A serine protease activity of human serum albumin towards 4-methylumbelliferyl-guanidinobenzoate (MUGB) and diisopropyl fluorophosphate (DEP): implications for the use of MUGB reactivity in amniotic fluid in prenatal diagnosis of cystic fibrosis

4-methylumbelliferylguanidinobenzoate (MUGB) reactivity in plasma from patients with cystic fibrosis and in amniotic fluid from pregnancies leading to children with cystic fibrosis, has been reported to be significantly decreased. We have so far been unable to confirm these findings and have therefore reexamined this reactivity using diisopropylfluorophosphate (DEF), another active site titrant of serine proteases. We have shown that MUGB and DFP are reacting with the same molecules in plasma and amniotic fluid. Using crossed immunoelectrophoresis and SDS-polyacrylamide gel electrophoresis of 3H-DFP labelled plasma and amniotic fluid we have obtained strong evidence that the main contribution of MUGB and DFP reactive protein in plasma and amniotic fluid is identical to serum albumin. The use of MUGB reactivity in amniotic fluid in pregnancies at risk for cystic fibrosis must therefore be reconsidered.