Protein chemical characterization of Gc globulin (vitamin D-binding protein) isoforms; Gc-1f, Gc-1s and Gc-2.

Gc globulin, also called vitamin D-binding protein, is a plasma protein involved in the extracellular actin-scavenger system, vitamin D transport and possibly also other biological activities. Low levels of Gc globulin have been found to correlate with multiple organ failure and non-survival of patients with fulminant hepatic failure and trauma. Here, we characterize the dominant isoforms of plasma-derived Gc globulin from Cohn fraction IV paste with respect to amino acid sequence and posttranslational modifications. Gc globulin was purified in large scale and the isoforms separated by ion exchange chromatography. The separated isoforms and several commercial preparations of individual isoforms were characterized by mass spectrometry. This revealed that the major isoforms were non-glycosylated. Compared to the Gc-1f isoform the other dominating isoforms represented an Asp/Glu substitution (Gc-1s) and a Thr/Lys substitution (Gc-2) in agreement with DNA sequencing studies. The commercial preparations were found to represent mainly one or two isoforms. An O-linked glycan with a mass of 656 Da and terminating with a sialic acid residue was detected on a minor proportion of Gc globulin molecules.