Is the proline-specific aminopeptidase P of the intestinal brush border an integral membrane enzyme?

It has been found that the microvillous membrane of rat enterocytes contains an aminopeptidase P which is one of the few enzymes capable to hydrolyze the peptide imido bond on the N-terminal side of proline residues. The enzyme was enriched 9-fold in chromatographically purified brush border membrane vesicles of the small intestine. Various extraction procedures and proteinase treatments yielded strong evidence that it is an integral part of the membrane without a stalked hydrophilic head exposed to the outer surface. It was solubilized by detergent and further enriched by ion exchange chromatography up to 73-fold.