Methylumbelliferyl-guanidinobenzoate reactive proteases and prenatal diagnosis of cystic fibrosis [letter].

For several reasons the supernatant fraction of amniotic fluid is not often 
used in the prenatal detection of inborn errors of metabolism. In 
early pregnancy the bulk of the protein in amniotic fluid is 
derived from the maternal circulation, and this can obscure the more 
critical contribution from the fetus needed for prenatal diagnosis. 10 - 
30% of amniotic fluids are significantly contaminated with maternal blood, and 
if the enzyme in question is present in this blood (as 
4-methylumbelliferyl guanidinobenzoate [MUGB] protease is), this can further distort diagnostic conclusions. 
For this reason we have examined the variation of MUGB protease 
in a heterogenous collection of amniotic fluids. No significant differences in 
titre between individual weeks in the second trimester were observed. A 
further 104 second-trimester amniotic fluids from pregnancies with abnormal outcome were 
also investigated. The means of the four groups were not significantly 
different from that of the normals and the ranges were very 
similar. We have not yet had the opportunity to examine an 
amniotic fluid from a known case of cystic fibrosis. Diagnosis of 
cystic fibrosis could probably be further refined by coupling amniotic-fluid protease 
titration and isoelectric focusing on polyacrylamide gel. 