Carboxypeptidase A activity of cultured skin fibroblasts and relationship to cystic fibrosis.

Properties of carboxypeptidase A of cultured skin fibroblasts from control and cystic 
fibrosis patients were studied using alpha-N- carbobenzoxy-L-glutamyl-L-tyrosine as substrate. Carboxypeptidase A 
was inhibited by thiomersal, cyanide, iodoacetate and N- ethylmaleimide in a 
similar manner for control and cystic fibrosis fibroblasts. Both trypsin and 
dithiothreitol treatment activated the enzyme, but 1,10-phenanthroline inhibited only in the 
presence of dithiothreitol. Both Zn2+ and Co2+ reversed this inhibition. Trypsin 
treatment of carboxypeptidase A produced a form of the enzyme having 
a higher KM value for both control and cystic fibrosis fibroblasts. 
Dithiothreitol treatment of control fibroblasts resulted in a form with similar 
properties to the trypsin activated form, but cystic fibrosis fibroblasts yielded 
a variant form with even higher KM and Vmax values. Since 
other properties were similar, it seems likely that this difference reflected 
binding of a molecule to the enzyme rather than of a 
defect in the enzyme. 