Reaction of 4-methylumbelliferylguanidinobenzoate with cultivated human skin fibroblasts derived from patients with cystic fibrosis.

Protease activity in cultivated human skin fibroblasts has been quantitated using 4-methylumbelliferylguanidinobenzoate (MUGB), an active site titrant of trypsin-like proteases (7). The reaction of the proteases with MUGB was complete in 1 hr, inhibited both by benzamidine and (p-nitrophenyl)-p'-guanidinobenzoate, but not by p-hydroxymercuribenzoate. The extent of reaction was proportional to protein concentration and independent of MUGB concentration. This activity was present in the particulate fraction of the cell. The mean "titre" values (nanomoles of 4-methylumbelliferone released per mg protein) of the proteases in fibroblasts from eight controls (N), 8 obligate heterozygotes (H), and 14 patients with cystic fibrosis (CF) were: N, 1.27 +/- 0.11; H, 0.82 +/- 0.12; CF, 0.66 +/- 0.10. The differences in the "titre" values for N:CF and N:H were significant (p less than 0.001) as were those for H:CF (p less than 0.01). The mean "titre" value obtained for cultivated control amniotic fluid cells was 1.29 +/- 0.17. These data indicate a reduction in the MUGB-reactive proteases in skin fibroblasts derived from patients with CF when compared either to control or to obligate heterozygotes. These data are consistent with our earlier suggestion (11, 15) that decreased proteolytic levels in the tissues and fluids of patients with CF may be a generalized phenomenon.