BIKUNIN IN RAT PLASMA, LYMPH AND BILE

Bikunin is a protease inhibitor consisting of a 16 kDa polypeptide and an 8 kDa chondroitin sulphate chain which has an apparent molecular m ass of 60-70 kDa upon gel filtration. It is synthesized by hepatocytes and occurs in plasma, both in free form, and in complex with other po lypeptides - mainly as the 180 kDa protein inter-alpha-inhibitor. Biku nin binds to proteases less avidly than other plasma inhibitors, makin g its role in the blood unclear. However, some observations indicate t hat bikunin has important functions outside the blood system. To asses s its capacity to reach extravascular spaces, we have determined the t otal concentration of bikunin in plasma (0.17 mg/ml), lymph (31 mu g/m l) and bile (0.2 mu g/ml). Quantitation after removal of complexed bik unin (inter-alpha-inhibitor) by acid precipitation showed that the con centration of free bikunin in those fluids was 3, 1.4 and 0.05 mu g/ml , respectively. These values yield a lymph/plasma ratio of free bikuni n of 0.5, which is higher than expected far a protein of the hydrodyna mic size and charge of bikunin. The bile/plasma ratio (0.02), however, is similar to that of other proteins of comparable size. The correspo nding values for inter-alpha-inhibitor, 0.16 and 0.001, respectively, indicate that its capacity to pass through the vascular endothelium is relatively high whereas transfer to bile is restricted. Furthermore, we have found that in a perfusate of an isolated rat liver, the ratio of free to complexed bikunin was 30-40 times higher than in plasma, co nsistent with previous observations showing that free bikunin is clear ed from the blood stream much more rapidly than inter-alpha-inhibitor.