Changes with age of the rat fetuin concentration in serum and its mRNA expression

Rat fetuin, a counterpart of human alpha2-HS glycoprotein and bovine fetuin, shows strong intermolecular binding and association with other serum proteins. Therefore, to measure its concentration in rat serum, we pretreated serum samples with 1% SDS plus 5% (ca. 0.7 M) 2-mercaptoethanol at 100 degrees C for 3 min, and then subjected them to SDS-PAGE under reducing conditions followed by Western blotting. We found that the fetuin concentrations in normal rat serum determined by Western blotting were 2.5-4.5 mg/ml. These concentrations were three orders of magnitude higher than the previously reported concentrations. We also tried to measure the fetuin concentration in rat serum by means of an enzyme-linked immunosorbent assay after treatment of the samples with 0.1% sodium dodecyl sulfate (SDS) plus 10 mM 2-mercaptoethylamine at 100 degrees C for 3 min, but it gave a value of about 1/4 of that on Western blotting. Rat fetuin is expressed mainly in the liver, with a peak 2-4 weeks after birth, as determined by Northern blot analysis. The fetuin mRNA level in the liver changes almost in parallel with its serum concentration. The tibia also expresses fetuin, but much less than the liver.