Human kidney alanine aminopeptidase: physical and kinetic properties of a sialic acid containing glycoprotein.

Human kidney alanine aminopeptidase has been purified to apparent homogeneity as judged by electrophoresis and sedimentation in the analytical ultracentrifuge. Amino acid analyses indicate that the enzyme is high in tryptophan content and low in cysteine content. The enzyme contains sialic acid, hexoses, and glucosamine, which make up 21% of its dry weight. In dilute buffer, the enzyme exhibits a molecular weight near 236 000, but in denaturing solvents the enzyme exhibits a molecular weight near 119 500. Zinc analyses by atomic absorption demonstrate 1 mol of zinc for 113 500 +/- 6900 g of protein. The zinc is firmly bound, since exhaustive dialysis against chelating agents does not remove the zinc or inactivate the enzyme. The enzyme is stimulated by Co2+ 1.65-fold, but, in contrast to the enzyme-zinc complex, the enzyme-cobalt complex dissociates upon dialysis. Kinetic studies with a series of aminoacyl-beta-napthylamides indicated that the highest kcat value was obtained for L-alanyl-beta-naphthylamide (8.43 X 10(3)s-1), whereas the lowest Km value was obtained for L-methionyl-beta-naphthylamide (1.4 X 10(-5) M).