Studies of the clinical application of serum leucine aminopeptidase (LAP) activity determined with leucinamide as substrate.

A new method was presented for the determination of leucine aminopeptidase (LAP). The principle of the method consisted in the measurement of ammonia liberated by the action of LAP with direct colorimetry. Leucine naphthylamide has been widely used as substrate for the determination of LAP (Nap-method), but leucineamide was used (NH3-method) in this study, and the enzyme activities determined by the both methods were compared in various diseases. Serum LAP activity in acute hepatitis was much higher in NH3-method than in Nap-method, but the activity in obstructive jaundice was much higher in the latter than in the former. It was demonstrated that the serum of normal rats and CCl4 treated rats contained isozymes (LAP-I and II) which showed the different substrate specificities toward leucinamide and leucine naphthylamide. LAP-I activity was much higher, but LAP-II activity was much lower in the NH3-method than in the Nap-method. LAP-I activity was remarkably elevated by the NH3-method, but not by the Nap-method in the serum of CCl4 treated rats. The results suggested that leucinamide was preferable substrate for the measurements of activities of serum LAP in the clinical examinations.