A family with a high serum aminopeptidase (microsomal) activity: properties of the enzyme from serum of the propositus.

We found a family with a high activity for hydrolyzing L-alanyl-beta-naphthylamide in their serum. This enzyme was confirmed to be aminopeptidase (microsomal) (EC 3.4.11.2) by means of immunological experiments involving anti-human kidney aminopeptidase (microsomal) antibody. We could not find the cause of the increased activity from the results of clinical and laboratory examinations. The enzyme from the propositus resembled that from the serum of normal subjects in molecular mass (240 000 Da), Km value (87 mumol/L), and degree of inhibition by antiserum to human kidney aminopeptidase (microsomal). It differed from the normal enzyme with respect to electrophoretic mobility (R1 value, 0.58; normal, 0.51), isoelectric point (pI, pH 3.4; normal, pH 3.8), and heat stability (more labile than normal). From information on this family and another one reported in the Japanese literature, we suggest that the mode of inheritance of a high activity of serum aminopeptidase (microsomal) may be autosomal dominant.