A novel mammalian high-molecular-weight aminopeptidase

Studies with the human lymphoma U937 cell line revealed the presence of two soluble aminopeptidase activities. Using specific antisera one of these was identified as the puromycin-specific aminopeptidase, while the other appeared to be a novel ~200-kDa activity. The kinetic properties of this high-molecular-weight aminopeptidase, referred to as Ap²⁰⁰, were similar to those of the puromycin-sensitive aminopeptidase, but showed quantitative differences. Ap²⁰⁰ is relatively insensitive to inhibition by both puromycin, K(i) = 27 μm, and bestatin, K(i) = 1.6 μm. Among the synthetic β-naphthylamides, Ap²⁰⁰ is more specific for alanine-β-naphthylamide compared to the puromycin-sensitive aminopeptidase. similarly, this enzyme cleaves a more limited number of a physiological peptides exhibiting a preference for the enkephalins. Ammonium sulfate, but not sodium chloride at the same ionic strength, was able to dissociate the high-molecular-weight aminopeptidase to a ~100-kDa active form. The high-molecular-weight aminopeptidase is found as a low abundant protein in a number of tissues including intestine, kidney, liver, lung, muscle, spleen, and testes, but could not be detected in adrenal, heart, or brain. Thus, it has a tissue which differs from the puromycin-sensitive aminopeptidase.