Serum Proprotein Convertase Subtilisin Kexin Type 9 Is Correlated Directly with Serum LDL

type 9 (PCSK9) is gaining attention as a key regulator of serum LDL-cholesterol (LDLC). This novel serine pro-tease causes the degradation of hepatic LDL receptors by an unknown mechanism. In humans, gain-of-func-tion mutations in the PCSK9 gene cause a form of familial hypercholesterolemia, whereas loss-of-function mutations result in significantly decreased LDLC and decreased cardiovascular risk. Relatively little is known about PCSK9 in human serum. Methods: We used recombinant human PCSK9 protein and 2 different anti-PCSK9 monoclonal antibodies to build a sandwich ELISA. We measured PCSK9 and lipids in 55 human serum samples and correlated the results. We used the anti-PCSK9 antibodies to assay lipoprotein particle fractions separated by sequentia