Alanine aminopeptidase in serum: biological variations and reference limits.

We studied the effect of various factors on the biological variation of alanine aminopeptidase (AAP, EC 3.4.11.2) in serum in a population of 2178 apparently healthy subjects and in subjects taking specific drugs. To measure AAP, we used an automated assay, with alanyl-4-nitroanilide as substrate. AAP activity concentrations were significantly higher in males than in females in all age groups between 10 and 55 years. The highest mean AAP values were found for children between 10 and 14 years. In male heavy smokers, AAP values were 8% higher than in moderate or nonsmokers (P less than 0.001); in females, this effect was less pronounced. AAP concentrations were higher in subjects consuming over 44 g of ethanol a day than in those consuming less (P less than 0.01 for men, P less than 0.05 for women). The use of oral contraceptives increased AAP values by 12% (P less than 0.001). Hypolipidemic drugs caused lower values for AAP in men (P less than 0.001). Subjects showing induction of gamma-glutamyltransferase (EC 2.3.2.2) by anticonvulsant drugs had also higher AAP activities (by 23%). Taking these data into account, we established reference limits for AAP in serum.