Certification of an enzyme reference material for alkaline phosphatase (CRM 371)

Production and certification of an enzyme reference material for adenosine deaminase 1 (BCR 647)

BACKGROUND

We describe the preparation of a lyophilised reference material containing purified human adenosine deaminase 1 and the certification of its catalytic concentration.

METHODS

The enzyme was purified from human erythrocytes.

RESULTS

The enzyme was >99% pure on polyacrylamide gel electrophoresis. Only trace amounts (<0.4%) of alanine aminotransferase, aspartate aminotransferase and L-lactate dehydrogenase were detected in the purified fraction. The purified adenosine deaminase had a molar mass of 41600 g/mol and an isoelectric pH at 4.7, 4.85 and 5.0. The material was prepared by diluting the purified adenosine deaminase in a matrix containing 50 mmol/l Tris-HCl buffer pH 7.4 and 30 g/l human serum albumin; dispensing in vials and freeze-drying. The batch was homogeneous and the predicted loss of adenosine deaminase activity per year on the basis of accelerated degradation studies was 0.006% at -20 degrees C and 0.04% at 4 degrees C. The certified value for adenosine deaminase catalytic concentration in the reconstituted reference material is (2.55+/-0.09) microkat/l when measured by the method that uses adenosine as substrate and glutamate dehydrogenase as auxiliary enzyme at 37 degrees C.

CONCLUSIONS

The material can be used to verify the comparability of results from different laboratories, for intra-laboratory quality control, or for calibration of the adenosine deaminase catalytic concentration measurements.

Preparation of enzyme calibration materials

Standardisation in clinical enzymology needs not only reference methods but also reference materials. While single-enzyme reference enzymes have been developed, a multienzyme certified reference material (MECRM) available in high amount remains to be produced. To transfer trueness from the value of the reference system to patients' results, validated enzyme calibrators (EC) are also needed. Both the MECRM and the ECs must exhibit the same catalytic properties as the corresponding enzymes in human plasma. Moreover, commutability of these materials with patients' samples must be experimentally tested for one or a set of methods defined by an analytical specificity equal to that of the reference method. Various experimental studies have shown that the commutability of an enzyme material depends on the source of enzyme and its purification process, the matrix (including cofactors, effectors, additives, stabilisers... ) and the mode of processing of the final material. To promote intermethod calibration in clinical enzymology, a collaborative programme between the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC), Institute for Reference Materials and Measurements (IRMM, Geel, Belgium) and IFCC corporate members is in progress for the development of a MECRM containing amylase, ALT, AST, ALP, CK, GGT, LDH, and lipase and exhibiting a wide and defined commutability.

Gene transfer technologies for the production of enzyme and protein reference materials

To maintain the success of recommended methods and to allow comparison among various methods of enzyme analysis, enzyme reference materials are required, having catalytic properties as close as possible to those of the corresponding human enzymes. Though human sources are preferable, ethical reasons require the extraction and purification from animal tissues. By providing theoretically unlimited amounts of material, gene transfer technologies and mass culture can overcome the need of human or mammalian tissues. We have used these technologies to produce human gamma-glutamyltransferase (GGT) and pancreatic lipase (PL) in various types of host cells. Different strategies were tested, especially for GGT, depending on the inherent properties and requirements of the human enzyme. Expression and purification protocols were optimized, yielding good amounts of recombinant enzymes which share many physico-chemical and catalytic features with their natural counterparts. Kinetic constants and catalytic behavior were very similar, demonstrating the usefulness of these products as reference materials. We assume recombinant DNA technologies could be successfully applied to most enzymes or proteins assayed in clinical chemistry laboratories.

Usefulness of reference materials in calibration of enzyme activities

alpha-Amylase, alkaline phosphatase and gamma-glutamyltransferase were studied in a multicentre evaluation. Analyses were performed on different patient samples. Each enzyme was assayed in two different laboratories at both 30 and 37 degrees C, with widely used reagent kits and with the IFCC reference method (if in existence). Results differed considerably according to the measurement procedure. Data also showed that it was not possible to employ a constant conversion factor for one enzyme and different techniques between 30 and 37 degrees C. Calibration with three reference materials extensively improved the intermethod consistency for most of the tested measurement procedures. It was possible to transfer accuracy from the method used for the certification of the reference material to routine procedures, by using the reference material as calibrator. Temperature did not seem to be a crucial variable for the implement of the enzyme calibrator approach.

Multi-enzyme reference material from established human cell lines and human sources

A multi-enzyme reference material was prepared from seven enzymes of asparatate aminotransferase (AST, EC 2.6.1.1), alanine aminotransferase (ALT, EC 2.6.1.2), alkaline phosphatase (ALP, EC 3.1.3.1), lactate dehydrogenase (LD, EC 1.1.1.27), creatine kinase (CK, EC 2.7.2.2), gamma-glutamyltranspeptidase (gamma-GT, EC 2.3.2.2) and amylase (AMY, EC 3.2.1.1) which were purified from human sources including established human cell lines. The enzymatic properties of the material closely resembled those of human serum. In lyophilized form the preparation was stable for at least 200 days when stored at 40 degrees C. Intermethod comparisons of the enzyme activities in 80 clinical specimens were done by correcting the mean values with calibration constants for different assay methods resulting from use of a human serum, the multi-enzyme reference and a commercial control serum. The results from the comparison for the six enzymes of AST, ALT, LD, CK, gamma-GT and AMY in use of the multi-enzyme reference were almost the same as those with use of a human serum as a calibrator, but were not satisfactory for ALP. Even though further search for more reliable material for ALP is required the multi-enzyme reference material can be used for standardization in clinical chemistry.