction.27... 

Very low concentrations of substrates may be assayed by recycling the test substrate for an appreciable but definite period of time and measuring the amount of product formed. The coenzyme NADPH, for instance, may be assayed using the two enzymes glutamate dehydrogenase (EC 1.4.1.3) and glucose-6-phosphate dehydrogenase (EC 1.1.1.49) ... 

First, the true physiological substrates of most esterases are unknown. It is, therefore, hardly practicable to systematically name esterases according to the recommendations of the Enzyme Nomenclature Committee [1], i.e., based on the definite (physiological) role of the enzyme. The difficulty is that the use of nonphysiological substrates during purification and in characterization assays does not contribute to discovering the physiological role of an enzyme. 

The simple definition of chitinase activity, EC 3.2.1.14, "hydrolysis of iV-acetyl-D-glucosaminide (l-4)-P-linkages in chitin and chitodextrins", belies the complexity and diversity of this group of enzymes. When chitinolytic organisms are investigated in detail, they are found to produce a range of chitinase activities. Usually these can be separated readily by chromatography or electrophoresis. However, it is more difficult to define their precise activities, chiefly because of the uncertain nature of the available assays for chitinases, with non-linear time courses... 

One of the difficulties of peroxidase studies is that the enzymes can react with a number of synthetic or natural substrates and that even the use of purified isozymes in assays for substrate specificity does not identify any definitive roles. A search for specific inhibitors represents another approach which is still poorly developed despite its potential utility. 

Introduction. Our aim is to obtain the physicochemical background in order to be able to set up an absolute viscosimetric method for the assay of endocellulases in enzyme mixtures, using a buffered HEC substrate and giving the definition of enzymic activities in katals, according to the recommendations of the International Union of Biochemistry. 

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