Protein standardization I: Protein purification procedure for the purification of human prealbumin, orosomucoid and transferrin as primary protein preparations

Protein standardization III: Method optimization basic principles for quantitative determination of human serum proteins on automated instruments based on turbidimetry or nephelometry

Quantitative protein determinations in routine laboratories are today most often carried out using automated instruments. However, slight variations in the assay principle, in the programming of the instrument or in the reagents may lead to different results. This has led to the prerequisite of method optimization and standardization. The basic principles of turbidimetry and nephelometry are discussed. The different reading principles are illustrated and investigated. Various problems are identified and a suggestion is made for an integrated, fast and convenient test system for the determination of a number of different proteins on the same instrument. An optimized test system for turbidimetry and nephelometry should comprise high-quality antibodies, calibrators, controls, and buffers and a protocol with detailed parameter settings in order to program the instrument correctly. A good user program takes full advantage of the optimal reading principles for the different instruments. This implies--for all suitable instruments--sample preincubation followed by real sample blanking, which automatically corrects for initial turbidity in the sample. Likewise it is recommended to measure the reagent blank, which represents any turbidity caused by the antibody itself. By correcting all signals with these two blank values the best possible signal is obtained for the specific analyte. An optimized test system should preferably offer a wide measuring range combined with a wide security range, which for the user means few re-runs and maximum security against antigen excess. A non-linear calibration curve based on six standards is obtained using a suitable mathematical fitting model, which normally is part of the instrument software.

Protein standardization II: Dry mass determination procedure for the determination of the dry mass of a pure protein preparation

A procedure has been developed for the determination of the dry mass of a pure protein preparation dissolved in one electrolyte. The procedure not only renders the concentration (in g/l), but additionally gives the partial specific volume of the protein (in ml/g). The latter is an important parameter for the characterization of a specific protein. Furthermore, the importance of extensive dialysis against one electrolyte is discussed. For the drying of human serum proteins it is clearly shown that KCl is preferred to NaCl due to its stable temperature curve. By observing the parallel fluctuations in the weight of the empty vials during drying, an average correction factor is introduced, which greatly minimizes these changes. The assay principle is discussed from a mathematical as well as from a practical point of view. A detailed procedure is described and the final results for three primary pure proteins (prealbumin, orosomucoid and transferrin) are presented. Finally, important parameters such as the wavelength of maximum absorbance, the absorption coefficient and the specific refractive increment are discussed and values for the three proteins are presented. When these parameters have been established the future determination of concentration and the characterization of pure protein solutions are greatly facilitated. These procedures were important tools for ascribing mass values for prealbumin, orosomucoid and transferrin to the international reference preparation CRM 470.

Protein standardization IV: Value transfer procedure for the assignment of serum protein values from a reference preparation to a target material

A new approach for the assignment of values to serum proteins in a target material using a reference preparation has been developed. The procedure describes the general as well as the practical principles involved in the value assignment (with examples). Two models have been developed: 1) The direct value transfer between serum matrices and 2) the indirect value transfer from a pure protein preparation to a serum protein material. The necessary mathematical equations are developed and explained. The data reduction and statistical evaluation are discussed. The practical procedure (the transfer protocol) is based on six dilutions of the reference preparation assayed together with six dilutions of the target material. In this way imprecision is reduced and the proportionality of the two materials (i.e. the presence or absence of matrix effects) can be assessed directly by evaluating a single regression plot. If no matrix effects are found, the regression line will pass through zero with a slope equal to the ratio of the concentrations of the two materials. The transfer protocol is based on a multiple point value assignment obtained by several measurements a day repeated on several days, an important prerequisite being that all reconstitutions and dilutions are controlled by weighing.