On the subunit structure of particulate aminopeptidase from pig kidney

Solubilization of particulate aminopeptidase (EC 3.4.11.2) from pig kidney with Triton X-100 yields an aggregate (mol. wt. approx. 10(6)) that decomposes into "free" aminopeptidase (mol. wt. 280 000) either upon autolysis at pH 5 or after exposure to trypsin. Both procedures yield free enzymes that are identical with respect to electrophoretic mobility, enzymatic activity and zinc content. After dissociation, the enzyme resulting from autolysis yields a single subunit of 140 000 molecular weight while the trypsin-treated enzyme produces three fragments (140 000, 95 000 and 48 000 mol. wt.). As the aggregate is formed by subunits 10 000 daltons heavier than those of the free enzyme, the existence of a hydrophobic portion anchoring the enzyme to the membrane might be postulated. Reactivation experiments carried out on the three purified fragments of urea-denatured aminopeptidase show that the 140 000 molecular weight subunit is the only one able to yield an active enzyme (after spontaneous dimerization). It can be concluded that the smaller fragments are artefacts resulting from trypsin degradation during purification.