Reaction of 4-methylumbelliferylguanidinobenzoate with proteases in plasma of patients with cystic fibrosis.

Protease activity, assayed using 4-methylumbelliferylguanidinobenzoate, an active site titrant of certain proteases, is significantly deficient in plasma of patients with cystic fibrosis. The deficiency can be demonstrated with both chloroform-ellagic acid activated plasma in which the proteases can hydrolyze esters of arginine and unactivated plasma in which the proteases have negligible activity towards these esters. The deficiency can also be demonstrated by separation of the proteases by isoelectric focusing on polyacrylamide gels or by chromatography on agarose columsn. Since protease deficiency can be demonstrated with unactivated plasma, the deficiency in cystic fibrosis is probably due to a reduced number of protease molecules rather than their decreased catalytic efficiency.