Enzyme reference materials

In many instances a selected and agreed reference measurement procedure forms the basis of the definition of the quantity and thereby represents the top of the calibration hierarchy. This is particularly true for establishing reference systems for the catalytic concentrations of enzymes. In 1999 members of the IFCC working group and some enzyme reference laboratories decided to establish new 37 C measurement procedures as IFCC reference procedures on the basis of the existing 30 C IFCC procedures and to certify enzyme reference materials for AFT, AST, GGT, CK, FD and amylase in collaboration with the IRMM. The enzymes having IFCC reference measurement procedures have catalytic concentra-... 

Moss DW. Enzyme reference materials their place in diagnostic enzymology. Ann Biol Chn 1994 52 143-6. 

In 1960, at the general assembly of the International Pharmaceutical Federation (FIP), the obsolescence of various national pharmacopeial methods for assaying pharmaceutical enzymes was demonstrated. An international commission on pharmaceutical enzymes was created to deal with this unsatisfactory situation and develop improved assay methods and guidelines for the preparation of pharmaceutical enzyme reference materials. The Center for Standards has a coordination function in organizing collaborative enzyme assays between academic, industrial, and national pharmaceutical control laboratories and in distributing FIP pharmaceutical enzyme standards. Since 1960, many FIP assay methods and standard preparations have been adopted by national and international pharmacopeias, such as the European Pharmacopoeia. The ultimate goal is to provide official, preferentially nonempirical, standardized assay methods by which comparison of commercially available pharmaceutical enzymes is made possible. The most desirable situation would be an international uniformity of enzyme standards and assay methods, which would allow physicians and clinicians to unambiguously compare the potencies of commercially available enzyme products. 

The requirements for, preparation of, and application of enzyme reference materials have been discussed extensively (134, 135, 151, 152, 153, 154, 155, 156, 157, 158). Cooperation among clinical enzvmologists in Europe and the United States over the last several years has resulted in the availability of only a few enzyme reference materials. SRM 8430 from NIST is a preparation of human erythrocyte aspartate aminotransferase in a human albumin matrix (144) certified reference material (CRM) 319 from the Community Bureau of Reference (BCR) of the Commission of the European Communities is a preparation of porcine y-glutamvltransferase in a bovine serum matrix (159). The working group of the BCR is in the process of establishing protocols and evaluating... 

R. Rej and R. Vanderlinde, Use of Purified Enzyme Reference Materials in Enzyme Activity Measurements, in Proc. Second International Symposium on Clinical Enzymology (eds. N. W. Tietz and A. Weinstock), American Association for Clinical Chemistry, Washington, DC, 1976, p. 249. 

R. E. Vanderlinde, Enzyme Reference Materials- Early Activities, in A Reference System for Clinical Enzymology Proceedings of the Workshop (ed. G. N. Bowers, Jr., G. C. Edwards, and R. N. Rand), The National Committee for Clinical Laboratory Standards, Villanova, 1986, p. 97. 

Though various methods for the determination of enzyme activity are used routinely in clinical laboratories, many of them are covered by patents, and the components of their test kits are not commercially available. In clinical chemistry, reference methods and certified enzyme reference materials (enzyme calibrators) for determination of activities of enzymes in human blood serum are recommended by international commissions (ISOTC/212, International Federation of Clinical Chemistry IFCC, Institute for Reference Materials and Measurements EU, etc.) at present for improved accuracy and establishment of traceability chains of the enzyme measurement system. By using these reference materials, the inaccuracy (imprecision between laboratories) has been minimized to within several per cent in several enzymes, such as lactate dehydrogenase (LDH) (EC 1.1.1.27), y-glutamyltrans-peptidase (y-GT) (EC 2.3.2.2), alanine aminotransferase (ALT) (EC 2.6.1.2), creatine kinase (CK) (EC 2.7.3.2), alkaline phosphatase (ALP) (EC 3.1.3.1), acid phosphatase (EC 3.1.3.2), and a-amylase (AMY)... 

