Impact of protein measurements on standardization of assays of apolipoproteins A-I and B1.

In 1989 the Committee on Apolipoproteins of the International Federation of Clinical Chemistry and the Centers for Disease Control conducted an international survey of total-protein measurements of isolated low-density lipoproteins (LDL) and delipidated high-density lipoproteins (HDL), and of their relationships to the National Institute of Standards and Technology (NIST) bovine serum albumin (BSA) Standard Reference Material (SRM). Most of the 93 apolipoprotein laboratories surveyed use the Lowry total-protein method. Results reported with the LDL preparations demonstrated a large bias and variation among methods; those with delipidated HDL were not as great, but were similar to those for BSA. Performance improved appreciably with use of the Lowry-sodium dodecyl sulfate method and the NIST BSA SRM for protein measurement. The total CVs, including among-laboratory and within-laboratory errors, averaged approximately 50% and 23% for LDL, 17% and 11% for HDL, and 18% and 11% for BSA solutions by all methods and by the Lowry methods, respectively. Regardless of the methods used, greater variability of the protein measurements was seen with the normally occurring LDL than with the nonlipoprotein BSA or delipidated HDL. The mean CV values for all samples among laboratories averaged between 10% and 15% with the modified Lowry methods; the biuret method gave the highest among-laboratory CV, 34%; the Kjeldahl had the lowest, 7.7%. Use of the same methodology and primary nonapolipoprotein standard is essential for comparability of protein results for apolipoprotein primary standard solutions. This is especially true for apolipoprotein B, because its inherent properties and lability make protein analysis difficult. This study supports the use of a standardized selected Lowry-sodium dodecyl sulfate method traceable to quantitative amino acid analysis as a point of reference for determining the protein concentration of primary calibration reference materials for apolipoproteins.