A family with high serum leucine aminopeptidase activity derived from a novel variant CD13

We investigated a family in which some individuals showed extremely high serum leucine aminopeptidase (LAP) (EC 3.4.11.2) activity, mainly derived from a variant CD13. The isoelectric points of the variant and normal CD13 were 3.3 and 4.1, respectively, and both points converged at 4.4 after treatment with neuraminidase, indicating that more sialic acids are bound to the variant CD13 than normal. The molecular masses of both CD13s were 144 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. After treatment with neuraminidase, N-glycosidase, and O-glycosidase, apparent molecular masses of the variant and normal CD13 were 106 kDa and 100 kDa, respectively, suggesting that the variant CD13 contains a longer peptide than normal. This is the first case of familial high serum LAP activity in which the origin could be demonstrated by anti-CD13 monoclonal antibodies to be a variant CD13 inherited in an autosomal dominant mode. The isoelectric point of the LAP activity after neuraminidase treatment was different from that previously reported.