Lipoprotein Lipase- The 5D2 Antibody
Now and again in science, a well-accepted dogma comes to be challenged, and researchers find themselves re-evaluating years of research. The history of Lipoprotein Lipase (LPL) is just one example of this.
What is LPL?
LPL is a triglyceride hydrolyzing enzyme that has been widely studied. It can mediate interactions of lipoproteins with cell surfaces and receptors, and deficiency or low activity can lead to hypertriglyceridaemia (a risk factor for atherosclerosis) or, in severe cases, to severe pancreatitis. The enzyme has been described as a central molecule in plasma lipid metabolism.
The 5D2 antibody
The 5D2 antibody specific for LPL is a reagent that is remarkably well characterised. In an era in which the specificity of many antibodies is being questioned, the data available for the 5D2 antibody is exemplary.
Back in 1998 Chang et al (1) published work that showed that the 5D2 antibody could inhibit the hydrolytic activity of LPL against long chain triglycerides. They showed that this was due to the antibody effectively sharing a binding site with that of the substrate triglycerides that LPL act upon, in a tryptophan rich loop of amino-acids. Importantly, they also published immunoassay data that indicated that the 5D2 antibody could detect functional LPL in a self-pairing ELISA assay (one in which the same antibody is used both as a capture and detection reagent). This assay indicated that functional LPL was therefore in homodimeric form, with two chains folding together, and thereby presenting two epitopes for binding. Monomeric LPL was not recognised by 5D2 to the same degree and was considered to be an inactive form. The dogma that LPL is only active in homodimeric form survived for more than 20 years.
The homodimer story began to change with a publication by Beigneux et al in 2018 (2). This group showed that freshly isolated monomeric LPL was indeed enzymatically active. Additionally, they found that the 5D2 antibody did not recognize monomeric LPL in a self-paired ELISA. Importantly, 5D2 does recognise monomeric LPL in applications such as Western Blot, confirming it’s specificity. How did they explain this after all of this time? It’s likely that an artefact from the presence of heparin in the samples used in the original analyses led to the confusion, as heparin is likely to lead to dimerization of LPL.
The 5D2 antibody now
Of course the 5D2 antibody itself hasn’t changed at all during this period. It remains a remarkably important reagent, and will continue to be used by researchers investigating the role of LPL for many years to come.
It is, however, a perfect example of how science moves on and that the interpretation of results may always be open to revision.
References
(2) Beigneux, A.P. et al (2019) Lipoprotein lipase is active as a monomer PNAS USA 116:6319