Changes of the Quaternary Structure of Microvillus Aminopeptidase in the Membrane

Abstract

Microvillus [pig intestine] aminopeptidase is an enzyme with a MW .apprx. 300,000. Normal preparations contain 3 different subunits (subunit A, MW 162,000; subunit B, MW 123,000; subunit C, MW 61,000). The relationship between the 3 subunits was studied by immunoelectrophoresis using specific antibodies against individual denatured subunits and by densitometric scanning of polyacrylamide gels after separation of the 3 subunits. Microvillus aminopeptidase probably initially appears in the membrane as a symmetric molecule built up to 2 identical A subunits. These subunits are then split into equimolar amounts of subunit B and subunit C by trypsin. Subunit B cannot generate subunit C but may be further degraded. The reaction sequence described is one which occurs in vivo. Treatment of purified aminopeptidase with trypsin increases the specific activity 2-fold. This phenomenon does not seem to be correlated to the generation of subunit B and subunit C or to the transformation of amphiphilic form into hydrophilic form.