Proteins of the kidney microvillar membrane. Enzymic and molecular properties of aminopeptidase W

Abstract

Aminopeptidase W is a newly discovered enzyme of the renal and intestinal brush borders, having been first isolated as a 130 kDa glycoprotein recognized by a monoclonal antibody [Gee & Kenny (1985) Biochem. J. 230, 753-764]. It is particularly effective in the hydrolysis of dipeptides, Glu-Trp (K(m) 0.57 mM; κ(cat.) 6770 min-1) being a favoured substrate. Dipeptides with tryptophan, phenylalanine or tyrosine in the P1' position were rapidly hydrolysed, but the requirements in respect of the P1 residue were not stringent. The activity of aminopeptidase W is markedly influenced by ionic conditions. The highest activity was observed in 100 mM-Tris/HCl, pH 8; phosphate ions were strongly inhibitory. Activity was also greatly affected by bivalent metal ions, and the magnitude and direction of the effects depended on the nature of the buffer anions and on pH. The most effective inhibitors were amastatin and bestatin. Some thiols also inhibited, but other chelating agents, EDTA and 1,10-phenanthroline, had no effect over the concentration range 1-10 mM. Other group-specific inhibitors, for cysteine, serine or aspartic peptidases, were also ineffective. Some molecular properties were studied. Deglycosylation by treatment with N-glycanase diminished the apparent subunit M(r) from 130,000 to 90,000. The enzyme contained zinc, 1.2 atoms/subunit, and in spite of the atypical properties of this enzyme in respect of chelating agent, a zinc-catalysed mechanism is the most probable. Its role in digestion and in renal function are not yet clear.