There are also RMs which are prepared for a specific application and are used for validation of relevant methods. Cobbaert et al. (1999) made use of Ion Selective Electrode (ISE)-protein-based materials when evaluating a procedure which used an electrode with an enzyme-linked biosensor to determine glucose and lactate in blood. Chance et al. (1999) are involved with the diagnosis of inherited disorders in newborn children and they prepared a series of reference materials consisting of blood spotted onto filter paper and dried, from which amino-acids can be eluted and... 

The first four materials (IRMM/IFCC-452, 453, 454, 455) are expected to be released during 2000. Projects on the certification of reference materials for cardiac marker (myoglobin) and total protein concentration in serum are under discussion. Even so the number of available CRMs for clinical chemistry and occupational toxicology is still limited. This has to do with the complexity of physiological compounds (e.g. proteins), the instabihty (e.g. enzymes), or the volatility (e.g. solvents). 

In order to measure the exact amount of a specific protein (analyte) by IHC signal intensity, a critical requirement is the availability of a standard reference material (present in a known amount by weight) that can be used to calibrate the assay (IHC stain). It is then possible to determine the amount of test analyte (protein) by a translation process from the intensity of IHC signals. In this respect it is helpful to consider the IHC stain as a tissue based ELISA assay (Enzyme Linked ImmunoSorbent Assay), noting that ELISA is used in the clinical laboratory as a standard quantitative method for measuring protein by weight in fluids, by reference to a calibrating reference standard. 

In Chapters 12 and 13 the individual reactions of metabolism are classified into these types and the enzymes that catalyze them are described in some detail. The chemistry of coenzymes and metalloenzymes are presented systematically in Chapters 14 to 16, and in Chapter 17 the logic of the combining of individual reactions into metabolic sequences is considered. It is not necessary to read Chapters 12-16 in their entirety since much of their content is reference material. In the later chapters on metabolism, cross-references point out the discussions of individual enzymes in Chapters 12-16. 

It is relevant to ask how often the routine measurement procedures currently used in laboratory medicine provide results that are traceable to high-level calibrators and reference measurement procedures (Lequin personal communication). It turns out that primary reference measurement procedures and primary calibrators are only available for about 30 types of quantity such as blood plasma concentration of bilirubins, cholesterols and sodium ion. International reference measurement procedures from the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) and corresponding certified reference material from BCR are available for the catalytic activity concentration of a few enzymes such as alkaline phosphatase and creatine kinase in plasma. For another 25 types of quantity, such... 

CK-MB can be measured in numerous ways. Immunoassays developed in recent years have improved on the analytical and clinical sensitivity and specificity of the earlier immunoinhibition and immunoprecipitation assays. These assays now (1) measure CK-MB directly and provide mass measurements, (2) are easily automated, and (3) provide rapid results (<30 minutes). Mass assays reliably measure low CK-MB concentrations in both samples with low total enzyme activity (<100 U/L) and with high total enzyme activity (>10,000 U/L). Furthermore, no interferences from other proteins have been documented. The majority of commercially available immunoassays that use monoclonal anti-CK-MB antibodies are the same as those listed in Table 5-2 for cardiac troponin assays. Excellent concordance has been shown between mass concentration and activity assays. A primary reference material is commercially available to assist in harmonization. If used for assay standardization, then this material allows... 

Type I water should be used in test methods requiring minimal interference and maximal precision and accuracy. Such procedures include trace metal, enzyme, and electrolyte measurements, and preparation of aU calibrators and solutions of reference materials. This water should be used immediately after production. No specifications for storage systems for type I water are given because it is not possible to maintain the high resistivity while drawing off water and storing it. 

