Biochemical differences in the mass and activity tests of lipoprotein-associated phospholipase A₂ explain the discordance in results between the two assay methods

Objectives: There are two platforms for the detection of Lp-PLA₂ in sera or plasmas: by its enzymatic activity (PLAC® activity test) and by its mass concentration (PLAC® mass test). It has been long recognized that these two platforms are not correlated well. The underlying cause for this is therefore investigated by the biochemical characterization of the two PLAC tests.

Design & methods: Human sera with and without the treatment by detergent were fractionated by using a Superose-6 column in phosphate buffered saline and the phospholipid associated Lp-PLA₂ was assessed by both PLAC mass and activity tests. The Lp-PLA₂ values of the two PLAC tests were compared under such conditions.

Results: Fractionation of sera and plasmas indicates that the association of Lp-PLA₂ with phospholipids, especially LDL and other large size phospholipid vesicles, may block the detection of the enzyme by antibodies in the immunoassay format under the conditions of the PLAC mass test. Inclusion of high concentration (>CMC, critical micelle concentration) of detergents in the assay buffer of PLAC mass test dissociates Lp-PLA₂ from phospholipid vesicles and results in the full detection of all Lp-PLA₂ in sera or plasmas for concentration. Such assay modification significantly improves the correlation between the PLAC mass and PLAC activity tests.

Conclusions: PLAC mass test only detects a small portion of the total Lp-PLA₂, mainly the Lp-PLA₂ associated with HDL. This is the main cause of the discordance and poor correlation between the PLAC mass and activity tests. Our results demonstrate the PLAC activity test is more accurate in assessing the total level of circulating Lp-PLA₂.