Sensitive Automated Elisa For Measurement Of Vitamin D-Binding Protein (Gc) In Human Urine

We report a sensitive automated ELISA for measurement of group-specific component (Gc; also known as Gc globulin and vitamin D-binding protein) that detects lower concentrations than any other published method. This new ELISA enables measurement of Gc in human urine. Gc is a 50- to 58-kDa multifunctional plasma protein synthesized mainly by hepatocytes and usually present in plasma in concentrations between 4 and 6 μmol/L. The functions of Gc are diverse. Gc is an important player in the actin-scavenger system, which prevents the harmful consequences of actin in the blood stream during tissue injury. It binds actin monomers with high affinity, and Gc–actin complexes are rapidly cleared from the circulation (1). Gc also has functions in the immune system, acting as a co-chemotactic factor together with complement C5a (2) and, after deglycosylation, as a very potent macrophage-activating factor (Gc-MAF) also capable of activating osteoclasts (3)(4). The name “vitamin D-binding protein” is derived from its ability to bind and transport vitamin D metabolites. Usually, <0.1% of 25-hydroxyvitamin D (25OHD) and <1% of 1,25-dihydroxyvitamin D [1,25(OH)2D] circulate in their free forms (5). Gc is probably also important for the renal activation of 25OHD to 1,25(OH)2D. Mice lacking the multifunctional receptor megalin lose Gc–25OHD complexes in the urine and are deficient in 1,25(OH)2D (6). Thus, there is evidence that, generally, Gc and Gc–25OHD complexes are filtered in the glomerulus and reabsorbed by megalin-mediated endocytosis into the proximal tubular cells, where vitamin D activation takes place. This theory postulates that, under healthy conditions, only trace amounts of Gc should be excreted in the urine, whereas urinary loss of Gc is expected to increase with decreases in the capacity for reabsorption in the proximal tubules. To evaluate the fate of Gc in humans with various kidney diseases, …