Vitamin D Binding Protein is a multi-functional plasma protein with many important functions. The most well known one, and the reason for its name, is the transport of vitamin D metabolites. Because VDBP is the primary transporter of vitamin D, it has a role in maintaining the total levels of vitamin D in the body and in regulating the amounts of free (unbound) vitamin D available for specific tissues and cell types to utilise.
Other functions of VDBP are to control bone development, binding of fatty acids, sequestration of actin and a range of less-defined roles in modulating immune and inflammatory responses. The actin removal is considered to be a very important role for VDBP, actins being toxic and released into the body following cell death.
Two amino acids on this protein chain have been shown to be very strongly associated with the activation of macrophages. In some publications, the vitamin D binding protein is shown to respond to enzymes released by inflammation. Studies show that this isoform not only has the potential to activate macrophages, it can turn them off when no longer needed, so can be classed as an immune system regulator.