The Power of Mass Spectroscopy as Arbiter for Immunoassays

In this issue of Clinical Chemistry , Henderson et al. present a new, elaborate method that uses mass spectrometry to analyze and quantify serum vitamin D binding protein (DBP)2 (1). This protein, initially discovered by its different electrophoretic mobility in various populations and therefore called group-specific component (GC), is used in population genetics and forensic medicine (2, 3). The genetic basis of the polymorphism is known to be 2 amino acid differences (aa 432 and 436) in the C domain of the protein, thereby generating 3 isoforms, GC1f, GC1s, and GC2, detectable by genetic analysis (single nucleotide polymorphisms RS7041 and RS4588). Because both alleles are codominant, this genetic diversity generates 3 homozygous and 3 heterozygous possibilities. A remarkably strange south-to-north gradient is evident: members of the original African population (and their descendants around the world) mainly express GC1f, whereas populations living further from the equator have gradually higher percentages of GC1s or GC2 alleles. Moreover, based on isoelectric focusing, >120 different types of DBP GCs have been identified, although the genetic or posttranslational origin of these isoforms remains largely unexplained. DBP is a glycoprotein with either 3 (GC1 isoforms) or 2 carbohydrate chains on some (2%–20%) of the circulating DBP molecules (4); it is a multifunctional protein probably belonging to the oldest member of the albumin gene family (1, 2). It has a very high affinity for actin and helps to depolymerize filamentous actin into globular actin when released in the bloodstream after cell damage (5). It is also the major binding protein for all vitamin D metabolites, with higher affinity for 25-hydroxyvitamin D \[25(OH)D\] (the prohormone) than for 1,25-dihydroxyvitamin D [1,25(OH)2D] (the vitamin D hormone). This is very similar to the situation for thyroid hormones, in which the T4 binds thyroxine-binding globulin much …