For a reference system to be capable of standardizing the results of different assays of a given en2yme activity, some conditions must be satisfied. First, the reference procedure used to assign the value of the reference material and the routine method(s) to be calibrated must have identical specificities for the analyte enzyme (i.e., for the specific isoenzyme or isoform under study). Second, the properties of the calibrator material must be the same as or closely similar to those of the analyte enzyme in its natural matrix, typically serum (i.e., the material must be commutable with human serum samples for that particular method). 

Siekmann L, Bonora R, Burtis CA, Ceriotti F, Clerc-Renaud P, et al. IIFCC primary reference procedures for the measurement of catalytic activity concentrations of enzymes at 37°C. International Federation of Clinical Chemistry and Laboratory Medicine. Part 7. Certification of four reference materials for the determination of enzymatic activity of gamma-glutamyltransferase, lactate dehydrogenase, alanine aminotransferase and creatine kinase accord. Clin Chem Lab Med 2002 40 739-45. 

Amplification assays can produce both qualitative answers (yes/no, more or less abundant than reference materials) and quantitative answers (the original concentration of target sequence in the sample). In order to use the amplification process for quantitative assessments, many variables need to be carefully controlled. Variations in extraction efficiency, presence of enzyme inhibitors, lot-to-lot variation in enzyme and reagent performance, and day-to-day variation in reaction and detection conditions need to be addressed in methods that attempt to yield a quantitative result. 

Currendy, measurement of enzymes is based on catalytic-activity measurements, and these depend on the experimental conditions under which the enzyme is measured. It is often difficult to compare enzyme results from different laboratories or to those reported in the literature. Efforts to improve interlaboratory comparability in enzyme measurements (129, 130, 131, 132, 133, 134) have concentrated on two aspects standardization of methodology and preparation of reference materials (135). 

Use of the combination of reference methodology and materials to convert incompatible enzyme activity results for aspartate aminotransferase (146), alkaline phosphatase (147), and lactate dehydrogenase (148) to compatible values by use of a single scale, termed by the authors the International Clinical Enzyme Scale (ICES) has been suggested (149). Application of the ICES concept to the 1970 Scandinavian interlaboratory surv ey decreased the interlaboratory coefficient of variation from 38% to 16% for the enzymes tested. Similarly, in the 1971 New York State aspartate aminotransferase survev, the interlaboratory CV decreased from 41 %-44% to 2%-5%, a major improvement. The Scandinavian Committee on Enzymes has expressed serious concerns about the philosophy of the ICES approach, and they again endorsed the widely accepted approach of ongoing development of reference methodologies and proper use of reference materials (150). Since this flurry of activity in 1984 and 1985, there has not been any further application or acceptance of the ICES concept. 

Immunological methods for enzymes, more specifically isoenzymes, such as lactate dehydrogenase-1 (167, 168), mitochondrial aspartate aminotransferase (169), prostatic acid phosphatase (170, 171,172), and creatine kinase-MB (173, 174, 175), have been in use in the clinical laboratory for 10 years. However, the use of the immunological rather than catalytic properties of enzymes has not provided the opportunities for standardization that was anticipated a number of years ago (176, 177, 178). It is only within the last year that a working group on CK-MB mass assay was formed under the auspices of the Standards Committee of the American Association for Clinical Chemistry (AACC). The objective of this working group is to prepare a reference material to calibrate methods that are based on the principle of CK-MB mass measurement. 

Entry Starting Material Product Enzyme References... 

There are no recognized enzyme standards or reference materials. The accepted basis for measurement is the rate of reduction of the substrate, commonly nicotinamide adenine dinucleotide (NAD) for many reactions. The reduced form, NADH, absorbs at 340 nm, and the rate of change of this absorbance is measured in an enzyme-activity assay. The absorptivity a in Beer s law) is known for NADH from this and the rate of change of absorbance per unit time, the activity of the enzyme can be calculated in micromoles of substrate converted per minute. This is referred to as an International Unit (lU), expressing the activity as lU/liter. Therefore, an accurate, absolute absorbance scale must be established in each case in order to make a valid assay. 

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