A continuous method for the determination of leucine aminopeptidase in human serum with L-leucinamide as substrate

A continuous procedure for the determination of leucine aminopeptidase is described. L-leucinamide is used as substrate and the liberated ammonia is determined with the glutamate dehydrogenase reaction. The enzyme is Mn2+-activated and 30 mumol/l MnCl2 is necessary for an optimal activity measurement. Influence of buffer type, buffer concentration and pH are reported together with the apparent Km values of leucine aminopeptidase for L-leucinamide and of glutamate dehydrogenase for 2-oxoglutarate. Amastatin, a potent inhibitor, inhibits the reaction of leucine aminopeptidase completely, whereas it has no inhibitory effect on the reaction with glutamate dehydrogenase. The normal reference interval for leucine aminopeptidase is 12-65 U/l at 37 degrees C. The properties of the enzyme are discussed.