Amidase activity of human Bence Jones proteins

Bence Jones proteins purified from urine of patients with multiple myeloma were found to be capable of hydrolyzing carbobenzoxy-L-valyl-glycyl-L-arginine p-nitroanilide (Chromozym TRY) and benzoyl-L-arginine p-nitroanilide (BApNA), synthetic chromogenic substrates for trypsin. The amidolytic activity obeyed classic Michaelis-Menten kinetics, exhibiting optimal activity around pH 8.4 and apparent Km of 140-730 microM and 18-27 microM for Chromozym TRY and BApNA, respectively. No activity was detected with intact IgG or Fab fragment, whereas the activity comparable to those of Bence Jones proteins was found with light chain derived from inactive IgG. Several lines of circumstantial evidence indicate that the observed activity was not due to contaminating enzyme.